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03-27-2012, 02:31 PM   #1
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Diffraction limiting?

I have a 645n, which I enjoy very much. I plan to visit the Atlanta Botanical Gardens with it and my 120mm macro lens. I am aware that diffraction will limit the resolution of the image, and closing down beyond a certain f stop will degrade sharpness.

My understanding is that generally f8 is a good rule of thumb for the sharpest image on a good lens, which the 120 macro is! Now the interesting part: As the lens is extended the effective f stop decreases, when the 120 is at its 1 to 1 closest focus the effective speed is considerably reduced. Is the diffraction limited f stop based on the physical size of the aperture, or the effective aperture?

I also have extension tubes for the 645. Using these with the 120 will give an even smaller effective aperture. I've looked on google for diffraction limits tables but haven't found any other than for digital sensors, these writers seem to imply that the problem is a bit different for digital than for film.

Any thoughts out there on this?

Thanks!

03-27-2012, 02:48 PM   #2
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Effective aperture.
03-27-2012, 03:01 PM   #3
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IMO, diffraction limits are mostly theoretical as to how they apply to real world photos. It's something that gets trotted out in discussions as a limiting factor, something commonly blamed for other, more obvious issues (like missing focus slightly or having too slow a shutter speed), but I've yet to take a shot that I could point to and say "This was softened from diffraction rather than any other factor". In short, I wouldn't even worry about it. I've got some pretty wild results at f./29-32, often shoot macro around f./20, and often have an effective aperture smaller than that due to extension. It shouldn't be a big concern - get the shot by any means necessary.
03-27-2012, 03:48 PM   #4
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On medium format you have at least one extra f-stop of range before the effect of diffraction sets in because the opening at a given f-stop is larger. Medium format lenses therefore typically have smaller min. apertures (f/32 rather than f/22 etc.). I usually just avoid the smallest aperture of a given lens.

03-27-2012, 04:00 PM   #5
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I had sharp results down to F16 not too bad at f22 but I genraly tried to avoid going above F16. This was using Bronica ETRS 645 and Bronica RF 645.
There was to me a notable differance between F16 and 22 with 22 being a bit too soft for my liking.
03-27-2012, 05:05 PM   #6
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You should not concern about diffraction in most real-life situations. Just don't stop down to the lowest f number.

The biggest variables that will affect the image quality will be accurate focus and motion blur. Unless you'll be using a tripod, it is very challenging to achieve and hold perfect focus handheld. Even if you use a very stable tripod, nearby traffic from Piedmont Road and other visitors walking around will introduce enough vibrations to produce some blur that will be equal or more than any diffraction artifacts. And outdoors there is always wind that will move your subjects.

Your best approach will be to use a tripod and a powerful flash set to 1/2 or 1/4 power to get a very high speed at F11 or F16 for good DOF.

Enjoy your visit, the tulips are spectacular.
03-27-2012, 07:14 PM   #7
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Many thanks to all. My hunch was that since the 120 and other lenses for the 645n will stop down further than those for my LX or K10d that Pentax in its wisdom felt that diffraction wasn't too much worry at "normal" apertures.

Demp10, the tulips are indeed spectacular, wonderful squirrel candy. I had a wonderful visit last weekend with the granddaughters.

In the Conservatory I probably won't have much worry from Piedmont Ave unless trying to shoot at slow speeds, although many seconds probably would work okay. But then other visitors could be a problem.I will of course take a tripod, and my AF400T flash. Wind? Whenever one is ready for the perfect shot the !@#%$W#$%^ wind kicks up. As Kurt Vonnegut would say, "And so it goes."
03-27-2012, 08:43 PM   #8
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In the macro range it is easy to see the effect on resolution--an effective f/150 is easy to get--and will reduce the resolution ten fold. On normal picture the smallest aperture is in the range f/16 to 22. And thus the effect is maybe twofold--and image dergradation due to motion, lack of focus etc. are typically as important or more so--so in real world non-macro photography it is unlikely to be important--unless you are critically careful of every part of the process. Again in macro (at m about 1/2 and larger) it is important (in "real world macro").

03-28-2012, 01:46 PM   #9
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Diffraction at smaller apertures causes pixels to blur on small or large format cameras. A larger format camera comes off better mainly because you don't have to blow up up the image as far to get the same resolution. As pointed out, there are other reasons to use a larger format.
03-28-2012, 01:47 PM   #10
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A couple of points. Where a lens becomes diffraction limited depends on how well it is corrected. The better corrected, the more wide open its diffraction limited aperture will be. Its diffraction limited aperture is where aberrations are no longer the cause of unsharpness but diffraction is. Great lenses become diffraction limited at f/4 but are by no means soft because the diffraction effect at this aperture is so small.

One of the formulas in the film days was 1380/f # to get a general idea of the maximum LP/mm due to diffraction. I do lots of macro work at f/45 with a 6x7 and am not really concerned about the slight loss due to diffraction.
03-28-2012, 02:12 PM   #11
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QuoteOriginally posted by desertscape Quote
A couple of points. Where a lens becomes diffraction limited depends on how well it is corrected. The better corrected, the more wide open its diffraction limited aperture will be. Its diffraction limited aperture is where aberrations are no longer the cause of unsharpness but diffraction is. Great lenses become diffraction limited at f/4 but are by no means soft because the diffraction effect at this aperture is so small.

One of the formulas in the film days was 1380/f # to get a general idea of the maximum LP/mm due to diffraction. I do lots of macro work at f/45 with a 6x7 and am not really concerned about the slight loss due to diffraction.
Very reassuring. Thanks to all for the quick responses.
03-29-2012, 02:53 PM   #12
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QuoteQuote:
One of the formulas in the film days was 1380/f # to get a general idea of the maximum LP/mm due to diffraction. I do lots of macro work at f/45 with a 6x7 and am not really concerned about the slight loss due to diffraction.
Your formula is f-stop vs line-pairs, which doesn't say anything about lens or film size. In other words, diffraction is not about lens size or camera format, only about aperture and pixel size.

You win big on a 6x7 camera because you have film that is more than 4.5x larger than 35mm! You scan it at 3000 dpi (56 mpixels equivalent) and print natively at 22x28 inches at 300 dpi. A 3000 dpi scan is about the same as 8.5 micron dots, which is like the Nikon D700, which has a 12 mpix, full frame sensor. Drop to a high-resolution crop-frame sensor, like the K5 and D800. These both have 4.8 micron pixels, and produce an 22x31 inch print at 150 dpi. No wonder diffraction doesn't show up in your 6x7 prints!

Digicams have very small pixels (under 2 microns); they show pixel-level loss of contrast due to diffraction by or above f/4. Pentax K5 and Nikon D800 both have 5.8 micron pixels, which may start to show diffraction by f/5.6.

The old rule of thumb for 35mm was f/8 and be there, which gave you a great, real-world balance between depth of field and diffraction. Pixel-peeping on today's small pixel cameras lets you see where you want to set your rule of thumb. Do a slideshow of a detailed image shot across all your apertures to see how depth of field and diffraction trade off with each other (Or subscribe to Diglloyd).

Pixel-level camera shake and mis-focus show up on these new, high-res cameras, also!

Last edited by MetaD; 03-29-2012 at 03:05 PM.
03-29-2012, 03:03 PM   #13
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Incidently, diffraction is an optical effect that causes details in the lens image to smear across several pixels. This first shows up as a loss of contrast, rather than blurry pixels.
03-29-2012, 03:39 PM   #14
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The diffraction limit is based on format size. That is it. Nothing to do with pixel pitch, resolving power, or bananas.
03-29-2012, 03:52 PM   #15
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I would say difraction is a lens thing and has nothing to do with format size ? F22 on 645 is very poor indeed !
Understanding Lens Diffraction
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