Practical diffraction

[Jim Rice]

I realize after searching "diffraction" that this is hot button issue (though for the life of me, I can't understand why). Rodenstock tells me that the optimum aperture for my 360 Caltar IIN is between 32 and 45 (a stop down from their recommendation for the 210 Sironar-N that never failed me on 4x5" at 22-32). May I just accept this? For my tastes, this issue has always been obfuscated here by either personalities (get well soon Jorge) or physics majors. I'm pretty sure that it varies with focal length, but I'm not certain.

I am also not interested in "good enough for contact printing". I'm sorry, but this seems like a fudge. OTOH, dissussions on CoC will just make my eyes glaze over.

So barring being warned off by the people i respect here, 32~45 is where I am.

The actual question: Am I full of it here?

[Eric Woodbury]

You need to wonder over to the home page HERE. Lots of good info. Start with:

http://www.largeformatphotography.info/fstop.html

and my favorite on optimum f/stop:

http://www.largeformatphotography.in...nsma-dof.1.gif

plus pages 2, 3, and 4, all referenced at the end of the first link.

[Brian Ellis]

If you're not making prints larger than 16x20 from 4x5 film don't worry about it.

[Lazybones]

In my experience, anything f45 and wider seems acceptable, as far as the tradeoff between depth of field and diffraction. I've only had a problem with f64 looking a bit soft on 4x5, and usually not worth the extra depth of field. Probably better to recompose, crop, or switch lenses (if possible) in this situation.

"I am also not interested in 'good enough for contact printing'. I'm sorry, but this seems like a fudge."

I couldn't agree more. So what I'm talking about is squeezing all that nice juicy quality out of that neg/chrome! I try to shoot for flexibility in print size, even if I just want, say, 11x14 for now, I may indeed want to print big in the future. It's when you don't have it and need it, that...

[Leonard Evens]

The usual rule of thumb is that an ideal diffraction limited lens provides you with a resolution in lp/mm approximately equal to 1500 divided by the f-number. So at f/45, such a lens should yield, 1500/45 ~ 33 lp/mm. Combining the lens with a modern film reduces the resolution further, but not by too much. Let's say you end up with about 30 lp/mm. The usual rule of thumb is that the human visual system can resolve between 5 and 10 lp/mm in a print, viewed from about 10-12 inches. If your vision is at the high end, you might see a difference at roughly a 3 X enlargement, but most of us wouldn't see a difference even in a 5 X or 6 X enlargement. One school of thought says that you should consider only a 2 X enlargment since that is the only size you would view that close. People are supposed to get proportionately further back for larger prints. On the other hand, if you want to satisfy 'grain sniffers', you have to worry about what any size print looks like close up.

If you start with the the above rule and consider the various considerations I raised above, you should be able to come up with a reasonable estimate of how far you can stop down without diffraction becoming a problem for the purposes you have in mind.

[Christopher Perez]

... and then there's the chemical effect of the developer with regards to micro contrast and edge enhancement.

I have a friend, Patrick Kolb, who shoots his 19inch Artar at f/45 and soups in PyroCat or ABC and swears his images appear sharper at f/45 than they do at f/22.

It's real easy to get wrapped around an axel about this stuff. The only time I've seen aperture play an important part in overall image quality is in the Biogon design wide angle lenses. Anything below f/32 has produced nothing but mush from lenses I've tried over the years.

[Jim Jones]

. . . The actual question: Am I full of it here?

Certainly not, if what you do satisfies you. Eric's first link is a great compilation of information I've wrestled with for decades. I enjoy such a wrestling match, even if I often lose. Leonard condenses it to a form easier to digest, and which agrees with my experience. The limitations of diffraction are not scientific absolutes when applied to photography. There are important esthetic considerations, too. Edward Weston used an obsolete lens with the iris modified to permit near pinhole apertures, and could produce better images than if he had limited the aperture to the traditional f/64.

A quick look of the effect of diffraction is at hand in an enlarger. Watch a fine grain pattern through a high power focusing magnifier as you adjust the aperture. With my El-Nikkor 50mm f/2.8 the grain in the center of the image is sharpest at about f/4, and is still fairly good at my normal aperture of f/8. By f/16 it is definitely fuzzy.

A rule of thumb suggests, for critical sharpness, a minimum camera lens entrance pupil of no smaller than maybe 5mm if the print is viewed from the distance that gives correct perspective. This is quite subjective. Remember, the most important rule in photography often is "@#$& the rules!"

[jim kitchen]

A few months ago, I decided to add an 8X10 to my backpack, and park my 4X5 for a while. I could not wait to see what brilliant images I could produce with this wonderful piece of equipment. I decided to set my 8x10 lenses to a middle f-stop, my normal practice with my 4X5, set the required tilts and swings, and produce the image. Unfortunately, and disappointingly, my images were extremely DOF challenged compared to my 4X5 images. My negatives were absolutely brilliant though from the tonal point of view, totally grain free, and the images were painfully sharp.

After reviewing, and enriching myself again with the archived notes on DOF and CoC to correct my issue with DOF, I understood I could probably use smaller f-stops, and I should also review my camera setup. I decided to make a few incremental changes, where the greatest change was the smaller f-stop. The smaller f-stop produced images with a better DOF wedge, but I noticed a subtle and uncharacteristic change in my image's tonality, and grain structure. I did not understand this change immediately, and I began to question the ability of my lenses at reproduce objects at infinity. I reviewed the original negative, which drew my attention to DOF, and I quickly realized I traded one problematic issue for another that being DOF for diffraction. I finally determined why my lenses performance changed at infinity, and only performed to my expectations at closer distances.

So, I decided to do an unscientific test with each lens I own on the weekend, to determine the difference in diffraction between the middle f-stop, and the smallest marked f-stop on each lens. My tests indicate diffraction positively affects each lens I own, and diffraction varies with each lens, while using smaller f-stops. How upsetting, but now the issue is understandable. Diffraction produces an incremental amount of grain across the entire 8X10 negative, and produces a softer image quality. I can improve the effects of diffraction, where I can isolate and correct this issue, to a certain degree within Photoshop. I can not however correct improper DOF and out of focus items. I must accept the use of smaller f-stops, to balance DOF, and I will continue to correct the increased grain and loss of image sharpness in Photoshop. I will however, try to improve my images using the middle f-stops, whenever possible.

Attached are two images, where the subject matter was approximately 200m away using f45 and f90 on my Rodenstock 480 Apo-Ronar, indicating the differences.

jim k

[Jeff Conrad]

The optimum f-number for the plane of focus is different from that for the limits of DoF; lens manufacturers' recommendations usually are for the former. With a large focus spread, there sometimes is a tradeoff between center and edge sharpness. Once a lens is essentially diffraction limited, stopping down further decreases sharpness in the plane of focus (though the loss of sharpness may not be apparent until you close down several more stops or examine very large prints at close distances). At the limits of DoF, closing down further reduces the defocus blur and increases sharpness, at least up to a point. Once the "maximum" f-number is reached, stopping down further decreases sharpness even at DoF limits, so there is no point in using a greater f-number.

For that reason, I consider Hansma's "optimum" f-number more of a maximum, because there is no benefit to using a greater f-number. Using a very different approach (calculated MTFs), described at http://www.largeformatphotography.in...DoFinDepth.pdf (PDF) under Diffraction, I get essentially the same results as Hansma. Both Hansma's results and mine seem consistent with what most of the others have said here.

I second the recommendation of Eric's first link. It has the most relevant information on this topic that I've seen in once place.

[Brian Ellis]

Jim Kitchen said:

"Diffraction produces an incremental amount of grain across the entire 8X10 negative, and produces a softer image quality."

I've never heard of diffraction increasing grain. Which doesn't mean it doesn't, there's lots of things I've never heard of. However, FWIW diffraction is caused by the fact that as the aperture gets smaller the proportion of light rays bouncing off the edges of the aperture before they strike the film (as opposed to striking the film directly) gets greater. An oversimplification but accurate enough for present purposes. I don't see how that would increase grain though I could be missing something.

I also don't understand the two thumbnails. Are they scans of an 8x10 print or a cropped section of an 8x10 print or something else? If they're a crop, what's the magnification factor of the crop? I ask because I've never seen diffraction create the degree of blurring shown in these thumbnails. Of course if any negative is enlarged by a big enough factor you'll eventually be able to see the effects of diffraction, even with a lens that's almost wide open, but with wider apertures the mag factor has to be unrealistically great. And even with smaller apertures such as f45 or even f90, in my experience (admittedly limited to my own work) the effects of diffraction don't look like these thumbnails, at least not until the mag factor gets way way up there. Perhaps you could explain what the thumbnails represent.