Michael has done it again

http://luminous-landscape.com/tutorials/un...ffraction.shtml

An up to date succinct comment on where we have reached in great MF/LF lenses for digital backs.

Who else actually has the 39MP Phase back and the Rodenstock lenses to explore for us.


I learned more about "diffraction-limited lenses" and that the HT lenses get into this rare group.

The next thing I want to know is how this in fact changes resolution?

I'm particularly interested in the wide angle lenses and would love to see further derivatives of this kind of very practical review on LL.

Kudos!

Asher

Hi!

This means essentially that you cannot stop down to much if you want to have optimum sharpness. All lenses go diffraction-limited sooner or later. A perfect lens has it's optimum sharpness fully open. Normal lenses need to be stopped down a couple of stops to reduce uncorrected aberrations, around f/8 or f/11 they would also get diffraction limited.

What this also means that there is a maximum "sharpness" you can achieve at a certain aperture, and that amount is sharpness increases with aperture. Any fairly decent lens should be pretty good at around f/11, but at that aperture it could not utilize all the resolution in a 39 MPix, back. So if you want the best resolution you can get you would need lenses which achieve maximum sharpness around f/5.6 or f/8 and avoid stopping down to more than that.

Putting it another way:

1) If you are using f/11 or f/16 on a 39 MP back you could probably get the same sharpness/resolution/MTF from a 25 MP back.

2) To utlize a 39 MP back you need lenses having optimum performance around f/8 or f/5.6, and not stop down any more that.

3) You see why "scheimpflug" is such a good idea :-)

4) This is nothing really new, many of Canon's top (L-class) telephoto lenses perform optimally at fully open aperture.

Best regards

Erik

Well, Distagons are supposed to be that way too!

I can see how one might need different optimization for portrait versus landscape. The latter id one has a fixed body camera where no Schleimflug is possible.

Now can the Rodenstock lenses actually fully resolve to show all the cpability of the 39MP sensor or is the camera back still lens-limited in resolution?

Asher

The pixel spacing for 6.8 micron pixels is 147 pixels/mm. The Nyquist limit is therefore 73 line pairs (lp)/mm (2 cycles per lp). In actual systems, the resolution is more like 3 cycles per lp so the actual resolution would be 50 lp/mm.

For a diffraction limited f/5.6 lens the resolution at MTF of 80% is 58 lp/mm and the MTF 50% is 140 lp/mm. The Rayliegh limit is 290 lp/mm. To really make full use of a diffraction limited f/5.6 lens, Michael would need a back with considerably more resolution. At least when 80MP backs come out, he won't have to buy new lenses.

I generally tend to stick with f8 for maximum sharpness. But ive used f16-f22 at times and results arent bad at all.

With the Pentax 6x7 I ussually stick to f11. Dof is pretty short in that format.

Another way of looking at this...

If you have to shoot at f11 for DOF reasons... is there any advantage to the HR lenses?

There is a tradeoff between i) Resolution of the back ii) Need for DOF iii) Image circle size sufficient to allow Scheimpflug movements

What I am wondering is, when you look at the diffraction limits, and the tiny image circle of the Rodenstock lenses... how big is the advantage over lenses with a greater image circle?

If you can't really use movements, you lose a lot of potential images. Is it worth it for a little extra sharpness (taking into account diffraction effects?)

Because of the extreme expense of these things, and their rarity (dealers wont stock 'em, certainly won't find their way into rental stock), I think it will be very difficult to test these tradeoffs and find a practical solution which fits with an individual photographers style and needs. And maybe it isn't worth it.

Looks to me like a law of diminishing returns is beginning to apply here, a bit like Hi-Fi buffs getting obsessed with cranking every last bit of frequency response from a loudspeaker.

A good point. Before we get to it, is there an advantage even without considering the image circle. Has anyone had a chance to do a side by side with the Digitars? I am consistantly awed by their sharpness and contrast.

That said, at 24, 35 and 47, depth of field on a P 25/45 at 5.6 and 8 is very workable even without tilt. At longer lengths it begins to take some careful consideration and planning.

As we all know, the Tilt-Flug effect is only helpful in some instances, but when it can be used, it is such a treat in delivering toes to horizon sharpness at the optimal lens ap. I would hesitate to sacrifice it.

Maybe he will . We're very fond of talking about 'laws of physics'. Perhaps the certainty gives us some comfort. But we should never forget that so called laws are not of physics but of man about physics. Every law that I know of (but to be truthful, I don't know much) that has been created by man, can be broken by man.

The so-called laws of diffraction as they relate to lenses, are in the process of being broken. Read here and again here .

I agree with bjanes that the P45 cannot make full use of a diffraction limited f5.6 lens. The lens could be said to 'out-resolve' the sensor. But such a lens will still serve a purpose because low contrast detail can be captured with less loss of contrast, and detail of really low contrast that simply couldn't be captured at all with a lens that is diffraction limited at a smaller aperture, will be able to be captured, at f5.6.

[quote=Ray,May 3 2006, 07:05 AM]
Maybe he will . We're very fond of talking about 'laws of physics'. Perhaps the certainty gives us some comfort. But we should never forget that so called laws are not of physics but of man about physics. Every law that I know of (but to be truthful, I don't know much) that has been created by man, can be broken by man.

The so-called laws of diffraction as they relate to lenses, are in the process of being broken. Read here and again here .

If you actually read those referrences, it appears that the laws of physics are not being violated. Rather, a negative index of refraction is responsible for these strange properties. Apparently, Snell's laws and Maxwell's equations still apply.

Reference 1:

"An intriguing property of the left-handed material is negative refraction. The optical properties of materials that are transparent to electromagnetic (EM) waves can be characterized by an index of refraction. Given the direction of the incident beam at the interface of vacuum and the material, the direction of the outgoing beam can be determined using Snell’s formula. All naturally available materials possess a positive refractive index. In lefthanded materials EM waves bend negatively and consequently a negative index of refractive index can be assigned to such materials without violating Maxwell’s equations. This negative bending allows considerable control over light propagation and opens the door for new approaches to a variety of applications from microwave to optical frequencies."

You know, bjanes, I actually thought after I'd written that, that perhaps I should edit my post because someone is sure to point out that the laws pertaining to optics haven't actually been broken, but rather extended. Of course you're right. I was trying to be smart and make a philosophical point . I imagine that at some point in the future, Maxwell's equations will prove to be as inadequate for certain applications as Newton's Laws of Motion have long since become.

I am a scientist at a major University, and just to clarify you are not correct. The laws of Physics exist and man works on trying to understand and explain these laws of physics as they exist in nature. Man has no ability to break these laws. For example no particle can travel faster than the speed of light. That is a law that we use to describe the physical universe and man is not able to break this law. In addition, there is no way a man made lens can break a law of physics. Mark

[quote=bjanes,May 4 2006, 03:23 PM]
[quote=Ray,May 3 2006, 07:05 AM]
Maybe he will . We're very fond of talking about 'laws of physics'. Perhaps the certainty gives us some comfort. But we should never forget that so called laws are not of physics but of man about physics. Every law that I know of (but to be truthful, I don't know much) that has been created by man, can be broken by man.

The so-called laws of diffraction as they relate to lenses, are in the process of being broken. Read here and again here .

If you actually read those referrences, it appears that the laws of physics are not being violated. Rather, a negative index of refraction is responsible for these strange properties. Apparently, Snell's laws and Maxwell's equations still apply.

Reference 1:

"An intriguing property of the left-handed material is negative refraction. The optical properties of materials that are transparent to electromagnetic (EM) waves can be characterized by an index of refraction. Given the direction of the incident beam at the interface of vacuum and the material, the direction of the outgoing beam can be determined using Snell’s formula. All naturally available materials possess a positive refractive index. In lefthanded materials EM waves bend negatively and consequently a negative index of refractive index can be assigned to such materials without violating Maxwell’s equations. This negative bending allows considerable control over light propagation and opens the door for new approaches to a variety of applications from microwave to optical frequencies."

[/quote]

You are a brave man to make such bold assertions. I would characerise them as a religious faith without ultimate foundation in logic and/or experience. Atoms, electrons, boson etc. know nothing about laws. They simply are . We, as humans, invent the laws (through experiment, trial and error, mathematical deduction etec etc) to contol and manipulate such phenomena, to great practical advantage. We delude ourselves if we think that these laws have any existence outside of human imagination.

I know this is getting way off topic but I just want to emphasize that nothing I said implies anything about religion or faith. I made no comment on such a view as that is a personal belief. You have brought religion into the discussion. I will remain neutral on this subject as it is irrelevant to the discussion. However, it is factually wrong to say that man invents the laws of nature. Moreover, I think it is arrogant to believe we (Man) can invent the laws of nature. We only define the laws of nature that exist---- for whatever reason. Sure we manipulate them to our advantage (e.g. digital optics) or disadvantage (nuclear bombs). Whether we are here on Earth or not, the laws will dictate the natural events in the Universe. Before man existed on Earth, laws of nature were still in full swing. You are correct that subatomic particles like boson, electrons, quarks etc know nothing about laws -- but they exist and follow the laws of physics.

Guys,

Don't get out of shape. We don't really know the laws of Physics only rough appoximations in the limited way we can observe things. Still we know enough to make clocks work, plains fly and measure events as fine as one can imagine.

The main thing to remember is that all our treasured gadgets are really pieces of outdated junk that haven't yet matured and we are just worm-food that hasn't yet served!

All we can do for now is simply pass on what little we know to others.

That is perhaps why we take pictures. That is what interests me!

Asher

Agreed

That's what Newton believed and hundreds of brilliant scientists after him. Even Einstein was beguiled by this notion. A deep flaw in Newton's theory of gravity, was why did not the solar planets and stars not collapse on each other? Explanation? God.

Einstein's first theories of Relativity assumed a non-expanding universe, in the tradition of Newton. He was later proved wrong, and admitted it was one of his greatest mistakes.

Arrogance is the assumtion that any scientific theory has any absolute truth (and for that matter, any religious theory).

Why do you keep bringing GOD into it. My statements are equally valid regardless on your or anyone elses views of God. I do not want to turn this into a religious discussion. I am not pushing my religious views on you.

NO scientist believes scientific theories are absolute truth. Theories are theories and not fact. Theories are proposed based on the best scientific knowledge. The theories are consistent with scientific knowledge and not fact. They are commonly wrong or need to be tweaked based on new scientific knowledge. That does not mean the laws of physics have changed. It just means we can better define them. Newton's theory of gravity is still valid. The reasons the planets do not collapse are based on laws of physics that were not understood at that time. Nonetheless, the laws of physics did not change-- just our understanding of those laws.

I am on this board to learn about photography so my comments will be limited to that topic from now on.

Fair enough! I will desist from further comment, except to point out that I only mentioned God once.