You can start seeing some softness with f64 contact prints (8x10) when you go past a 1:1 enlargement.
You can start seeing some softness with f64 contact prints (8x10) when you go past a 1:1 enlargement.
Mark, I've studied physical optics and can sling the math around. But the best practical insight has come from experience working in different formats and seeing what happens under different conditions.
I've seen diffraction effects have a visible impact on pictures under a wide range of circumstances. "Ruin" is a pretty strong word, though. For example, the performance of the 75mm lens for my Mamiya 6 gets distinctly worse at f/22, for example, and these days I'll generally stay away from anything beyond f/16 with that lens, but I have printed negatives exposed at f/22 and lived to tell about it. When I'm contact printing big negatives (e.g., 8x10) f/64 definitely loses just a hair of crispness compared with f/22, but the DOF advantages are almost always worth it. OTOH, f/90 starts to get a bit soft for my taste on the few lenses I've tested that far. In 4x5 negatives taken with a modern 135 and printed with modest enlargement, I can see some loss at f/45 with ultrasmooth films like TMX; it's a much closer call with a grittier film like HP5 Plus.
Just to see, I also did an experiment once with one of my enlarging lenses, where I printed the same negative using every aperture from wide open on down. The effect was gradual, but the loss of crispness at the smallest apertures was quite apparent.
And so on. The effects are real, but you just have to see for yourself what the practical tradeoffs are with the lenses and in the formats you use, and whether and when the effect gets large enough to be a problem for your purposes.
Lots of quantifying, but I can't find the physics of what's going on.
Mark, the math that captures the Huygens-Fresnel formalism is the physics, at least the classical physics. But one good way to develop a qualitative intuition for what's going on is to play with an interactive wave tutorial. Here's an interesting and quite powerful one:
jlearn.mit.edu/simulations/waves/waves2d.htm
It takes a few moments of fiddling to figure out how to use it, but stick with it - you can do a lot of nifty things with it.
Thanks for the input, Oren! I agree, one just has to test the limits and see what's acceptable. Contact printing 8x10 with a fairly grainy film lets me get away with more than most, which is why I haven't bumped too hard into the diffraction limits, I suppose. Looking at some samples of pronounced airy discs, I've noticed that most of them still have a strong spike of defined illumination in the middle. I wonder if this, coupled with a grainier film and *perhaps* (big perhaps) adjacency effects from stand developing help mask the diffraction.
jlearn.mit.edu/simulations/waves/waves2d.htm
That was definitely one of the coolest things I've seen on the web for quite a while! And after building a couple of different apertures with the walls, I think I can better understand (in theory) what's happening. It's just a matter of getting my tiny mind to comprehend electromagnetic radiation (light) acting as what *looks* like fluid dynamics.
Mark, for me the easiest way to think of this is to use Huygens' original geometric conception of wave propagation. All points on the wavefront are regarded as little tiny emitters of new waves acting in time with each other. With a large aperture, light spread sideways from one such tiny emitter gets cancelled by the light from another elsewhere on the aperture. Near the edges you lose this cancellation and you get interference patterns as some of the light spills sideways. With a small aperture you cannot help but be near the edges, so light spills sideways even from the little emitter in the center of the stop.
To me, the amazing thing is not that light spreads out after passing through a hole, but rather that plane waves propagate as plane waves.
Like Struan, that is conceptually the way it makes the most sense to me. For the wavefront to continue to propogate smoothly, the different parts of the wavefront must be in sync with each other. Now think of what happens when you place an obstruction in the way such that it obstructs part of the beam. The part of the wavefront that just brushes past the aperture no longer is in balance because the parts that were doing the balancing on one side got cut off by the obstruction. Since this front is now out of sync, it also affects the wavefronts next to it to a smaller extent and so on. Therefore the spread.
In terms of why smaller apertures create more diffraction, based on the above, you can see that it is the ratio of the area of the aperture to the perimeter of the aperture that is important - that is, the area passes the light beam without much problem, the perimeter creates the edge and creates teh imbalance along the perimeter. As you reduce the radius of a circle, the area (pi *r*r) decreases much more rapidly than the perimeter (2*pi*r). In other words, as you reduce tha radius of the aperture, the perimeter is starting to contribute more and more to the image making light.
Diffraction is quite real - all you need to do is put a negative in your enlarger and examine the aeriel image as you stop down - you will see the grain turn to mush. However, I do agree with the previous posts in terms of the impact it has. If contact printing 8x10, you would be hard-pressed to ruin a photograph by diffraction (if by ruin one means unacceptably softness). Also, diffraction results in a gradual softening of image detail. That does not mean you cannot make a print from the negative - just that the degree of enlargment that is possible for a required CoC reduces. For many kinds of work, DOF trumps diffraction - the somewhat lower resolution is preferable to live with (at least all parts of the picture are equally out of focus, which if you think about it is pretty much what we always have - we never have perfect resolution) compared to having some parts of the picture being very obviously out of focus.
Cheers, DJ
Here's an interesting page on diffraction:
www.cambridgeincolour.com/tutorials/diffraction-photography.htm
Diffraction is quite real - all you need to do is put a negative in your enlarger and examine the aeriel image as you stop down - you will see the grain turn to mush.
Is that really an accurate measure?
Consider - is there a difference in the outcome of the enlarger's diffraction compared to using the same focal length lens at infinity in a camera?
Some further comments, agreements, etc.
1. At f/64, the diffraction limit should be about 1500/64 ~ 23 lp/mm. Few large format lenses have markings below f/64, and a contact print of an 8 x 10 would invovle no enlargement. I don't know that anyone claims to be able to see 23 lp/mm in near vision of a print. So you would seem pretty safe in any practical situation to ignore diffraction if you are contact printing 8 x 10 negatives.
2. The effect of diffraction is usually expressed in terms of the relative aperture or f-number, not the absolute size of the aperture. When you do that, there is no dependence on focal length. As noted previously, it depends just on the relative aperture and wavelength.
3. The intutitive explanation of diffraction given by Struan is as good as any. You might also note that the diffraction pattern from a single linear slit is a bright central band surrounded by less fainter bands in both directions. So it makes physical sense, to me at least, that the corresponding pattern for a circular aperture is a central spot surrounded by concentric rings. In the end, of course, you have to look at the mathematical theory in physical optics. This is even more true if you try to understand how diffraction and defocus combine to affect the image, which is the issue in photography.
Posting Permissions
Reply With Quote
Bookmarks