Fairy tales about 'diffraction'

[pakodominguez]

For 35-mm format, that's f/16 - f/22. Beyond these apertures, diffraction blur will start to cross the boundary from 'just barely visible' to 'objectionable'. Macro lenses offer somewhat smaller apertures still because in the context of extremely narrow depth-of-field at high magnifications, the objectionability of diffration blur will shift a bit.

Hi Olaf,
A little bit of practical application on all this (interesting...) discussion: I want to reproduce slides with my digital DSRL (most probably the A850 and Minolta 50mm f2.8; or the Minolta MD 50mm f3.5; or a Nikkor EL 50mm f2.8...) I already did a couple of test, mostly at f11 or f16, and I'm not convinced by the results because the lack of sharpness -specially with the Nikkor EL 50 f2.8. Do you think it is a matter of diffraction, and that I'll get a better result at f8? or it is just my (bad) technique?

[01af]

I want to reproduce slides with my digital DSLR (most probably the A850 and Minolta AF 50 mm 1:2.8; or the Minolta MD 50 mm 1:3.5; or a Nikkor EL 50 mm 1:2.8 ...) I already did a couple of tests, mostly at f/11 or f/16, and I'm not convinced by the results because the lack of sharpness—specially with the Nikkor EL 50/2.8. Do you think it is a matter of diffraction, and that I'll get a better result at f/8?

Are your slides in 35-mm format? If so then with the A850 you're workíng at 1:1 magnification. So at a nominal aperture of f/16, the effective aperture will be at f/32 (or somewhere near f/22 with the AF Macro 50 mm 1:2.8 ). That means diffraction blur that will be clearly visible and, in this application, definitely objectionable. I'd expect much better results at nominal apertures of f/4 to f/8 (i. e. f/5.6 - f/16 effectively). Of course, at these comparatively wide apertures, focus precision must be perfect; the slightest mis-focusing will result in a visible loss of sharpness.

I'd also try to copy the slides in regular and in reversed orientation. Macro lenses (and enlarger lenses, too) usually have a very flat field—but there might be some minor residual field curvature still. Slides usually aren't perfectly flat either. So to achieve good sharpness in the copy across the whole field, one orientation of the slide may work better than the other ...

[agorabasta]

Pako, are you using collimated backlight for your slides? I'm asking because backlight properties do heavily affect the results.

[01af]

agorabasta is right—the lighting will affect the character of your results ... in particular, visual sharpness, among other things. Basically, for slide copying you have the same choices as in an enlarger: diffuse or direct light. For diffuse light, use the white, non-transparent but translucent screen that comes with all slide copying devices I am aware of. For direct light, don't use it—but then, the light should be collimated (just like in enlargers or slide projectors).

Diffuse light is easier to handle; it will keep contrast down and suppress the apearance of dust particles. Direct light will emphasise acutance (apparent sharpness of edges and details) but will also make any dust particle to stand out; it will also lead to a higher contrast in the copy. If you primarily are after sharpness then you may want to try direct—and collimated—light.

To build your own collimator, all you need is a (preferably large) close-up lens and some adhesive tape. Put the close-up lens standing on its rim on your desktop and fix it with adhesive tape so it won't roll off or trip over. Place your (small to medium-sized) flash gun on the front side of the lens and place your camera with the slide copier behind the lens so the flash light will hit the slide to copy through the close-up lens. Adjust the distance between the close-up lens and the flash gun's reflector so that it's equal to the close-up lens' focal length. You can check the proper distance with your camera and an arbitrary lens set to infinity: when aiming through the close-up lens at the flash gun, the reflector should appear sharp. The distance between close-up lens and the slide to copy doesn't matter; choose it at your convenience. If the corners of the copy are getting too dark then use a close-up lens with a wider diameter or reduce the distance between flash gun and close-up lens by a few millimeters. Make sure everything is neatly lined up along one axis. You may want to shield off any flash light that is not going through the close-up lens, to avoid stray light and possible loss of contrast.

For the close-up lens to work as a collimator, any diopter strength will do. A higher strength will just lead to a brighter collimated beam of light ... so if the flash gun is too strong, use a weaker close-up lens (and re-adjust the distance to the flash gun accordingly).

[pakodominguez]

Thanks for the lighting information. I use the film/slide holder's white diffuser. And indirect light -a flash bounced or a strobo with a softbox. I stop using direct light because the high contrast and loss of detail in the dark areas, I prefer to control the contrast on Lightroom.

Regards

[agorabasta]

Pako, I believe you would be much better off using a remote flash in tele position facing a deeply defocused long tele lens set at its medium aperture. The most important requirement for the lens properties is that it must be as much 'image side telecentric' as possible. That's why a long, bulky, preferably old and manual, tele lens has the best chance to safely collimate your backlight flux.

With a diffuse backlight, the lens on the camera has a very small chance to be the primary limiting factor in your setup.

[01af]

Pako, I believe you would be much better off using a remote flash in tele position ...

The flash being in tele position won't make any difference beyond brightness. In wide-angle position, a greater portion of the flash gun's output would go astray and be wasted, that's all. Provided the flash has a zoom reflector to begin with.

... facing a deeply defocused long tele lens set at its medium aperture.

Huh!? Either you want collimation—then better focus accurately—, or you don't. And don't use any other aperture than the widest, or you risk vignetting. By the way, if you really want to use a telephoto lens (or any taking lens) as a collimator then the 'focusing' thereof won't get accomplished by the lens' focusing ring but by adjusting the lens' distance to the flash gun with the focusing ring firmly set to infinity (and the lens' rear end pointed towards the flash gun). But then, it would be a foolish idea to use a telephoto lens for this purpose anyway. Better use a close-up lens—as a collimator, that's the same thing as a telephoto lens optically but much easier to set up and to handle, and less likely to cause vignetting.

The most important requirement for the lens properties is that it must be as much 'image side telecentric' as possible. That's why a long, bulky, preferably old and manual, tele lens has the best chance to safely collimate your backlight flux.

Except that a close-up lens has an even better chance. And no, image-side telecentricity is not required.

And what would 'unsafe collimation' be like?

With a diffuse backlight, the lens on the camera has a very small chance to be the primary limiting factor in your setup.

Well ... if with direct light contrast becomes uncontrollable then diffuse lighting simply is better. After all, there's a reason why all slide copiers include such a white translucent diffusor screen. On the other hand, there also is a reason why these diffusor screens are removable. Pako, when you tried using direct light for copying before, was it with or without collimation?

And oh, by the way—in my previous post I said the collimator-to-slide distance didn't matter. I am afraid that's not entirely true. While this distance isn't critical, it still mustn't be too short or too long. A distance between approximately 1× to 2× (maybe 3×) the collimator's focal length should work fine ... provided you're willing to try direct collimated light in the first place ...

[pakodominguez]

With a diffuse backlight, the lens on the camera has a very small chance to be the primary limiting factor in your setup.

Well ... if with direct light contrast becomes uncontrollable then diffuse lighting simply is better. After all, there's a reason why all slide copiers include such a white translucent diffusor screen. On the other hand, there also is a reason why these diffusor screens are removable. Pako, when you tried using direct light for copying before, was it with or without collimation?

Hi Olaf,
No collimation. In the film era (...) I used to put the flash straight to the slide holder, that have the white-translucid diffuser. Most of the times I did reproduce slides with negative film in order to print the pictures (80's and early 90's, no easy way to print from slides in Argentina at that time, and Cybachrome was way overpriced) Then, in the 90's I used to duplicate my own slides. Same method and/or I start bouncing the flash to a white surface in front of the diffuser, then I picked the one that looks better.

With digital I did bounced the light to a white surface, or I used a Monolight with a Softbox -Bounced light works better/softer/less contrast. I never used collimation before, and I think I'll stick with my trusty method

I understand contrast increase the perception of sharpness (I did work with Nikon equipment for a while, Nikon lenses provide high contrast results, Minolta lenses are less contrasty, but have better "micro-contrast", so more detail everywhere.

I made the original question about diffraction because I was not satisfied with the results of a batch of duplication I did a little while ago (A850 + Minolta 50mm f2.8 at f11/f16) and I was wandering if working at f8 will help. Then this conversation came back and it was the right forum for asking this question

BTW, Peter Ganzel shows, on his PBase website another technique, when he synchronize his slide projector with his digital camera -apparently that works fine for him. I know some people "digitalize' Super * or 35 mm film by projecting it to a screen and doing a "screener' with a video camera, with mixed results... I don't have a slide projector or a good screen.

Regards

[agorabasta]

Telecentricity means the exit pupil at infinity, by definition. Exit pupil at infinity means the output flux is always collimated, regardless if lens is focused or not. Using mid aperture helps against the lens vignetting. Defocusing is necessary to avoid catching the image of the lamp and reflector.

All's plain, simple and logical. Quite unlike the contradicting 'advices'...

[agorabasta]

Just tried another setup with a single point light source. That source was a tiny white LED on my keychain

Put it as close as possible to the front element of Samyang 85f1.4 and got a nice output light cone diverging at 6 degrees only. Sure the lens is quite defocused with the source that close, so the focus may be adjusted only to get the smoothest view of the aperture edge. The lens itself may then stay wide open, stopping it down to f/4 may be used to mark the central area in the projected spot where the slide better be placed, then put it back wide open. Then the slide target in that setup should be placed at about 80cm from the lens front element/LED.

Works like a charm, simple as a stick.

[Greg Beetham]

Agorabasta just thinking out loud here….couldn’t the image degradation that follows stopping down to small apertures be described, among other things, as a ‘choking’ of the all important front element group, making it much smaller in effective diameter than it was with the iris wide open, resulting in a loss of angular resolution; and perhaps increasing the ratio of diffraction/diffusion.
Also a question without notice: The front element group is still transmitting the same amount of light to the interior of the lens and up to the iris regardless of what f-stop is selected. What effect does this light have on the light that does actually pass through the small aperture? Is there an interference pattern from the blocked/reflected light in front of the iris?
Greg

[agorabasta]

Greg,

Yes you actually say the same what I was trying to explain in this thread, just in slightly different wording.

My understanding is that at tight apertures the 'hole' cross-section becomes so small, that every imperfection in there leads to a build-up of 'choking'. And it works so not only for the front element itself, but also for the dust and density variations in the air on the way from the object to the front element; so that the farthest objects in the image degrade faster at aperture tightening.

And sure the aperture blades are pretty reflective, and the light dispersed in the front part creates some haze for sure. But wouldn't look for diffraction/interference from that source.

[01af]

Telecentricity means the exit pupil at infinity, by definition. Exit pupil at infinity means the output flux is always collimated, regardless if lens is focused or not.

Sure. But first—telecentric lenses usually are not found in the average hobbyist photographer's closet. And second—they're not required anyway, because nothing is keeping you from properly adjusting the distance between light source and collimator.

Using mid aperture helps against the lens vignetting.

To the contrary. In this application, it will cause vignetting. Remember—we're not casting a real image; we're collimating a light flux. You know what collimation is, don't you? I don't need to explain the difference to you, do I?

Defocusing is necessary to avoid catching the image of the lamp and reflector.

Don't worry about the image of the lamp and reflector. When properly collimated, that image is at infinite distance. With the slide copier just several inches away from the collimator, that's all the defocusing ever needed.

Works like a charm, simple as a stick.

Sure it will work that way. But that doesn't mean it couldn't work any better. If, for example, you'd move your point light source backwards by a few millimeters or centimeters, away from the front element, then you can reduce the divergence of the light cone from 6° down to 0°, i. e. perfect collimation.

And for the life of me I cannot understand why someone would want to place the light source before the front element and let the collimated light go out the rear element. Reverse the set-up, get even better collimation and a wider diameter of the collimated flux, and still simple as a stick.

Temporarily stopping down the collimator lens as an aid for aligning the set-up is a good idea. That's an advantage over using just a close-up lens indeed.

[Greg Beetham]

Yeah agorabasta that’s an effect I hadn’t ever realized before, the degrading of the boundary of dof at a progressive rate. Btw, both you and Olaf mentioned airy disks on numerous occasions in the ‘discussion’ I thought airy disks were only a theoretical performance aspect of an optical design from an imaginary point source (or a real one if performing an actual test), I didn’t think that extended objects produced airy disks, or do they?
Greg

[01af]

I didn’t think that extended objects produced Airy disks, or do they?

Of course they don't—simply because extended objects are no points. Still, Airy disks are more that just a theoretical concept. Any image in fact is composed of a virtually infinite number of partially superimposed Airy disks of very different shapes and sizes. That's why, unlike agorabasta is suggesting, the exact shape of a single Airy disk is totally irrelevant to our topic. Relevant is just one thing: Images of points can never be points again; they will always have a certain diameter which cannot be less than a certain limit. And this limit is the greater, the smaller the aperture is. That's all.