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Author Topic: Lens Diffraction - When is it an issue with Medium Format Lenses?  (Read 11594 times)
BartvanderWolf
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« Reply #80 on: December 09, 2011, 02:46:47 PM »
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I repeated my experiment at f/22, but omitted capture sharpening. The resolution calculated from your target was 57 lp/mm and the MTF 10 calculated by Imatest was 59 lp/mm, confirming the approx. 10% figure you quoted. It is interesting to note that the MTF10 at f/22 is at Nyquist, which would imply that deconvolution sharpening could restore some of the lost detail imposed by shooting at this small aperture.

Hi Bill,

That's correct. Unfortunately we'll not be able to recover all detail all the way up to Nyquist in your example because the actual image contrast gets too low (in case of an adequate Low-pass filter). Image contrast multiplied by MTF response approaches zero. This is also where a low glare lens helps to maintain contrast in the microdetail. I expect the image contrast of a MF system without low-pass filter to reach Nyquist without much attenuation other than that from an area sample (and produce aliasing).

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I am somewhat confused by your statement that MTF at Rayleigh is around 20%, since I had always thought that it was aroung 10% as stated by Roger Clark. However, on checking in Wikipedia, they do mention a figure of 20%. How does one resolve these conflicting values?

Frankly, I traditionally assumed Roger's 9% number to be correct, but when checking Wikipedia found a higher contrast being quoted. To verify, I made a simulation in ImageJ of two (f/16, 564 nm) diffraction patterns I already had, offset by the radius to the first zero, and I find something like a bit over 26% contrast (depending on sensel size (!) and alignment with the sensel grid). This simulation is without the effect of a low-pas filter.

Cheers,
Bart

P.S.  The sensel's aperture size (or microlens) will quickly reduce the above oversampled (1 micron) rendering of the neighboring diffraction patterns (Rayleigh criterion) into a complete blur with zero contrast. I'm preparing a demonstration of it, but it takes the computer a while to calculate the base images.
« Last Edit: December 10, 2011, 07:55:37 AM by BartvanderWolf » Logged
Alan Klein
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« Reply #81 on: December 10, 2011, 01:32:51 PM »
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When I shot 67 on film I normally used the f/8 markers for depth of field when shooting at f/16.

Eric:  Basically you're doing the same thing I described I do except you're using a two stop margin rather than my one stop.  I checked that link you provided on Hyper-focal.  It appears a method for calculating for digital cameras and lenses rather than for anything for my film RB67.  But thanks for the link.  It'll be useful with my DSLR (as soon as I get one!) Wink
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ErikKaffehr
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« Reply #82 on: December 10, 2011, 03:03:25 PM »
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Alain,

A few observations:

1) When I started shooting 67 I discovered that I had issues with sharpness, and realized that those were caused by the "optimistic" DoF scales. I didn't buy MF to get same sharpness as from 135, but to have better images.

2) I sort of revisited the issue of stopping down. The philosophy I have right now is essentially to focus on what is important and stop down as much as needed. But I try to avoid going to f/22. I found out that of my around 58000 images about 300 are shot at f/22, many of those are tests. But after reconsidering my images I'd say that stopping down to f/22 is more OK than I expected. It's still  a waste of resolution, but resolution may less important than what I (we) may believe.

3) Sharpening matters a lot. Cannot replace the real stuff but matters a lot.

I hope you are going to enjoy digital once you have it. For many of us digital is a revelation but for some it's just a steep learning curve.

Best regards
Erik

Eric:  Basically you're doing the same thing I described I do except you're using a two stop margin rather than my one stop.  I checked that link you provided on Hyper-focal.  It appears a method for calculating for digital cameras and lenses rather than for anything for my film RB67.  But thanks for the link.  It'll be useful with my DSLR (as soon as I get one!) Wink
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