Diffraction apertures and pixel pitch sizes

[ampguy]

Many folks stop down as much as possible to sharp photos. However, in several cases, due to diffraction, you may be getting less sharp, lower resolution, and higher noise photos as you stop down.

Here is what is going on:

as you stop down, the light rays are increasingly hitting the edges of the aperture blades instead of going through the center directly to the media.

Depending on the wavelength of the color, the photons of light hitting the aperture sides tend to diffract over multiple pixel pitches or on film, onto multiple emulsion elements.

The result of the diffracted light projecting over multiple pixel pitches results in loss of detail, and a smoothing or blurring effect, similar to the effects of high iso noise.

There are online diffraction calculators, and overviews is here:

http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm

With the Leica M8, I find that going beyond 5.6 with most lenses can start to show diffraction, which makes the fast 1/8000 shutter very useful.

Independent of diffraction, the best optical resolution for most of my lenses also happens to be between 2.8 and 5.6, which works out well.

 

[antiquark]

Interesting to play with the numbers. If you plug in f/8 for 35mm film, the Airy disk diameter is 10.7 um. If you equate the disk diameter to pixels, that works out to a sensor size of 7.5 megapixels.

If you stop down to f/16, the Airy disk is 21.5 um, which works out to a sensor of 2 megapixels! And that's across the whole image, even the in-focus part.

Numbers like that are part of the reason why manufacturers aren't keen on releasing cameras with more than, say, 12 mp. The extra pixels would go to waste in most circumstances.

 

[Mike Meal]

I was thinking about this a while ago but just how much difference does diffraction make?
I shoot and develop my own B+W neg (120) these days I scan rather than make enlargements in a darkroom and the largest print I would ever wish to make would be 12x12" so is it really worth worrying about considering the small output size?

 

[semilog]

Interesting to play with the numbers. If you plug in f/8 for 35mm film, the Airy disk diameter is 10.7 um. If you equate the disk diameter to pixels, that works out to a sensor size of 7.5 megapixels....

Numbers like that are part of the reason why manufacturers aren't keen on releasing cameras with more than, say, 12 mp. The extra pixels would go to waste in most circumstances.

Yup. And if the lens is really superlative (that is, sufficiently well-corrected at wide apertures), you can see diffraction effects kicking in amazingly early -- in the linked example, there is measurable (though likely not visible) degradation at f/4 !

 

[ampguy]

some p&s's are diffraction limited

some p&s's are diffraction limited

even before their widest aperture - Canon G7 is limited at f2, but min f stop on the lens is 2.8.

At f22, you will get images with high-end digitals, it's just that they're not 10-20mp effective, they're more like 2mp.

Read the comments at the end of this article:

http://www.luminous-landscape.com/essays/Equivalent-Lenses.shtml

but better yet do your own tests. I'm happy if my lens go to F4, but on film I need that extra bit to avoid ND filters.

Interesting to play with the numbers. If you plug in f/8 for 35mm film, the Airy disk diameter is 10.7 um. If you equate the disk diameter to pixels, that works out to a sensor size of 7.5 megapixels.

If you stop down to f/16, the Airy disk is 21.5 um, which works out to a sensor of 2 megapixels! And that's across the whole image, even the in-focus part.

Numbers like that are part of the reason why manufacturers aren't keen on releasing cameras with more than, say, 12 mp. The extra pixels would go to waste in most circumstances.