Whenever I need to know where my maximum DoF will be in relation to my aperture and focal lengths, I use a chart I carry with me. However, I now there is a way to calculate this using the distance scale and lens length, plus the aperture setting on zoom lenses. Given a Canon lens such as the 24-70, how exactly is this method performed? I've tried to find a detailed answer to this question, but have failed.

I don't know the answer to your question ... but I'd like to advert to the format issue. Is your DOF chart actually designed for your camera's image format? For instance, you cannot use a chart meant for 35-mm format when you're shooting with an APS-C-format camera. The same applies to the DOF scales on 35-mm-format lenses used on APS-C-format cameras (if your lens happens to sport a DOF scale in the first place). Just a point to be aware of ...

-- Olaf

Also keep in mind that you need to take the intended print size into account when calculating depth of field.

If you plan to print larger for a given shot, then you need to consider that the depth of field will be reduced (assuming the same viewing distance for the print). Conversely, if the print is just going to be small you might not need to stop down as far since the smaller print will have more depth of field.

DOF is focal length/distance dependent, not format size.

Of course the larger format you go the longer lenses you need to use to equal the same FOV.



example: if we are trying to get a 80mm FOV on 4 different digital formats we would use the following lenses:

50mm lens used on a apsc dslr will give the fov of 80mm
80mm lens used on a FF 35mm will give fov of 80mm
100mm lens on a smaller MF sensor( p31, a17, etc will fov of 80mm
110mm lens on a near FF MF sensor( p25, a22) will give fov of 80mm

Longer the lens the more shallow your dof becomes.

However, I cant say that the dof charts on lenses are accurate with digital sensors because the digital sensors are perfectly flat unlike film.

Hope this helps.

Kyle

I found this article helpful.

http://www.trenholm.org/hmmerk/DOFR.html

Older primes had DOF scales on the lens, but zooms don't have them since they'd have to have one for each focal length.

Aside from your charts, one other way is to use DOF calculator software on a PDA or mobile phone. I wrote one for my Windows Mobile SmartPhone that I find pretty handy.

I used the online (and down loadable) DoF calculator using my cameras specifications.

Yeah the chart I made is essentially the same thing. I created it using 17mm to around 100mm.

Also as another poster commented, yeah, the primes used to and still do probably have that, but like you said, you can't do that for zooms. Nevertheless, the foot/meter distance window is still on the zoom lenses, and for some reason I would think. So exactly what are they there for? My 70-200mm has a mark for 70 and 100 on the outside of the foot/meter mark window, where the infinity mark is. You know what I'm talking about. I'm pretty sure it's not there for looks, or maybe it is?

The distance window tells you your focus distance, although I've always thought these were of very little use on anything but a macro lens, because the distance scale is so coarse you can't really tell what the distance is to any meaninful degree of acurracy.

I'm not sure what you're describing here, I'd have to see a picture.

Jeff,

The distance scale tells you your focus distance, but that affected by your aperture to get maximum DoF--I see no difference in where to focus no matter what aperture I'm at when looking at the scale. Or have I missed something here.

The 70mm and 100mm mark I'm referring too are just small marks with lines leading to the distance scale on the lens, so you'll know where you are between 70-200, but it's a rough guide since they are very close together.

I believe the marks (in red) indicate the focus offset for infra-red film and are not relevant for normal focussing.

The problem with these zooms and the distance scales is that they are incredibly course (as already mentioned), are logarithmic and only relate to one focal length. Not forgetting the old curved focus field which just compounds the problem further.

e.g. set your 24-70 to 24mm and the focus to between 3m and infinity. What distance have you set the lens to? Do the same but at 70mm. Same problem - it's any ones guess what the actual focus distance is.

Because of this a DOF chart is pretty useless at getting consistent results.

What you can do though is do some tests and work out where on the scale will give you acceptable results at each focal length and aperture (and across the frame from centre to edge).

e.g on my 24-70 at f11 and 1Ds2 the following settings give me minimum focus distance while just maintaining acceptable sharpness at infinity:

24mm 10 feet
28mm half way between 5 + 10 feet
35mm 10 feet
50mm third way between 10 feet + infinity

Sounds screwy but it gives consistent results with near far compositions and gives a base reference point to avoid messing the focus up and missing shots.

This statement is utterly wrong.



This statement is not wrong but meaningless in this context.

-- Olaf

Time,

No, you don't know exactly what your setting is on a zoom, nor do you know exactly how many feet from the front of the lens to the foreground, unless you take a measurement using a tape or some other measuring device. The chart allows you to make a very close estimate. I use the chart as an estimate, and then when I'm close to something, I take out my tape and actually measure the distance. Then I shoot several shots for a fudge factor. If I'm not that close, say I need the foreground to be sharp at 15 feet, I just guess the distance and shoot several shots to make sure at least one will be the sharpest at the given focal length and aperture. Then I usually smoke a joint while I walk back to my truck. In other words, I don't worry about it anymore after I've done that.

So what I'm getting here is that the distance marks on zoom lenses are pretty much useless for anything other than looks? What about the infinity mark on that same scale? Same story, useless?

It's an approximation close enough to be somewhat useful, but not accurate enough for you to measure the distance and then dial in the focus setting on the lens.

OK assuming it was accurate enough, how would you use it exactly? The reason I'm asking is that I can get good at estimating using the marks, which as you say will be helpful as opposed to pulling out my DoF chart.

I think it would be difficult to come up with a calculation on a zoom.

If you know the exact distance I suppose you could figure the angles of the cone of light from that focus point and the diameter of the aperture. Maybe you can figure the cone of light to your 1 pixel size assuming a thin lens. Then decide how many pixels of OOF light will you accept as soft but good enough. Look at the relative angles of that cone to your focus cone. Then the same angle on the distance side is acceptable as within DOF. Sounds messy.

Zoom in after the shot to see if the areas you wanted in focus are in fact focused at the pixel level. I think experience with similar shots will have to tell you what f stop to start at.

Try doing what your attempting with adapted lenses with JUST the wrong thickness adapter. The distance scales on the lens become 'for decorative purposes only'.

A cheap laser range finder from B&H or an older optical RF one used by golfers + one of those circular cardboard DoF calculators (can't recall who sells them). I would assume the calculator is available for different formats or can be set for the format in use. Always meant to buy one myself - better than lugging around a binder of lens charts.

One sample, but not the only one.

http://www.bhphotovideo.com/c/product/1551...Calculator.html

BINGO

http://www.bhphotovideo.com/c/product/4966...eld_Guide_.html

First of all, the distance scale isn't that accurate, so measuring the distance with a tape measure and then dialing in the focus adjustment simply is not practical. You're much better off using AF with a single point where you want sharpest focus.

Second, if you're really going to be that anal about controlling DOF, you're going to need the tape measure and the DOF spreadsheet regardless. Measure your subject distance and input that (and your best estimate of focal length if using a zoom) into the sheet, and select aperture accordingly.

Not to say some of the solutions proposed aren't viable, but wouldn't the easiest approach be to bang out a couple bracketed frames? Set up the shot and shoot f/8, f/11, f/16 then move on. You can pick which frame has the DOF you like best when you get home.

Messing with a calculator/chart/laser rangefinder seems a little bit of overkill, and perhaps more prone to error or miscalculation.

That's what I've been doing, yes. I also think hitting the spot you want in focus, like you say, and then using your smallest aperture setting that renders enough detail is probably the best way. I say "best" because if you want X in focus and you want the background in focus too, and you spot focus on X and use your smallest aperture, then you either got it or you didn't, and it doesn't matter--your camera / lens is not capable of it. And it's faster than all the calculating and measuring.

Thanks Jon.

What I said--yep.

How is that utterly wrong?

please explain.

1. DOF does depend on format size.

2. Whether DOF becomes wider, remains the same, or becomes shallower with a longer focal length depends on a wealth of other factors. So a blunt statement like yours, without providing context, is meaningless.

-- Olaf

OK the second statement is a bit out of context, I see that.

But the first statement I feel as if we are talking about the same thing and wording it different.

Here is what im trying to say:

If we take two different to match a fov lets say FF 35mm sensor(5d) and MF sensor(A22)

We would need a 50mm for the 5d, we need a 70mm to match that on the A22

Shot at the same aperture and distance the 70mm on the A22 will produce a shallower dof.

I can understand what you are saying since by using a larger format we have to get closer to the subject to equal the same field of view, but my statement is correct as the 70mm is going give a thinner dof at the same distance and given aperture than the 50mm.

I hope that makes more sense.

That's right.



Actually it makes no sense at all. Your statement was, DOF does not depend on format size. And that's definitely wrong. I cannot see how your example above is supposed to provide evidence for your erroneous statement.

-- Olaf

Changing format size does affect DOF, and it affects it in the opposite direction that you think it would.

If you hold all things constant (focal length, aperture, focus distance, print size) and only change the size of the format, then the larger film format will have more depth of field than the smaller format. This is because the larger film format requires less enlargement to reach the desired print size, and therefore uses a less stringent COC.

It is only when you change focal lengths to match the FOV between formats that the larger format has less depth of field. The effect of lens focal length overpowers the effect of format size when calculating DOF.

A simple way to state it is:

Larger formats require the use of longer focal length lenses to accomplish any given photographic purpose, and that use of longer focal length lenses leads to functionally reduced depth of field.

Thats excactly what I was trying to say, thank you!