I know from hearing snippets of conversations that reduced sized sensors have different DoF characteristics than that of a FF sensor with same lens, f/stop, etc. But I don't know why. Could I persuade anyone to enlighten me, please?

While there's a lot of optical theory involved in an exact answer, most of which is over my head (things such as circle of confusion, degree of enlargement, etc), the primary reason is relatively simple.

To get the equivalent field of view and image framing with a larger format camera, you must use a longer lens at the same focus distance. Think of it this way, if you're shooting 35mm with a 200mm lens at 8 feet distance, your depth of field is very shallow, even at f/8. Now a large format shooter has to do the same thing (200mm lens, 8 feet) just to shoot a waist up portrait. They're still stuck with the depth of field of a 200mm lens, focused at 8 feet and f/8, even though the field of view is closer to a 55mm lens (in 35mm format).

Hope this helps!

Sheldon

DOF is proportional to magnification. Not just at the sensor, but also in relation to display size. When looking at an 8x10 print, which image was enlarged more, an APS size one, or a full-frame 35mm one? So when using the same focal length, the APS image is being enlarged more to reach the same display size and suffers a slight loss of DOF - notice, the angle of view is not the same. (If the display sizes are changed in relation to the different sensor sizes then you are back to the same DOF.)

However, when at the same angle of view, which means different focal lengths and image plane magnifications, the smaller sensor has greater depth of field.

BTW, this assumes the same viewing distance. DOF increases with viewing distance of the display as well.

Anon E Mouse is right that the issue of degree of enlargement affects depth of field. It is a little confusing in that the principle works in what you would think is the wrong direction. For example, small formats have to be enlarged more to reach a given print size, so they in theory would have less depth of field (more enlargement = less depth of field).

However, having to use a longer focal length lens to achieve an equivalent angular field of view has a much larger effect, so the net result is that larger formats have functionally less depth of field at comparable angular fields of view, focus distance, and aperture.

For a sense of the scale of the issue, here are some rough guidelines to think about in terms of depth of field.

A 1.6 crop camera has roughly 1 1/3 stop more depth of field than a full frame camera. (ie. f/1.8 on 20D with 50mm lens = ~f/2.8 on 5D with 80mm lens).

A 35mm full frame camera has about 4 1/2 stops more depth of field than a 4x5 camera. (ie. f/1.4 on 5D with 35mm lens = ~f/6.4 on 4x5 with 135mm lens)

This conversation suggests that a less-than full frame sensor increases subject magnification and affects DOF. It does not.

It is a misnomer to refer to the effect of a smaller sensor as a multiplier, a multiplication effect, etc. Camera manufacturers label it that way, and that is not at all what happens. It confuses people.

A 100mm lens still has a 100mm focal length, whether it is used on a full frame sensor or not. The sensor size merely affects the field of view, not the subject magnification.

I have used a Canon 180mm "L" macro lens for years. Whether I use it on my 1Ds MkII or my Canon 20D, I get exactly the same size subject if I use a tripod and keep the subject distance fixed. Since the subject distance and subject magnification are unaffected, so is the DOF. All of this is very easy to demonstrate.

What will be different is that the 1Ds MkII image will show a wider FOV. The FOV when the same lens is used on a 20D is much narrower. It is approximately the same as if the lens was 270mm. This is often a benefit for macro shots because it makes it easier to separate the subject from the background.

When you use a smaller sensor but keep the focal length the same with a 35mm lens, the smaller sensor captures a smaller part of the image from the lens. The lens is still designed (and used by the camera) to create an image the size required for 35mm film. You just get the central portion of that image with a smaller sensor. The rest falls outside the sensor. The portion you do get is identical to what you get with a full-frame sensor.

Cheers,

Mitch

If we try to compare depth of field performance on two different cameras, we need to adopt some premises for comparison.

Here is one set that is often used:

1. We use focal lengths on the two cameras that yield the same field of view.

2. We have the subject at the same distance.

3. We use an apertureof the same f/number.

4. We use, as our criterion of acceptable blurring, circle of confusion diameter limit (COCDL) values that are the same fraction of the camera's format size.

Note that condition 4 is consistent with saying that we will judge blurring on the same size print, viewed from the same distance, for both cameras.

If we adopt those premises for comparison, then the camera with the larger format will have less depth of field.

As to why this is, it is easiest to say "it comes form the depth of field equations". The parameters that are different between the two situations are:

A. The focal length is greater on the larger-format camera (per item 1). That greatly reduces the depth of field (it goes roughly as the inverse of the square of the focal length). The reason has to do with longitudinal magnfication and the relationship between depth of focus and depth of field, but I won't bore you with that right now.

B. Our tolerance for the size of the blur figure is increased (per item 4). That of course leads to an increased depth of field, but not to the degree required to overcome item A (it goes roughly as the COCDL).

Let's not confuse our discussion with unnecesary talk about the circle of confusion.

The circle of confusion is the same when both cameras are 35mm DSLRs. Their sensors are positioned the same distance from the lens. The lens has the same focal length, etc.

With a less than full-size sensor, you are getting a smaller central portion of that same circle of confusion. That's all.

Imagine that you took 35mm film and painted an opaque back ink frame around the film so that only the central 2/3 of the film frame is exposed to the light during exposure. Do you believe you get subject magnification or affect DOF? Well, you wouldn't. Ask anyone who's had the film transport in a 35mm film camera go screwy.

A less than full-size sensor has no sensors around the periphery of a 35mm frame, just as if you blocked the extra sensors of a full-frame sensor with something opaque. That's the only difference in a 35mm DSLR. All you get is reduced FOV. You do not get a boost in magnification, subject or otherwise.

Cheers,

Mitch

Mitch -

You cannot separate the issue of depth of field from the concept of a circle of confusion. Depth of field is a construct that is defined by the application of a chosen circle of confusion. You change the arbitrary number that is the circle of confusion, and you change the depth of field. Talking about one without the other is like trying to write in English without using the alphabet.

Fortunately, we have standarized on a circle of confusion that represents the average person's ability to see detail in an 8x10 print at normal viewing distances. Change the print size, viewing distance, or the degree to which you have to enlarge your "negative" to get to an 8x10 print, and you have to change the circle of confusion, which changes the depth of field. Of course, it changes no true optical properties of the lens/camera/film, it just changes what we perceive to be in focus.

You are correct that sensor/negative size has nothing to do with "magnification" as though a smaller sensor was a magic teleconverter. However, it does have everything to do with enlargement. The more you have to enlarge a negative to get to a standard print size, the more stringent a circle of confusion you have to use on the negative to have the standard print be acceptably sharp (acceptable DOF). In the context of this parameter, smaller formats actually have less depth of field, because they must be enlarged more.

However, this is overwhelmingly outweighed by the key issue (which is the answer to the original question). When you use a smaller format, you tend to use a shorter focal length lens and stand farther away with that lens than you would with a larger format.

Shorter focal length lens = more depth of field.
Standing farther away = more depth of field.

I hate DOF threads, why do I let myself get sucked in?

That's adding other factors besides the size of the sensor. That shifts the argument from a discussion of sensor size and DOF to sensor size, DOF, and other stuff.

The discussion was about whether an APS-C size sensor in a 35mm DSLR alters DOF or subject magnification. We both agree, it does not.

Alter anything else, of course things will change. But that misses the "key" issue in these DOF threads.

If you alter the distance to change the FOV, of course you will alter the DOF. But that has nothing to do with the sensor size. It has everything to do with repositioning the camera. Why confuse the issue?

You cannot get the same subject magnification and FOV with a full-frame sensor and a smaller sensor at the same subject distance. If you want the same subject size, you have to sacrifice FOV for the smaller sensor. Want the same FOV, you have to sacrifice subject magnification.

Same lens and same subject distance, full-frame or not, you get the same reproduction ratio. Use the same enlargement for both images, and nothing changes. Use different enlargements for your images and any differences in image appearance owe to those different enlargement ratios, not the differences in the size of the sensors.

I believe it is helpful, in discussions like this, to focus on what changes and what does not. If we hold everything else constant and change only the sensor size, the only thing that changes is FOV. Subject magnification and DOF are completely unaffected.

I do not tend to stand any further back with my 20D than I do with my 1Ds MkII. The relationship between subject and background and the overall effect I want the image to have on the viewer determine what focal length lens I use and where I stand.

Many people assume they get less DOF with a smaller sensor because of a telephoto multiplier effect. It is that widely held misunderstanding that I have been addressing.

The conversation about Circle of Confusion just adds to the confusion. That is a factor determined by the lens manufacturer for the expected enlargement factor. Medium format film cameras use a lower Circle of Confusion because the expected enlargement ratio is smaller. Same lens on two 35mm DSLRs with the same subject distance and aperture, one full-frame and one not, yields the same Circle of Confusion and the same DOF. You just see less of it with the samller sensor.

I see this argument all over the different forums. With a 10D or a 20D or whatever, you get a 1.6x multiplier effect, so your 300mm lens becomes the equivalent of a 480mm lens. (It does, but in FOV only.) The logic is then extended to DOF. Since the lens is the equivalent of a 480mm lens, DOF must be the same as a 480mm lens. Nope. For subject magnification and DOF, the lens is still the equivalent of 300mm.

Michael Reichmann has a really nice essay on DOF, Circle of confusion, etc.:

http://www.luminous-landscape.com/tutorial...eries/dof.shtml

Cheers,

Mitch

I agree with everything you wrote.

The only issue is that to measure DOF, you must choose a circle of confusion. In reality there is no DOF, only focus plane then gradually increasing blur. DOF simply is a measurement of how much blur we will tolerate before something becomes "out of focus". You have to draw the line between "acceptable focus" and "out of focus" somewhere, which is done by picking a circle of confusion. You can pick one, I can pick one, the lens manufacturer can pick one, but it's still an arbitrarily chosen number that's the determining factor of how much "DOF" you have.

I guess I should also clarify my statement on the "standing further back" point. You only stand further back with a smaller format if you hold the lens focal length constant. I should have said that with a smaller format you use a shorter focal length lens OR you stand further back.

Another good article on DOF, more relating to the issue of full frame 35mm vs. 1.6 crop digital:

http://www.photo.net/learn/optics/dofdigital/

I'm more in the mindspace of 4x5 vs DSLR. I was just outside in the front yard trying to do a 1:1 macro shot with 4x5. Talk about murderous DOF issues!

To determine the width of the DOF, yes, I agree. But that has nothing whatsoever to do with DOF and sensor size.

That's why I say, discussion of Circle of Confusion only adds confusion here. It is a constant when we talk about full-frame sensor v. smaller sensor. It's a relevant factor in other contexts. But not when we compare the results we can expect with a full-frame sensor or a smaller sensor using the same lens.

Nice chatting with you.

Cheers,

Mitch

I can agree with that, if you just want to say that something has more DOF or less DOF without indicating what that DOF is, you can toss out the circle of confusion.

Amazing, a DOF thread that ends with agreement! What a great forum this is!

Hi, Mitch,



I was not talking about some circle of confusion, or even the diameter of some circle of confusion. I was speaking of the circle of confusion diameter limit (COCDL) which we adopt as a criterion of acceptable blurring, without which concept there is no such thing as depth of field. (The ambiguity in terminlogy is why I don't like calling this adopted number "the circle of confusion", and I don't call it that.)



Would a 35-mm dSLR be one whose sensor size is 36 mm x 24 mm?

Indeed, if we have a camera with a certain focal length lens, focused at a certain distance, with a certain aperture (as an f/number), then for a point on an object at some specific distance other than the focus distance, the diameter of the circle of confusion (its actual diameter, not a limit we place on that diameter as a criterion of "acceptable blurring") will be the same regardless of format size. (The optical system of course "has no idea" how large is the film mask or digital sensor array at the focal plane.)

But maybe that's not what you mean.

Maybe you mean, "for a given format size, many people use the same circle of confusion diameter limit for reckoning depth of field." But hardly always.


I can't imagine what you mean by that. Are you by any chance confusing "circle of confusion" with "image circle"?

Regarding magnification, (image) magnfication is a function of the focal length and the distance to the subject. I certainly didn't say anything about (image) magnfication. I discussed longitudinal magnfication, which is a different matter althogether. It is involved in relating depth of focus to the corresponding depth of field.

In any case, I'm afraid I'm not able to follow the point you are making.

Best regards,

Doug

What do I have to lose? I'll jump in to the fray

1) DoF is a function of aperture, focal length and subject distance from the camera, or more simply stated: aperture and subject magnification factor. Note that sensor size has nothing to do with either equation.

2) However, to obtain a similar FRAMING from cameras with different sensor sizes but using the same focal length lens, we must change our shooting position -- which in turn changes the magnification factor, which in turn changes the DoF. So it is ALWAYS true that smaller sensor cameras will have MORE DoF when generating framing comparable to a larger-sensor camera, all else equal.

3) CoC diameter is a CHOICE and hence is a variable in the DoF equation. As a result it always generates some confusion when discussed. Simply stated, for a given PRINT size and resolution desired (photographer's choice and hence variables), CoC will need to change based on sensor size; smaller sensors will require a smaller CoC to generate the same print resolution as the larger sensor, but the change is not proportional.

3a) IMO the confusion arises because the CoC required does NOT change proprtionately with subject magnification factor, it changes as the inverse square -- which again is why a smaller sensor will generate MORE DoF in a given image when all else is equal.

Agreed. Except that in #3a you state that the change in the CoC is why the smaller format has more DOF. Actually, the CoC works against the smaller format (opposite direction), but the reasons you stated in #2 completely outweigh that and lead to the smaller format having less DOF.

You misunderstood -- and I agree the smaller CoC works against the smaller format. As I said, they did not change proportionately but CoC changes at a slower rate, the inverse square. As such, the change required for a smaller CoC with the smaller format progresses less than the subject magnification change does to generate the same framing as the larger-sensor image.

A 35mm DSL does not need to have a full frame 35mm sensor. It needs to be able to use optics designed for 35mm film cameras. The Canon D30/D60/10D/20D/etc. are all 35mm DSLRs. They use the same Canon EOS lenses used with 35mm film cameras and full-frame 35mm DLRs like the Canon 1Ds MkII.

The optical system in 35mm DSLRs are designed to create images for 35mm film cameras or 35mm DSLRs. As you say, the lens knows nothing about the medium capturing the image.

Some of the 35mm DSLRs cameras just happen to capture only a smaller portion of the image than 35mm film or a full-frame 35mm sensor would capture.

In my opinion, talk about Circle of Confusion, Circle of Confusion Diameter Limit, and the like contribute nothing to a beginning digital photographers understanding about how FOV, DOF, and subject magnification are affected by the APS-C size sensor in a Canon D30/D60/10D/20D/etc. relative to a full-frame sensor.

Camera sales people often tell them that their telephoto lenses get a 1.6x multiplier effect. They tell them that their 300mm lens becomes the equivalent of a 480mm lens. They tell them you get more DOF with the smaller sensor. Etc. At best, the camera sale people misunderstand. At worse, they downright defraud people.

You do not need to know anything about CoC or COCDL to understand that the only thing that changes is FOV when the sensor is smaller.

What you do about that smaller FOV is a separate issue.

How you calculate DOF and the role of CoC is also a separate issue.

The only difference between a larger sensor and a smaller sensor -- everything else being constant (i.e., the same distance to the subject, the same aperture, the same lens, the same focal length) -- is just one thing: FOV.

You can say, most people will reframe. I have no idea what "most" people will do. But when they do reframe -- large sensor or small sensor -- things are gonna change. That's not because of sensor size. That's because you reframed: something completely separate and independent of sensor size.

Cheers,

Mitch

Sensor size doesn't affect DoF; the Nikon D2X in effect has two sensor sizes, the 12MP APS-C sensor, and in "High-speed crop mode", a smaller 6MP sensor obtained by masking out about half the sensor area. The resulting image, in terms of DoF and everything else, is completely equivalent to doing an identical crop in Photoshop of the full-sensor image. (Nikon's purpose in doing the crop in-camera is to reduce file size to increase the fps rate.) The same is true in comparing a full-frame sensor with an APS-sized one.

This has been discussed many times in this forum, and I believe there is a consensus that:

1) If you take photos from the same distance and compare DOF on equal sized prints viewed from an equal distance, the DOF will be the same for any combination of focal length f and f-stop N that gives the same effective aperture diameter, A=f/N. For example, f=50mm, f/1.4, f=70mm f/2 and f=100mm f/2.8 all have effective aperture diameter about A=35mm, and so all give about equal DOF in a "same print size, same distance" comparison.

In other words, adjust f-stop in proportion to focal length to get equal DOF.

2) Smaller formats typically involve using shorter focal lengths to get the same framing of the subject (same FOV), and so will get the same DOF with a proportionately lower f-stop (and thus potentially with a lower ISO speed), or will give more DOF with the same f-stop.

Sometimes the "shorter, brighter" lens needed for equally low DOF does not exist for a smaller format (50/1.4, 85/1.2 or 85/1.4 in 35mm have no matches for for smaller formats). Other times a "match" does exist and then is usually similar in cost and weight, due to having similar sized lens elements, due to the similar aperture diameter (135/2 vs 85/1.4, 200/2.8 vs 135/2, 300/4 vs 200/2.8, 300/2.8 vs 200/2, 600/4 vs 400/2.8).


Historically, lenses for larger formats have typically had longer focal lengths and higher minimum f-stops, and have been most often used with higher f-stops in order to meet DOF needs, leading to the pattern of larger formats typically being used with lower shutter speeds, or alternatively higher ISO speeds to get the same DOF and shutter speed.

What I say is, 'roll on the new technology currently under development to circumvent the laws of diffraction'.

Mitch,
To continue with your scenario and keep the "comparison" valid (i.e. not introduce new variables), you have to switch painted and unpainted frames in the same enlarger (and not change anything else in it) to make the prints. Guess what, the cropped frame print is smaller. Yup, same as cutting down the original print to leave the central 2/3. It was the same image coming through the lens, so this is rational.

Now, from the same viewing distance, you are correct that each will have the same parts appear to be in focus and nothing will be different about the central 2/3 of the image. So the "All you get is reduced FoV" comment must apply to the print as well. DoF as a physical thing doesn't exist until you view the image because HOW you view the image determines what will appear in focus aka acceptably sharp.

"A beginning digital photographer" , perhaps someone coming from FF 35mm such as film, to cropped sensor such as digital needs to appreciate this consequence, don't you think? Should there be a warning like "Attention, this camera makes smaller prints than you are used to"

Andy

It's odd that there still seems to be confusion over the effects of a cropped sensor after all this time. It's probably due to the unfortunate terminology of 'crop factor' which has resulted from the fact that APS-C format cameras all use 35mm lenses.

If you could fit Medium Format lenses to 35mm bodies, then we could call the 35mm format a 'cropped MF format' and we'd all be confused about which lens to use.

I'm no DOF expert and don't even own a digital camera. So I'm qualified to jump in as well. <g>

Your statement makes most common sense to me, and is what I would expect from digital cameras with small or full frame sensors.

Lets leave small and full frame sensors out for a minute. All else being equal, will the same lens result in different DOFs between a film camera and a digital camera with a full frame sensor?

Your question is confused. In the first sentence you write, 'let's leave small and full frame sensors out'. In the second sentence you bring 'full frame sensors' back in. Which is it? Out or in?

I meant instead of comparing DOF differences between small and full frame sensors, will there be any DOF differences for a lens when used on a 35mm film camera and when used on a 35mm digital camera with full frame sensor? That's a mouthful, hope I got it right this time.

Only in respect of differences in resolution at the plane of focus. It's theoretically possible that a fine grain B&W film could deliver more resolution than, say a 5D, but color film wouldn't stand a chance.

DoF calculators do not bring such matters into their calculations, but it seems clear to me that either a lens or film that is able to deliver greater resolution at the plane of focus than a lesser lens or lower grade film (or coarser sensor), will produce an effect of less DoF because the difference between what's in focus and what's out of focus will be greater.

The correct answer is a definite "maybe". Because of it's higher resolving power, the digital sensor has a smaller effective circle of confusion, and therefore less DOF, but only if the lens used is as good as the sensor. If your lens is a cheap coke bottle that is outresolved by the film and digital sensor, DOF will be identical between digital and film. But if you have a good lens that outresolves the film but not the digital sensor, then the more sharply-focused areas found only in the digital image will raise the bar of what appears to be "in focus", and areas that appear "in focus" in the film image will suffer in comparison to other areas in the digital image that are even more sharply focused, and therefore will appear "out of focus".

What we perceive as "in focus" is based on comparison to other parts of the image, so any area that is very sharply focused will cause less sharply-focused areas to appear "out of focus". So the more resolution you extract from a given imaging area (like 24x36mm), the narrower your DOF will be--you're effectively decreasing your CoC size when you do so. As prints get larger, the difference will become more noticeable. Here's an example:



In the web-sized JPEG above, everything appears to be "in focus" because the entire image is equally sharp--resolution is limited by downsampling to 800x640 pixels, not the lens or sensor. But when looking at the full-resolution version of this image, you can see that the right edge of the image is slightly OOF compared to the center and left side. By reducing overall resolution when downsizing this image for web display, I increased the effective CoC and increased DoF. But when increasing resolution, (going back to the original sized image) I decrease the effective CoC and DoF decreases proportionately.

Now it's my turn to be confused. When "less DOF" is used, I always take it to mean that the distance range within which everything is sharp is *shorter*. But in your explanation above, you seem to say that a digital camera will have a *longer* distant range within which everything is sharp than a film camera, everything else being equal.

Re-read what I wrote: Digital achieves higher resolution, smaller CoC, narrower DOF. The sharper the sharpest focus is, the narrower the distance range that will be that sharply focused in the image.

In broad terms, if the lens and f/stop is the same and the distance to subject is the same then the DoF will be the same, whether film or sensor. However, if you wish to put a fine point on it, there will be subtle differences in DoF depending upon the resolving power of the film or sensor being used. For example, a 1Ds2 should produce a marginally shallower DoF than the 5D, but so marginally shallower that you probably wouldn't notice it.

Likewise, a T-Max ISO 100 B&W film, that can apparently resolve 100 lp/mm at 60% MTF would produce a shallower DoF (everything else being the same) than an average slide or color film, given a big enough print.

Depth of field is simple. The mathematical formula for final sharpness in the print has depth of field inversely proportional to magnification from sensor to print (you make bigger prints, you have less depth of field, you use smaller sensor to make same size print, you have less depth of field), BUT, depth of field is inversely proportional to the SQUARE of the focal length, thus changes in the focal length of the lens used (the actual focal length, not some fudged 35 mm. equivalrent) have a much larger effect on depth of field.

This applies no matter what sensor size you have, whether it's film or digital, contact print or enlargement, sharpness fanatic or prebyopic baby boomer who's lost his reading glasses.

DoF is not simple. It's complicated by all sorts of factors which have an influence one way or another, including print size, lens quality, focal length, sensor quality, sensor size, objective factors and subjective factors.

There have been many protracted debates on this forum about these issues, some arguments addressing only the properties of a perfect lens using simplistic mathematical formulas and other arguments taking in a broader spectrum of variables that have both a direct and indirect influence on DoF.

Some folks will argue that sensor size has no bearing on DoF, for example, completely ignoring the practical reality that sensor size determines choice of focal length and therefore indirectly affects DoF. But because sensor or film format does not appear in the DoF formulas, such people will strenuously deny that sensor size has any bearing on DoF. It's a sort of photographic fundamentalism.

Most of the debates I see about "DOF" could be sum up as "how do you define DOF?". Many people use different meanings... without even being aware of this...

I like the traditional way (ie: DOF formulae and CoC=1/1730 or whatever of the diagonal format) because its mainstream since years and provides also a good comparison basis for Background blur, even between different formats.
In another forum, I had to define "useful DOF" because people wanted to take into account the system real limitations and even "absolute DOF" to please one who considered DOF to be a characteristic of the lenses only... Oh well...

Olivier

Olivier,
I take it you mean CoC should be 1/1730 of the diagonal of the format. I suppose that's good enough for an 8x12" print, although some demanding people will claim the CoC should be equal to the pixel pitch of the sensor if maximum sharpness at the plane of focus is desired on the maximum size print the camera can produce, without interpolation, at 240 ppi.

Where I find these DoF calculators begin to break down is in the area of lens quality and sensor pixel pitch. For example, DoF Master treats the D30 the same as the 30D. The sensors are the same size. However, the D30 has 3mp and the 30D 8mp, yet DoF Master uses the same CoC of 0.019mm to get the same result at the same aperture and focal length of lens and same distance to subject.

I suppose this is fair enough if you make an assumption that the D30 with an average lens will deliver detail as sharp as the eye can see when viewing an 8x12" print from a distance of 10" or so, and that any sharper result the 30D might produce with a better lens on a larger print is irrelevant for this size of 8x12" print.

Perhaps someone who owns both a D30 and 30D could test this .

I notice that DoF master does not emphasise that their figures apply only to a print size of 8x10" (or 12"). In fact I can't see it mentioned. But maybe I haven't looked hard enough.

Ray, I started with a D30 and have a 20D now. Without formally doing any comparisons , my recollection/experience tells me the following. I would say that 8x12's from the D30 with a good (enough) lens - I had a good 28-135 - came close to that performance. I also had some poor lenses that didn't - suggesting something about system resolution and that it didn't take much loss in lens resolution for the overall loss to be noticeable; i.e. the D30 was at its limit at 8x12 at 240ppi original pixels, my (preferred) limit. The 20D goes beyond 8x12 at 240.

The DoF concept is definitely a moving target, given the variability in print size, degree of crop/enlargement that's so easy with digital. With the temptation to use original pixels at the printer's limit making it so easy to either crop/enlarge or just print bigger, it makes sense to use the pixel (or close to it) as the CoC to guide capture apertures during exposure

Andy

Andy,
As you describe it is pretty much as I imagined, but a thorough camparison of the D30 and 30D on an A4 size print would be interesting, especially if the D30 shot was taken with a zom at, say 50mm, and the 30D shot was taken with the Canon 50/1.4 prime.

Ray, it's just that "DOF" is defined without those.
Historically, DOF has been defined by using the 'perfect lens' formulae and a 1/x ratio of diagonal format that matched both an "acceptable" quality for viewing print AND the resolution of film used at that time.
Resolution of lenses and films and then digital cameras has improved, but this hasn't changed DOF definition... and for good reasons: simplicity, relationship too Out Of Focus Blur and probably the most important one => photos are meant to be viewed, not to be scrutinized with a microscope at 100%...
That's why we keep finding it in most litterature and in all online DOF calculators. This is also what is used by all manufacturers (Canon, Schneider, Leica, etc...).
Using a different 1/x ratio of the format depending on one's needs is still within this definition.

Taking into account the Lens resolution, Sensor resolution, Diffraction, more accurate formulae depending on lens design, non planeity of focus area, whatelse... is a different modelization.
Anyone can use and refine such modelization if they want.
The only thing I am asking is to use a different name than "DOF/Depth Of Field" which is assumed to be the traditional definition (and is really mainstream) in order to avoid misunderstandings. This is why I suggested "Useful DOF", etc... for those different concepts.

Olivier

If you are interested in more comparisons, have a look at those shots by Jean-Marie Sepulchre and Michel Denis-Huot (in french). They are quite interesting about 24x36 vs APS, although Jean-Marie and I didn't agree on the conclusions...

If you want to go down the 'Lens&Sensor quality' road, don't forget:
- you won't have any formulae to calculate that "useful DOF" (change name here...)
- using an excellent lens and a bad one won't give you the same "useful DOF" even when using exactly the same settings
- of course, "useful DOF" will be different in the center and in the corners where lenses are less sharp
- there won't be any relationship between DOF and OOF blur
- Diffraction will have to be taken into account (and will you get more DOF with larger CoC or something else like DOF=0... you choose)
- it will be sensor+lens dependant... and will have to be updated with every change
- don't count me in!!!

Olivier
PS: if you do so, please don't forget to use a different name than just "DOF/Depth Of Field" so that I can understand what you're talking about and leave you alone in that mess...

Olivier,
I assure you I have no intention of going down that road. The DoF issue for me is mainly a conceptual one of academic interest and a matter of semantics and meaningful definition. As a practical photographer who uses zoom lenses most of the time, outdoors and in the field, I have no way, nor the time, to make precise DoF measurements which take into consideration lens resolution, pixel density or print size. Even using a basic DoF calculator such as DoF Master would be too cumbersome for me. For example, if the nearest point which is reasonably sharp is 12.8 metres, how do I measure that distance? I'm not interested in carrying around a 30 metre measuring tape or wading acroos a river to make a precise measurement.

It is sufficient for me simply to be aware of the factors that can influence the perception of DoF on the final print. For example, with my D60 and 20D I was more reluctant to use f16 than I am with my 5D because the greater pixel density of the 20D revealed the softening effects of f16 more. With the 5D the softening effect is hardly noticeable. F16 is very usable with the 5D because the pixel density is less. At the same time, I need to use f16 with the 5D more than I do with the 20D because DoF is shallower with the 5D at the same f stop and FoV.

These are issues I need to be aware of to make reasonably accurate guesses in the field.

Perhaps instead of using the "focal length multiplier factor" we should simply call it "crop factor". For that's what it is.

Take your 4x5 camera, put a 200mm lens on it. Then cut out a 24x36mm piece. The result is EXACTLY THE SAME as if you used the same film in a 35mm camera with a 200mm lens at the same f/stop, from the same viewpoint.

It's a good thing that our large format lenses go up to f/64, f/96, and further. That allows us to get more depth of field from the same angle of view.

So an APS-C sensor is the same as if you cut out an APS-C size bit of film from a 35mm camera.

The "normal" lens for 24x36 is usually 50mm (it's actually 43mm, but that's another whole subject). The "normal" lens for APS-C is 31mm or thereabouts. So to get the same field of view, from the same spot, you're using a wider angle lens on the smaller sensor. Thus more depth of field, assuming the same f/sop.

Which is true, as far as it goes, and completely misses the point.

Why would someone do that? Do you do that? What people do is frame the shot they want through the viewfinder and then enlarge it to the print size they want. If they can't get the framing they want then, if at all possible, they either change lens or move their camera. What they don't want to do is crop a bit out of the middle of their picture and thereby lose a significant percentage of the available resolution on whatever format they happen to be using.

Focal length multiplier is a far more useful term, not because anyone cares very much about focal lengths, but because everyone is really far more interested in angle-of-view, and people know (in broad terms) what angle-of-view corresponds to a particular focal length on a 35mm camera.

So insisting that 200mm is 200mm is 200mm is pedantic and misleading and frankly has nothing to do with the way pictures are taken.

But if it makes you feel intellectually superior...

(The bit that is unspoken and wrong in what you wrote is that you're implicitly assuming that the print size varies between the crop and non-crop. If you make an 8x10 print from a 200mm lens on APS-C and an 8x10 print from a 200mm lens on 24x36mm sensor and crop out the equivalent bit, you are not left with an 8x10 print. If you enlarge that middle bit you have lost your "identical" DOF.)

Actually, "focal length multiplier" is a bad term because focal length changes more than angle of view - "crop factor" is far better. This problem of different formats is only a problem with amateurs with a singular knowledge of 35mm cameras. Photographers used to other format do not have any isses with this. Manufacturers have taken this "35mm equivalent" thing because they have found their customers do not understand fairly basic concepts in photography. They have even gone so far as to give magnification in "35mm equivalents."

This thread is a result of the "dumbing down" of photography inforced by manufacturers and the trade publications. The issue here is not that complex, but the subject come up time and again. Actually, there should not be any reason to compare formats. Learn to use the format as it is and not compared it to something else.

"There was a query ... as to whether Depth of Field was calculated any differently for digital Vs. film. The answer is, no. There is no difference whosesoever. DOF doesn't care about the recording media type or size, though a lower COF is used for medium and large format, since the amount of magnification to make a decent sized print is much less than for 35mm."

I think that should answer the original question.

The last part of the last sentence in the quot refers to the degree of enlargement. Medium and large formats usually require less enlargement. Therefore, I think the quote should be that a higher COF is used, not a lower.

Oh, the quote is from Michael Reichmann on the Luminous-Landscape.

If one were to place on a multi-camera bar a 20D and a 5D using the same mm lens at the same f stop the picture would be exactly the same except for the crop factor of the aps sensor, right? At the same time, the DOF is only a function of the distance from the sensor to the subject at a given f stop, right? The shorter the distance from the sensor to the subject the shalower the DOF , the longer the distance the deeper the DOF. Have I got it?

Jerry

Almost. You are right that depth of field increases with object distance and larger f-numbers. However, if the images are displayed the same size, the depth of field will be sligthly less with the smaller sensor due to the greater image magnification.

Normally, photographer are only interested in one camera at a time so cross format comparisons are not normally an issue. But note, if the focal length is different where the angle of view is the same, the smaller sensor will have the greater depth of field.

This is an excellent example of the sort of statement that brings together subjective and objective concepts of DoF which are the source of much confusion.



That's almost like saying, 'these two pictures are exactly the same apart from the fact they are different'.

If you compare a 5D image with a 20D image in the manner you suggest, what is the impression you get if the 5D image, with its wider FoV, has a few out-of-focus rocks in the foreground which have been cropped out of the 20D image?

Subjectively, whether 2 images are completely different or similar, the image containing blurred objects in the foreground, background or to one side, will be perceived as having shallower DoF. If you crop the 5D image so the FoV is exactly the same as that of the 20D, you are comparing a 5mp image with an 8mp image. You don't have to be a genius to work out which image is likely to be sharper.

To the extent that the 20D image is sharper than the 5D image at the plane of focus, the 20D image will have a shallower DoF.

Your above statement would be more true if the comparison was between a cropped 1Ds2 image and a D60 image, same lens, same f stop, same distance to subject, same pixel density.

A couple of days ago I used my TS-E 24mm with the 5D to get an extended panoramic shot of one of these magnificant ruins in the jungle at Siem Reap (Preah Khan, actually), by extending the width 22mm using shift. Back at the hotel, having downloaded my day's shooting to the laptop, I was dismayed to find the corners and edges were not sharp. I've been used to using this lens with my D60 and 20D where there's no problem with resolution fall-off at the edges.

It's really not acceptable. I wish Canon would upgrade this lens. It's expensive and we deserve better. I felt compelled to return to the same venue this morning and reshoot using my 20D with the same lens but from a greater distance.

Ray makes a good point. DoF is a visual thing. But it can calculated. He also brings up some things that are second order effects.

When breaking down something with several variables to see the effect of s single variable or each variable, it is very useful to seperate the variables and change only one at a time. I am afraid Ray and others are guilty of confusing the issue by changing more than one variable at a time. If you want to see the effect of format (sensor size), change just the sensor size. Looking at different size prints (showing the out of focus rocks) and making prints of different subject image magnification same size prints from different size sensors) only confuses the answer and hides the effect of sensor size among other factors.

According to Michael Reichmann, changing only sensor size does not change depth of field. I believe that is true.

The original question as posed changes more than one varible (sensor size and sensor resolution by using two different kinds of cameras). If the original question is what effect does sensor size have on depth of field, the answer is none whatsoever.

As an aside, pinhole cameras may have huge depth of field for at least two reasons. The f/stop is huge (tiny aperture with long focal length). The resolution is poor so it is difficult for people to distinguish betweem focused and out of focus parts of the image (the very essence of depth of field). Depth of field is judged by what the viewer sees as acceptable sharp. Some may say a pinhole camera (or even a Canon 1ds with a cheapo coke bottle lens) has no acceptably sharp image, even at the plane of focus. Then the depth of field is undefined (for that viewer).

I'm familiar with this view. There is a theoretical DoF in relation to a perfect or standard lens that can be calculated with a simple mathematical formula. The result is the DoF of the imaginary lens, not the printed image.

It's ironic that we should spend so much time worrying about the performance of our lenses, yet when it comes to DoF, many of us (not me, however), accept that all lenses are the same.

Changing only sensor size does not change the characteristics of the lens, if the lens is not changed, but it may certainly change the charcteristics of the sensor (pixel pitch, for example) and it certainly, without doubt, changes the composition of the scene being photographed.

When Michael made that statement, in my opinion he should have emphasised that he was only referring to the characteristics of the lens in an objective manner. Clearly, changing sensor size cannot affect the properties of the lens. Such properties are independent of the photograph or composition.

DOF is calculated based on standard print size not based on the image at the image plane of the camera (you can do it, but it is still in relation to display size). This is done because DOF is based on the average angular resolution of the human visual system - there is no absolute definition of sharpness. To say DOF is exactly the same at the image plane regardless of the format size is a pointless comment; are you expecting the resulting DOF in my full-frame print from a 55mm Grandagon on my 4x5 view camera to appear the same from my 55mm macro Nikkor on my F3? Or do you always view images at the sensor dimensions - I would suggest you are very, very near sighted as well as that limit being really tough for folks with compact digital cameras.