Confused about diffraction?

[joshdaskew]

Hi, Was just hoping that someone out there could help me.. From reading a couple of posts, I have a rough idea about diffraction and that your images can get softer if using small apertures. As a rough guide, it seems that f22 seems to be this limit and then using smaller apertures than this (f32, f45) can soften the image. I also read in another post that someone was getting soft images shot with a 75mm lens at f22 and that this was apparently past the diffraction limit for that focal length. So I guess my question is, does the diffraction limit depend on the focal length or is it more format based? I am using a Rodenstock GRANDAGON 75mm f/6.8, Schneider Super Angulon
90mm f8, Rodenstock 135mm Sironar-N, Rodenstock Apo Sironar-N, Caltar 210mm II-N and a Nikon M 300mm. If someone could suggest the diffraction limits for each of these lenses that would be greatly appreciated!

Also, I am planning on printing a folio with the dimensions of 34 cm ( length ) x 28 cm ( width ) and was just wondering if diffraction would play a part in print sizes that small (even if closed right down to f45) compared to an image that was shot within diffraction limits.. Also, at that size, would there be any difference between the lenses I currently own and the latest offerings from Schneider, Rodenstock etc etc..

Ok, thanks so much, much appreciated! Best Regards Josh.

P.S Am not particuarly technically minded so please keep this in mind when answering. Thanks again.

[ic-racer]

In general it is 'format based' because the smaller formats are usually enlarged more to match the larger formats final print size.

You can also compare a 4x5 contact to an 11x14 contact, but you need to decide if they will have the same viewing distance, as the viewing distance influences the acceptable Airy disks sizes.


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

[N Dhananjay]

Diffraction depends upon the f stop. However, the sweet spot for sharpness in a photograph depends upon a tradeoff between diffraaction limitations and aberrations in a lens. Typicall, the larger the aperture, the aberrations tend to be high while diffraction limitations are low. Smaller apertures tend to reduce certain aberrations but increase diffraction limitations. So, the sweet spot is when you have stopped down enough to reduce aberrations significantly but not so much that diffraction becomes a huge issue. The idea of being diffraction limited is that beyond this point, lens aberrations are not decreased sufficiently to compensate for the diffraction costs. So, you would imagine that a lot depends upon lens design etc and saying something like 'f/22 is the sweet spot for all lenses' might be a bit of a sweeping generalization. But others have argued that manufacturers aim at that spot in their design.

I would suggest using the ground glass as the final arbiter. You shouold also keep in mind that we are dealing with different kinds of unsharpness, differences that you might care about. The diffraction limitation argument holds for whatever is in sharp focus. That is, if you were photographing a brick wall, what sort of aperture would give you the best sharpness? In reality, there are further trade-offs, the most prominent being depth of field (DOF). Diffraction affects the resolving power of a lens - it shows up a softening of the entire image. Shallow DOF shows up most prominently in the out-of-focus areas. Certain kinds/styles of photographs suffer more from lack of DOF than diffraction. For example, if you make portraits where you want the sitter in focus but would like to throw the background out of focus, you would probably tend towards larger apertures. You may still want to stop down some to reduce aberrations, but not so much that DOF includes background. At the other extreme, if you are shooting a landscape where you want near, middle and distance in sharp focus, you may have to use small apertures for sufficient DOF. That is, the slight softening of detail overall may be preferable to having some parts in focus and other parts blatantly blurred.

All of this also needs to be tied to what size print you are making, whether you are working in B&W or color etc. For e.g., color images typically need less sharpness than B&W to look similarly sharp (I'm talking about subjective sensations here) because the human eye-mind combination interprets color as information. On the other hand, getting colors of different wavelengths to focus on the same spot is also a fiendishly difficult problem for lens designers. Similarly, if one is contact printing, diffraction is much less of an issue - if it looks adequately sharp on the GG and negative, it should look adequately sharp on the print. When enlarging, you need to consider the fact that the diffraction blurs (and the aberration blurs) are being enlarged. At some point, they will be large enough to be unacceptable.

So, in practice, I think it makes sense to marry all this knowledge with how you want your photograph to look. It probably makes sense to aim at using the sweet spot, but sometimes conditions dictate otherwise. In other words, try to use movements so that you can work around the sweet spot, but also be prepared to make the tradeoff, when required. I enjoy learning about all this but I have concluded that it is all in the wash and one is better served by concentrating on visual concerns (while having the knowledge for those occasional situations that warrant it).

Cheers, DJ

[Rick Olson]

Hello Josh,

Does diffraction exist? The physics of optics says "yes." Do I see it in my 5 x 7 black and white negatives? No. I just developed several 5 x 7 sheets where I needed f32 and f45 to achieve some deep focus. I checked them with a 10x magnifier and they were tack sharp to me, as expected. I used to get all wrapped up in this "best f-stop" mentality and did whatever I could to stay at f22 or less for razor-sharp images. I was not able to achieve what I wanted so I ventured into the "risky waters" of lens diffraction. Didn't see a difference. Maybe my 1960s vintage, single-coated and scratched Fuji 210 lens is a supreme example of optical excellence and sharp at all f-stops. Anyway, I doubt I would see any diffraction effects when contacting printing, but maybe some can.

Use the f-stop you need to achieve the results YOU want and enjoy!


Rick

[Bob Salomon]

Hello Josh,

Does diffraction exist? The physics of optics says "yes." Do I see it in my 5 x 7 black and white negatives? No. I just developed several 5 x 7 sheets where I needed f32 and f45 to achieve some deep focus. I checked them with a 10x magnifier and they were tack sharp to me, as expected. I used to get all wrapped up in this "best f-stop" mentality and did whatever I could to stay at f22 or less for razor-sharp images. I was not able to achieve what I wanted so I ventured into the "risky waters" of lens diffraction. Didn't see a difference. Maybe my 1960s vintage, single-coated and scratched Fuji 210 lens is a supreme example of optical excellence and sharp at all f-stops. Anyway, I doubt I would see any diffraction effects when contacting printing, but maybe some can.

Use the f-stop you need to achieve the results YOU want and enjoy!


Rick

What kind of fine detail did you check at the edges, corners and the center? Were they equally sharp?

[Brian Ellis]

The effects of diffraction show up as an overall softness of the image caused by the fact that when a photograph is made some of the light passes through the lens aperture directly to the film without striking the aperture blades. The rest of the light first strikes the edges of the aperture blades and is bent ("diffracted") by them before the light strikes the film. The higher the proportion of bent light to direct light the greater the diffraction. And the smaller the aperture the greater the proportion of bent light to direct light. Which is why the effects of diffraction become more apparent as the aperture gets smaller.

N Dhananjay's explanation is very good and tells you about all you need to know. I'd add only a couple things to what he's said. While the size of the aperture controls the degree of diffraction regardless of film format or lens focal length, diffraction isn't a practical problem IMHO with 4x5 and larger film unless the print is enlarged beyond about 5x. Usually the effects of insufficient depth of field will be much more noticeable in the print than the effects of diffraction with LF film. So I agree with Rick. It's better to stop down as far as you need to go to get the depth of field you want than it is to worry about diffraction with LF film.

The situation is entirely different with smaller formats, especially 35mm, because smaller formats are typically enlarged much more than LF. That's why the smallest aperture on most 35mm lenses is f22 or even f16, whereas most LF photograhers are just getting started at apertures that wide.

The effects of diffraction aren't going to be any different in the edges or corners of film or a print than in the center. Diffraction might exacerbate differences that otherwise exist. But the differences are caused by lens optics or something else, not by diffraction because the effects of diffraction show up as an overall softness, not a softness in any particular area of the film or print. At least that's my understanding, someone more knowledgeable about the science involved can correct me if I'm wrong.

[aduncanson]

A note on usage:

"Diffraction limited" is a term used to describe telescopes and, less frequently, lenses meaning that the effects of the residual aberrations on lens resolution are masked by the maximum resolution allowed by diffraction for a perfect lens.

What are those maximum resolution limits? A complete answer is beyond my comprehension and attention span, but a useful rule of thumb is that the Max Resolution (in lp/mm) = 1600/ F-No. The actual limits depend on wavelength, position off the optical axis and whether resolution is measured radially or tangentially. However, the rule of thumb above can be used as an upper bound, or max value, for diffraction limited resolution under ideal conditions.

To tabulate those upper bounds:

f/16 - 100 lp/mm
f/22 - 71 lp/mm
f/32 - 50 lp/mm
f/45 - 35 lp/mm

As stated by several posters above, whether or not these diffraction limits will affect you, depends on the magnification of your print and the conditions under which you will inspect it. If the eye can resolve 6 lp/mm, you can use f/45 and make a 6X enlargement (24x30 inches) from your 4x5 and probably get by just fine (assuming everything else in your enlarging chain is working perfectly.

[Daniel_Buck]

I've done some fairly lax tests on this to satisfy my own curiosity, and for me and my tolerances, I've not seen much 4x5 and practically none on 8x10 at print sizes less 20" wide and smaller. Granted I haven't tried any f90+ exposures, but at f32 and f45 (as stopped down as I would ever really need to go) I've not noticed anything soft enough that would make me not want to stop down that much.

Do a few quick tests, and you'll probably find out quickly if you can or can't tolerate shooting f45 or even f64 at your film and print size.

[Rick Olson]

Hello Bob,

I was shooting some abandoned railroad equipment this weekend in Chama NM on the Cumbres and Toltec railroad. The fine detail checked in the lower edges/corners was track ballast (rocks/gravel), rails and ties. Tilted to get this area sharp. Nearby was a track switch-stand at about 8' tall and some railroad cars down the track. Had to get those in focus. Behind that in the distance was a strand of tall trees that extended to the top of the image. Had to get that in focus also, so I adjusted the focus plane about mid-way up and stopped down. I tried a similar shot like this before at f-22 and it didn't cut it. This time, I shot at f-45 and it was sharp from the track ballast, a few feet in front of me (image edge/corner), the switch-stand and railroad cars (image center), to the tops of the trees. My 10x lupe certainly presented a sharp negative at an equivalent print size of 50" x 70", so I think my 5" x 7" contact print should look pretty good!

Rick

What kind of fine detail did you check at the edges, corners and the center? Were they equally sharp?

[Bob Salomon]

Hello Bob,

I was shooting some abandoned railroad equipment this weekend in Chama NM on the Cumbres and Toltec railroad. The fine detail checked in the lower edges/corners was track ballast (rocks/gravel), rails and ties. Tilted to get this area sharp. Nearby was a track switch-stand at about 8' tall and some railroad cars down the track. Had to get those in focus. Behind that in the distance was a strand of tall trees that extended to the top of the image. Had to get that in focus also, so I adjusted the focus plane about mid-way up and stopped down. I tried a similar shot like this before at f-22 and it didn't cut it. This time, I shot at f-45 and it was sharp from the track ballast, a few feet in front of me (image edge/corner), the switch-stand and railroad cars (image center), to the tops of the trees. My 10x lupe certainly presented a sharp negative at an equivalent print size of 50" x 70", so I think my 5" x 7" contact print should look pretty good!

Rick

Now, have you ever compared the sharpness, contrast, color, saturation, distortion and overall performance with a modern lens with modern coatings at the same magnification?