Luminous Landscape's example of diffraction-limited lens performance

[Mark Sawyer]

Looking at the "Luminous Landscape" site recently, (I've been refered to it several times in the past as a "great resource"...), I found the following tutorial on lens diffraction, using a 180mm APO Sironar lens:

http://www.luminous-landscape.com/tu...fraction.shtml

The tutorial notes: "It doesn't take an experienced eye to see how f/5.6 has a small but visible edge. f/8 is very close, but f/11, though still usable, is starting to deteriorate. After that resolution declines rapidly. I regard f/32 and f/45 as unusable.

Optical theory says that a perfect lens will be perfect wide open, and that diffraction will start to take its toll as the lens is stopped down."

There is a series of images made at varying f/stops, with the image degrading with each stop smaller. I've worked at very small stops before, and never seen this sort of degradation, especially at the mid-stops. And I'd heard most lenses perform opyimally around f/16.

Btw, the depth of field in this image series seems unchanged from f/5.6 through f/45.

I do have some understanding of diffraction limits, largely through discussions here... Still...

Is it just me, or is this "tutorial" a bunch of bs?

[Oren Grad]

Mark, remember that the linked article is referring to the 180 Apo-Sironar-Digital lens, not the more familiar Apo-Sironar-N or -S. The Rodenstock digital lenses cover a smaller image field but in tradeoff are designed to achieve optimal performance at f/8-f/11, and in the center of the field even at f/5.6. So Michael Reichmann's reported results aren't surprising at all.

[Mark Sawyer]

I'm unfamiliar with the HR lenses, so I'll take a more knowledgeable photographer's word, especially since it makes sense! Still, the unchanging depth of field bugs me. Perhaps there's much less depth than there appears...

[Oren Grad]

Actually, the "HR" designation in the article must be incorrect - there is no 180 in the Apo-Sironar-Digital HR series, though there is in the regular Apo-Sironar-Digital series.

I don't know about the DOF. Bear in mind, though, that 180 is quite a long lens for the 36x48 sensor that MR used for these, so the small crop he's showing at 100% could reflect a subject that's at a pretty healthy distance from the camera.

[QT Luong]

The reason why you didn't see this degradation is that you didn't expose the same frame at different apertures and then compare at high magnification the in-focus area.

I've done a lot of MTF measurements on 35mm lenses (it's digi, so I don't have to burn film), and I can confirm that the drop in performance past f8 is easily measurable at f11 with most good lenses, and very clear at f16 at f22.

[shileshjani]

No BS here. This real and dictated by physics of diffraction. Just remember one thing though: As an example, for the reduced sized sensor on a Nikon D2X (12 MP), at 100% screen magnification, you are seeing the equivalent of more than 60X magnification. The format used by MR will have a lower magnification factor than 60X, but a quick guesstimate suggests it will still be more than 20X magnification. If you were to subject ANY lens to this kind of 20X magnification , you will see the effect. That is why us LF users can gloat about image detail, a 16x20 in print is ONLY a 4X magnification for 4x5 film.

[Colin Robertson]

"These 100% crops of the full frame have not been processed in any way".
Okay. Maybe I'm overtired, but what is this 'magnification' we're allowing for?? This says, clearly, 'full frame'. What am I missing??

[shileshjani]

"These 100% crops of the full frame"

They are very small sections of the full frame image - in this case the physical size of sensor (full frame) is 49.1 x 36.8 mm with 7216 x 5412 pixels. At 72 dpi screen resolution (100%), the full frame would be be a 2545 x 1909 mm (or 100 x 75 inch) image. 2545 divided by 49.1 is approximately 53X. Someone please check my math.

So in fact, my guesstimate of 20X was way off. Even a theoretically PERFECT lens will show the effect when scaled up more than 50X.

MR is not full of BS, but neither is it rocket science. It is akin to saying 2+2 = 4-0. Trivial really.

[shileshjani]

Another note echoing QT: The fact you see visible degradation at each stopping down of the lens is actually the hallmark of a GOOD lens. It means that the lens does not have other major issues which degrade it's performance at large apertures. In other words, a lens that has a "sweet-spot" aperture stopped down from wide open is indicative of "less than perfect" lens. In real life, real lenses, compromises......compromises.

[David Luttmann]

This is getting all too typical on the internet. Someone stating what another has said as "BS"....yet they have not used the lens or even tested as MR had. Maybe in the future, rather than announcing to the world what you think is a "BS" test, you should actually try it yourself. That would have given you an informed opinion and would have spared us the bandwidth!