Resolution Limits of 35mm Photography

Dan Fromm said:

Ed, I've done the experiment. At 1:1 a known good 100/6.3 Luminar is sharper at f/6.3 than at f/8. I've got the same result with a 100/6.3 Neupolar. If you can't accept my results, the best thing for you to do is try to duplicate them.

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... But still - no MTF diagrams to support that work.

Who did I direct to my post of 03-06-2007 (#26 permalink)? If it wasn't you, then I will suggest you go there anyway, and report back to me!!!.

As is, you won't accept anyone's word for anything,...

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Not blindly, I won't - and certainly not the word of an inflexible bully - so insecure that they must resort to insults, lacking anything remotely resembling a coherent argument.

... you won't do your own testing, you won't open a book or go to a library.

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How much "testing" would you consider adequate? I HAVE done a fair amount of that in the past. Not open a book ??? I did revisit Van Heel and Velzel (ah memories..) and a number of others - and my notes - from my own library ...

If I had a 2x4 and could reach you I'd address you smartly with it. This is said to work on mules, might work on you.

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Well ... that may be a true statement - defining your character. I will confess to being a little surprised ... I would have given you credit for more maturity and courage than that. I've been open about my location - where, exactly are you from - or after making that statement, would you rather not say, and go into hiding?

One discovery I've made - EVERY bully that ever existed is/ was, at his core, a CRAVEN coward. If he was not, there would be no reason to be a bully.

Dan Fromm said:

Ed, I've done the experiment. At 1:1 a known good 100/6.3 Luminar is sharper at f/6.3 than at f/8. I've got the same result with a 100/6.3 Neupolar. If you can't accept my results, the best thing for you to do is try to duplicate them.

As is, you won't accept anyone's word for anything, you won't do your own testing, you won't open a book or go to a library. If I had a 2x4 and could reach you I'd address you smartly with it. This is said to work on mules, might work on you.

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g'day Dan

so any lens should only be used at a pre-determined aperture?

isn't that just a little too restrictive?

can any normal person notice the difference?

what about the many other image making variables?

Dan Fromm; ... If I had a 2x4 and could reach you I'd address you smartly with it. This is said to work on mules, might work on you.

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On a more serious note - the moderators here might want to take notice here.

Mr. Fromm doesn't bother me one bit - but I think consideration of the suitability of that remark and its effect on the tone and image of APUG might be warranted.

Many posts ago, it was stated that the physical aperture of the lens (rather than the f-stop) determines the effects of diffraction on the image. This is incorrect: the f-stop used is the relevant factor, regardless of focal length. The reason large format images can be very sharp at very high f-stop values is that the negatives are enlarged much less.

Resolution is a rather subjective metric. MTF curves, properly understood, offer a much better understanding of image quality and sharpness. Interestingly, the best 35 mm lenses show roughly similar MTF curves (weighted for the higher enlargement factor) as the best medium and large format lenses. In other words, the information projected by a high quality 35 mm lens is roughly the same as that projected by a high quality large format lens. Of course the 35 mm lens will need to be at a lower f-stop compared to the larger lenses.

The difference in image quality between 35 mm and larger formats is almost entirely due to the different enlargement factors required, and therefore the resolution (and grain, etc.) of the film itself. There are also a great many mediocre lenses for 35 mm cameras (almost all zooms for a start), whereas few modern lenses for medium and large format are truly woeful.

Photography has nothing to do with resolution, hate to break this to you. Dry your tampons, and get over it.

Cheers!

Ray Heath said:

g'day Dan

so any lens should only be used at a pre-determined aperture?

isn't that just a little too restrictive?

can any normal person notice the difference?

what about the many other image making variables?

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Ray, it makes no sense at all to shoot a lens only at its "best" aperture, whatever that is. We all trade off sharpness in the plane of best focus for sharpness in depth. We all sometimes use selective focus to isolate the main subject from its surroundings. There are times when it makes best sense to shoot wide open. Why should anyone restrict itself to shooting at the one true aperture?

I suggested to Ed Sukach that he buy a copy of John Williams' book Image Clarity. You may enjoy reading it.

People who mainly play with lenses think sharpness, by which they often mean something like resolution at five or ten percent contrast, is the end-all. People who take pictures understand that producing a print that works is the end-all.

Most of the lenses I own have gone through acceptance testing on arrival. The point of this is not to find the lens' one true aperture, but to find out if it is good enough to use. Not all of the lenses I've bought, e.g., my Industar-51, seem good enough to use. I find insignificant differences among my lenses that pass acceptance testing, and this is consistent with Chris Perez' results.

That said, in photomacrography it is often the case that stopping down reduces sharpness everywhere in the image, i.e., hurts, doesn't help, image quality. In my limited experience, and I don't think that, e.g., Ted Harris agrees, this sets in around 5:1. For a deeper discussion of this apparent paradox, with illustrations, see H. Lou Gibson's little book Photomacrography. It was also published without Mr. Gibson's name as, IIRC, Kodak Publication N-12b.

Cheers,

Dan

Ed,

Why are you so hung up on MTF curves? All of the ones I've seen published by major lens manufacturers, viz., Rodenstock, Schneider, and Zeiss, plot image contrast as a fraction of subject contrast against distance from the center of the field for a handful of spatial frequencies and apertures. The spatial frequencies selected are always well below the theoretical diffraction limit (~ 1500/f number) at the aperture.
I don't see how MTF curves as conventionally published can be read to find the aperture at which residual aberrations are swamped by diffraction. Please teach me how to do this.

When I read published MTF curves, I see two things in them. How well the lens reproduces contrast at fairly low spatial frequencies, by aperture and distance from the center of the field, and how far out in the field the lens produces usable image quality.

If you want to see a really scary set of MTF curves, look at the set that Rodenstock published for the Apo Rodagon D 75/4 M 1:1. A great lens, no doubt, but of very limited usefulness. I sold mine because I couldn't work within its limits.

So why do you insist that I produce MTF curves?

Dorian Gray said:

Many posts ago, it was stated that the physical aperture of the lens (rather than the f-stop) determines the effects of diffraction on the image. This is incorrect: the f-stop used is the relevant factor, regardless of focal length. The reason large format images can be very sharp at very high f-stop values is that the negatives are enlarged much less.

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Not true.
There is no direct link between "f/stop" and diffraction - the ratio Ole uses, ad:f (aperture diameter in relation to focal length) is proper.
It is not possible to determine the amount of diffraction solely from the single value of f/stop; focal length MUST be considered.
The amount of diffraction observed at f/22 will be far different in a 50mm lens than a 500mm lens... and it is NOT linear.

This was the subject of an extensive discussion a while ago here on APUG. I don't know if it is still in the archives, but a web site explaining all this was listed. I'll see if I can retrieve the address and post it.

Dan Fromm said:

I don't see how MTF curves as conventionally published can be read to find the aperture at which residual aberrations are swamped by diffraction.

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Well, the MTF values at 5-40 cycles/mm are vastly more indicative of actual image quality than theoretical diffraction-limited "resolution" figures, or even tested "resolution" figures. And from MTF curves plotted for various f-stops, one can of course find the best aperture for sharpness, ignoring depth of field requirements, etc. (which in actual photography cannot be ignored of course). The MTF curves show at which f-stop diffraction begins to degrade image quality more than the remaining optical aberrations at that f-stop.

Ed Sukach said:

It is not possible to determine the amount of diffraction solely from the single value of f/stop; focal length MUST be considered.
The amount of diffraction observed at f/22 will be far different in a 50mm lens than a 500mm lens... and it is NOT linear.

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By definition, f-stop values take focal length into account. All lenses at f/22 will exhibit almost identical diffraction effects, regardless of focal length. Anecdotal evidence supports this: all 35 mm lenses start to suffer strong diffraction effects at around f/11, whether the focal length is 21 mm or 300 mm. I'll read the archived thread if you can find it, but it sounds like it's wrong.

Dan Fromm said:

Ed,

Why are you so hung up on MTF curves?...

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I am not hung up on MTF data.

I don't see how MTF curves as conventionally published can be read to find the aperture at which residual aberrations are swamped by diffraction. Please teach me how to do this.

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Nor can I.

I realize that a great attention span is required to keep up with this thread ... Let me try to review:

A message was posted claiming that the performance of a lens was *ONLY* limited by the aperture; that diffraction due to - and SOLELY due to an decrease in aperture size was the ONLY factor limiting performance, and that information was necessary for every photographer trying to produce a "serious" image.

I disagreed, not in that there was not some point when diffraction WOULD affect the performance of a lens, but that the manufacturers had recognized this, and had limited the selection of minimum aperture to reflect this, and prevent the use of their product beyond its design limits.

The reply came back: "No, no ... we all have - MUST - be aware of that minimum aperture" - and suggesting that somehow, the manufacturers were withholding that information from us `ragged masses' - "if we are to produce `acceptable' work."

I next questioned HOW that information could be determined: and WHY the manufacturers did not publish that critical information in their Lens Data sheets.

The answer was returned: "No need for them to publish that `diffraction limit'... it is a SIMPLE matter to determine that from the published MTF curves."

One way to test that last statement was to post MTF curves, and ask for the "least minimum acceptable aperture" based on the information contained therein. A "simple" matter ...

No one has yet done so ... and while I DO think that some inference may be obtained from the MTF curves, it is, apparently, as I have suggested ... NOTHING like a "simple" matter.

Now, to summarize ... I think it is folly to worry about the effect of diffraction in the ordinary - and even not-so-ordinary uses of the lenses in employed by the overwhelming majority here. Period. The manufacturers have already done all the "worrying" necessary - well, maybe excepting Holga, but even there ... WHO CARES?? - See "The f/63 Group Manifesto".

I am adamantly convinced of this ... and the threat of being beaten like a mule with a 2x4 (do you beat these mules often?) will be NOT be instrumental in changing my OPINION.
Nor will this threat cause me to rush out and buy these lenses and testing equipment and spend hours of Library research to re-learn what I've already learned - and for which I have passed the formal testing... NOR write reports.

I do not seek physical battle... but ... How about arranging a "Joust" at the Medieval Revival Site in Carver, MA - King Arthur's Faire. We could use 2x4s instead of lances....

Ed, I think you know we can't strictly tell anything about optimum apertures from the MTF graphs you posted (although we know that Zeiss provides curves for full aperture and at an f-stop they consider optimum). The impact of diffraction on the image increases as the lens is stopped down; by contrast, optical aberrations tend to diminish as the lens is stopped down. At some point the two "cross over", giving an optimum aperture. But it's not that simple, because on-axis performance is better than the edges, therefore optimum on-axis performance often occurs just one stop down from wide open (or even wide open in some cases), whereas the extreme corners may improve all the way down to minimum aperture in some cases. Therefore Zeiss' choice of "optimum aperture" in their published curves is somewhat subjective: it necessarily incorporates a weighting towards centre or edge performance.

To make a judgement about optimum aperture, one needs access to MTF curves across the f-stop range, and one needs to make a decision about whether performance in the centre, corners or some in-between balance is most important.

Dorian Gray said:

To make a judgement about optimum aperture, one needs access to MTF curves across the f-stop range, and one needs to make a decision about whether performance in the centre, corners or some in-between balance is most important.

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Not only that: I've talked to Zeiss lens designers (who tend to be seriously enthusiastic about their trade) and they reckon that an MTF curve can't tell you what a lens really does. Nor does a computer simulation: you have to build it, and try it, and see how you like it.

They have quite clear (and supportable) views on how much resolution you can get on the film with white light, and they reckon it's one useful indicator of likely performance. But it is only one; it is only useful, not decisive; and it is only an indicator. They say much the same about MTF curves.

Cheers,

R.

At some level, I dislike this site - too much D.....l reference, but it contains a calculator that will determine the f/stop where diffraction will theoretically be greater than "geometrical optics" errors. It is not all that accurate .. diffraction is dependent on the physical distance from aperture to film plane, not traditional "focal length" (consider "retrofocus" and "telephoto" lenses), but serves as a fairly good comparison, based on format. Go to:

http://www.cambridgeincolour.com ; Enter the site; Select "tutorial"; scroll down to "Advanced topics"; Select "Understanding Diffraction, Pixel Size, and Airy Disks", then "Calculating the Diffraction Limit"; - I would ignore all the selections here, except ... choose "Camera Type", and then "Select Aperture" ... and "Calculate".

Note that the "Diffraction Limit" - in my book, "The Diaphram Where One Has to Start Being Concerned About Diffraction", is f/16 for 35mm; f/32 for 6x6 and 6x7; f/64 for both 5x4 and 10x8 inch formats - all of which seem to be reasonably close to the minimum apertures that I can access on the lenses I own... Limited by the manufacturers.

Other sites of interest:

http://www.mrpinhole.com/index.php (Note here the various OPTIMUM pinhole (aperture diameters) based on aperture-to-fim distance and their calculated f/stops - f/stops themselves are only useful in determining exposure - not resolution).

[ http://www.mines.edu/~mmyoung/PHCamera.pdf ]

I have two others, but I can't seem to get the addresses right ... I'l keep trying and will post them later.

http://www.cambridgeincolour.com/ appears to be armchair physics. Despite what it says, a good lens at f/22 on a 35mm camera IS diffraction limited. If the authors of the diffraction limit calculator don't want to be bothered with taking photographs, someone could lead them into a darkroom and show them what stopping an enlarging lens down to its minimum aperture does to sharpness in a fine grain 35mm negative.

As for http://www.mrpinhole.com/index.php, it is often recommended on the F295 Pinhole Forum by those who prefer simple answers to a complex question. A far better calculator is Pinhole Designer, which allows one to select critical factors such as the predominant wavelength of film or light and an appropriate user constant. These sites apply to pinhole photography, but may be of interest to lens users. Another site with much good information on pinhole photography is Dead Link Removed.

The Matt Young link is one of the most intelligent articles I've found in considerable reading on diffraction limited lensless photography.

A fairly simple experiment here is to photograph a test target using a tripod and sharp film -- something you'd only do if you're an obsessive with nothing better to do, or are being paid to do a review -- and see what happens in the real world. Generally, for a top-flight lens, resolution builds fairly fast; peaks at around f/5.6 centrally, maybe f/8 edges (+/- 1 stop, depending on the lens); is more or less the same at f/11, or maybe a tiny bit worse; then falls off detectably at f/16 and significantly at f/22.

R.

Roger Hicks said:

A fairly simple experiment here is to photograph a test target using a tripod and sharp film -- something you'd only do if you're an obsessive with nothing better to do, or are being paid to do a review -- and see what happens in the real world. Generally, for a top-flight lens, resolution builds fairly fast; peaks at around f/5.6 centrally, maybe f/8 edges (+/- 1 stop, depending on the lens); is more or less the same at f/11, or maybe a tiny bit worse; then falls off detectably at f/16 and significantly at f/22.

R.

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Roger has a good point, but some others here would rather disagree and argue ad infinitum just to see there name on the computer screen.

Did I burn someone?? My bad.

Steve

His response will be, as it has been time and again in this thread, that that doesn't prove that it's diffraction limiting the resolution. He'll argue that maybe you can strongly demonstrate a correlation between higher f/number and lower resolution, but that isn't inductive proof of diffraction being responsible. He'll then ask us why he should care about resolution.

Achieving an unsharp print is much easier than achieving a very sharp print. For those of us who value sharpness and detail rendition from time to time, I find it extremely valuable to understand this and apply it in my decision making process. If that means I take some things for granted, well, I do that at my own risk. There are much higher stakes things in life that we have to take for granted beyond a point. And these reductio ad absurdum exercises in epistomology are frankly not all that useful either.

Jim Jones said:

http://www.cambridgeincolour.com/ appears to be armchair physics. Despite what it says, a good lens at f/22 on a 35mm camera IS diffraction limited.

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Despite? That IS exactly what it says ... With a 35mm format and f/22, the lens Is diffraction limited.

... If the authors of the diffraction limit calculator don't want to be bothered with taking photographs, someone could lead them into a darkroom and show them what stopping an enlarging lens down to its minimum aperture does to sharpness in a fine grain 35mm negative.

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II don't think it does anything to the "sharpness" in the NEGATIVE; It probably would in the PRINT. However I rarely will stop down that far, unless I'm tilting the easel for perspective correction... in which case, the loss of definition will probably be less important that depth of focus. Wait ... will my 50mm Enlaging lens stop doen that far???? I'll have to look.

Pinhole Designer, which allows one to select critical factors such as the predominant wavelength of film or light and an appropriate user constant. These sites apply to pinhole photography, but may be of interest to lens users. Another site with much good information on pinhole photography is Dead Link Removed.

The Matt Young link is one of the most intelligent articles I've found in considerable reading on diffraction limited lensless photography.[/QUOTE]

The "cz.pinhole" site was one of the addresses I was trying to correct - Thanks.

I'll try the other site.

DrPablo said:

His response will be, as it has been time and again in this thread, that that doesn't prove that it's diffraction limiting the resolution. He'll argue that maybe you can strongly demonstrate a correlation between higher f/number and lower resolution, but that isn't inductive proof of diffraction being responsible. He'll then ask us why he should care about resolution.

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May I ask who you are talking about here?

"It doesn't prove that it is diffraction limiting the resolution"? What "doesn't prove?" At some point, diffraction will; I've tried to make the point that that aperture will be beyond the range allowed by the manufacturer ... and given that restriction, we do not have to worry about diffraction.

I never said that we should not care about diffraction itself, only that its effect at common f/stops is negligible - and not worth the worrying effort.

And even if it is NOT me - I think that the batteries in your crystal ball - or whatever other device/ means you use to predict what others WILL "say" are in dire need of replacement.

Ed Sukach said:

Despite? That IS exactly what it says ... With a 35mm format and f/22, the lens Is diffraction limited.

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Wooops!! My ERROR - It does indicate that a lens for the 35mm format is NOT "diffraction limited" at f/22 ... and it does not become so until f/32. That WAS a surprise!!!

I know that one of the 50mm f/2 lenses for the new Zeiss Contax does stop down to f/22.

I wonder if the diaphragms for all 50mm lenses are located, in fact, at 50mm from the film plane? ... Or if there is some degree of "telephoto-ness" involved...?

94 posts down the line, what the HECK are you guys still bickering about? We all know that stopping down a lens corrects certain aberrations, thus helping resolution, but also that past a "sweet spot" that is relative to each lens, more stopping down will in fact decrease resolution because of diffraction.

What is there more to argue about?

Well, I know I received quite an education on diffraction and the issues associated with it.

mhv said:

... We all know that stopping down a lens corrects certain aberrations, thus helping resolution, but also that past a "sweet spot" that is relative to each lens, more stopping down will in fact decrease resolution because of diffraction.

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No, we DON'T all know that beyond the "sweet spot" it is ALL diffraction ... because it ISN'T.

Check out the sites listed ... to determine when diffraction is significant.

Ed Sukach said:

No, we DON'T all know that beyond the "sweet spot" it is ALL diffraction ... because it ISN'T.

Check out the sites listed ... to determine when diffraction is significant.

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Dear Ed,

At the risk of appearing combative -- and I have to confess myself genuinely ignorant here, so I do not wish to tread upon your toes -- which aberrations commonly become worse on stopping down?

Cheers,

R.

I don't take you as "combative".

There are a number of factors that affect the focal plane of a lens... glasses with different refractive indices are used with different curves, both spherical and aspherical to converge all light rays onto the same plane ... as perfectly as possible (within reason, in any event). The image seen at "full aperture" is a collection of many ray traces, some in good places - others not. We see, and the film is affected by, an average, or root-mean-square ... or something like that ... of all of them.

Limiting the ray traces to those in the center of the lens CAN - and usually is, to a point - a "good thing". Depending on the design, favorable ray traces MAY be eliminated as well, lessening the overall quality. At some aperture, diffraction, which is separate from refraction, will be the dominant factor. Rarely is the aperture affording the "best" quality, a.k.a. "sweet spot", located close to the point of "diffraction dominance," Rarely, although possible. Most of the time, the design will converge the rays best at the aperture expected to be used the most ... near "wide open" for fast lenses; near the "middle" for ordinary camera and enlarging lenses.

As a side comment ... Optical Design Engineers are, as a class, the most intelligent, best damn engineers of all. Every one I've ever met seemed to be a cut above the rest.... They have to be ... Optics is one of, if not THE most, difficult field of all.