Stopping down and "Diffraction"

[Cletus]

So, I've always been told, or read somewhere, never to stop down past, say, errrr, about f32-45 'cause anything at, or smaller than about f45 is getting into "diffraction limits" of the lens, or some such hokum.

Now, I know there is indeed such a thing as diffraction. I know it's something you need to be aware of, especially when printing using an enlarger with a lens with an aperture that can stop down past f45, or f32, or whatever the number is.

So here's my main question(s):

1. It seems like I can get a little more rise, or shift, etc. on my 8x10 with a certain lens the more I stop down. This is noral isn't it? True for all lenses, right? The more you stop down, the larger the IC of a given lens? Or at least I can see the hole in the diaphragm through the corners of the GG better when stopped waayyy down...

2. So on a rare occasion, when I need to stop down to f45, f64, or even off the scale toward f128 for MAX coverage (or even for any other reason to stop way down), is diffraction limiting really something I need to be....cognizant of? Will it really be apparent in my contact print made from an 8x10 neg? Will it really make a visible difference in sharpness for a small (11x14) enlargement from a 4x5?

I hear about "diffraction limiting" all the time, I'm just trying to imagine how much impact it might actually, practically speaking, have on a typical large format photograph?

Anybody here stop down to f64 and beyond with impunity?

[Leigh]

Diffraction limiting is one of those threats that mothers use to get their kids to eat Brussels sprouts, as in:
"If you don't eat your sprouts you'll be diffraction limited for the rest of your life."

Yes, it is a real physical phenomenon that can be measured in the laboratory.
It should be left in the laboratory.

Process lenses, which are the highest quality optics available, typically go down to f/128 or smaller.

The image degradation attributable to DL is generally indistinguishable from defocus.
It's interesting that DL appears to increase defocus while smaller apertures increase DoF.
For a given sharp point removed some distance from the plane of focus, DL will defocus it but DoF will sharpen it.

I use whatever aperture is appropriate to the situation, as did famous photographers of the past.

Yes, image circle diameter increases as you stop down.

- Leigh

[rdenney]

Generally, insufficient depth of field is a more noticeable effect than diffraction. But subject motion blur is a bigger effect still.

No effect is always a defect, of course.

I stop down as necessary for depth of field. That may force so long a shutter time that I can't control motion blur. That's when I either try to turn motion blur to my advantage, choose a different composition, wait for a calmer day, use faster film, or spend more effort finding the optimum focus plane position using tilts and swings.

But I never worry about diffraction. If diffraction is severe enough to undermine a given enlargement, I just don't make as big a print. That has happened only a couple of times in my decades of large-format work, though generally I don't make big prints. It happens more frequently with small format.

Rick "unwilling to eliminate a fine effect by creating a coarse effect" Denney

[Leigh]

Permit me a short discourse on diffraction, if you please.
Those disinterested in theory can skip this post.

Diffraction occurs when a beam of energy hits an edge that's opaque to that energy.

Some years ago I had the opportunity to work with diffraction in a very different setting, that being a circular
beam passing over a straight edge, rather than going through a hole.

In that situation it's very easy to see that the degradation due to diffraction depends entirely on the ratio of
the energy hitting the edge to the energy passing freely through the system.

Putting this in a photographic context, it's the ratio of the area of the aperture to its circumference that's important.
That's why long FL lenses are "diffraction limited" at smaller apertures (larger f/ numbers) than short FL lenses.

This also explains why small formats are more susceptible to DL issues than large format, since they use shorter lenses.

The problem with the usual explanation of DL and the Airy disc is that it is modeled using a small hole. Thus almost all
of the energy hits the edge, resulting in a worst-case effect.

- Leigh

[N Dhananjay]

Diffraction is real enough - as you say, you can see the grain turn to mush with a grain focuser under an enlarger lens. How big an issue it is varies according to taste and intention.

The usual trade-off is DOF vs. diffraction. Diffraction results in a uniform softening of the image whereas inadequate DOF shows up as areas of very good sharpness contrasted against areas of absolutely inadequate sharpness. Different people can, and do, make different calls about the appeal of these two 'looks'.

If you are shooting in a smaller format, you probably need to worry about it since the resolution drop can easily become noticeable when you enlarge the negative. The usual number bandied about is that you require about 5-10 lpmm resolution (let's say 7.5 lpmm) at a minimum on the print. Assuming about a 10X enlargement from 35mm, that will require a resolution of about 75 lpmm on film, making the unrealistic assumption of no losses elsewhere in the system - let us make the more reasonable assumption of requiring about 100-150 lpmm - this would mean probably the best lenses, very careful technique etc and I seriously doubt that the upper end is feasible but let's play along. Using a formula such as approx. 1500/f-stop to calculate diffraction limits, you can see that by about f/11 or f/16, you no longer get sufficient resolution on film. Now you will not see this when viewing the image because the 100 lpmm is well above what the human eye's limits are, so you might judge that you have got the DOF you need and are getting away without diffraction effects, but once enlarged, you will see the softness in the image. The problem is that by now the softness can get attributed to lots of other things (camera shake etc.)

With a typical contact printing process, you see the impact on the ground glass. Also, large negatives (and especially contact printing) is forgiving of many sins. So, if it looks good on the ground glass, (let's say a resolution 15 lpmm), the print is gong to look fine to the eye. You can get that level of resolution at f/90 - f/128. Beyond this point, of course, you are essentially using a pinhole and that money you spent on the expensive glass is a waste (at least for this image).

So, what does this mean in summary? If you are contact printing, you typically do not worry about diffraction because it is highly unlikely that you will see the effects on the print, even at extreme f-stops. With smaller formats, I would say you do need to worry about this because a) you do not see the effect when viewing the image - it looks fine to the eye (because you are seeing an image of say 50 lpmm - well above the requirements for the human eye) but when enlarged, the resolution drop is easy to discern.

Also, the IC increasing with greater stopping down depends upon the lens deign. What you are referring to (in terms of seeing the aperture from the corners - does it look 'football' shaped or round?) is a function of mechanical vignetting. Unless you've used extreme movements, it should not require stopping down more than a couple of stops to get past this region. On some lens designs (such as the Dagor), the lens can throw out a big cone of light but the resolution off-axis might be very poor and unusable - in fact, this is often the reason to mechanically vignette a lens). Off-axis resolution gets better as you stop down, which means more of that cone of light becomes usable as you stop down - i.e., the image circle does not get bigger but a larger part of it becomes usable.

Cheers, DJ

[Cletus]

Okay, so in relation to lenses, depth of field, image circle and coverage -

Does a smaller aperture, or f-stop ALWAYS increase the size of the image circle projected by the lens?

Does a smaller aperture, or f-stop ALWAYS increase the depth of field of the image?

And last, regardless of shutter speed (assume a bright, sunny day with no wind at all) at what point will stopping the lens down begin to impact the overall sharpness of the image?

I'm imagining me of those "infinity focus" landscapes of a road in the open desert, diminishing toward the horizon. I happen to be looking at a print I made like this right now, and it's not very 'sharp' at all - it occurred to me that I probably used a fairly small aperture, maybe f45, and the overall image just isn't very sharp in the details. If that makes any sense....I'm sure the other variables were under control - stable support, no wnd, etc...I was thinking the sharpness issue might have been related to diffraction limitation,or whatever the correct term is.

[N Dhananjay]

Permit me a short discourse on diffraction, if you please.
Those disinterested in theory can skip this post.

Diffraction occurs when a beam of energy hits an edge that's opaque to that energy.

Some years ago I had the opportunity to work with diffraction in a very different setting, that being a circular
beam passing over a straight edge, rather than going through a hole.

In that situation it's very easy to see that the degradation due to diffraction depends entirely on the ratio of
the energy hitting the edge to the energy passing freely through the system.

Putting this in a photographic context, it's the ratio of the area of the aperture to its circumference that's important.
That's why long FL lenses are "diffraction limited" at smaller apertures (larger f/ numbers) than short FL lenses.

This also explains why small formats are more susceptible to DL issues than large format, since they use shorter lenses.

The problem with the usual explanation of DL and the Airy disc is that it is modeled using a small hole. Thus almost all
of the energy hits the edge, resulting in a worst-case effect.

- Leigh

This is true. I find it helpful to think of a beam of light as a number of waveforms balanced with each other. So, each wave has wave besides it keeping it in check and preventing it from 'spreading' (speaking loosely). However, when an obstruction is placed in the path of the beam, the waveforms are unbalanced. The central wavelets are still balanced by other wavelets on the sides but the ones at the periphery (which got cut off by the obstruction) are not balanced and held in check anymore and are now free to spread.

It is not only a function of the ratio of area to circumference (which yields a function of r/2 - pi*r^2/2*pi*r). You also need to consider the fact that the resolution results from angular functions. So, a resolution limit at the diaphragm spreads as an angular function towards the film. The longer the distance travelled, the greater the angular spread. Which is why diffraction is usually defined in photography as only dependent on f-stop and not the format size or focal length.

Cheers, DJ

[BrianShaw]

I've believe diffraction to be a real phenomonon of optical physics, but in practice I've never seen it, especially at prints 11x14 and under. I suppose if one prints bigger than that, stands too close to the print to view, or scrutinizes with a microscope it might be detectable... but in terms of practicality I doubt it is something to fear.

p.s. I always ate my sprouts as a kid, and still do!

[Cletus]

Thanks DJ, nice explanation! And thanks Leigh and Rick too, just that some of the theoretical stuff is a tad over my head. If we want to talk tech about electrical theory and principles, I would be on more solid ground! Optical, not so much..

[N Dhananjay]

Okay, so in relation to lenses, depth of field, image circle and coverage -

Does a smaller aperture, or f-stop ALWAYS increase the size of the image circle projected by the lens?

>>>>> I think it is more fair to say that smaller apertures increase the uniformity of resolution across the image area (i.e., on-axis and off-axis).

Does a smaller aperture, or f-stop ALWAYS increase the depth of field of the image?

>>>>> Yes.

And last, regardless of shutter speed (assume a bright, sunny day with no wind at all) at what point will stopping the lens down begin to impact the overall sharpness of the image? >>>>> This is a difficult question to answer without more details. If you were contact printing, it is highly unlikely that diffraction is an issue , even at f/90 or f/128. And it certainly won't be at f/45.

I'm imagining me of those "infinity focus" landscapes of a road in the open desert, diminishing toward the horizon. I happen to be looking at a print I made like this right now, and it's not very 'sharp' at all - it occurred to me that I probably used a fairly small aperture, maybe f45, and the overall image just isn't very sharp in the details. If that makes any sense....I'm sure the other variables were under control - stable support, no wnd, etc...I was thinking the sharpness issue might have been related to diffraction limitation,or whatever the correct term is.

>>>>> Again, assuming you are talking about a contact print, at f/45 I very much doubt diffraction is causing the softness. I would suspect other problems - shake, vibration, improper ground glass - film plane registration etc.

Cheers, DJ