Confused about diffraction?

[Rodney Polden]

...... remember that a lens at f/22 has an aperture that is on the order of 20,000 wavelengths of light ....................

Wouldn't that number depend on the focal length AND aperture of the lens, rather than just the aperture? It seems to me that it's the area of the aperture rather than the f stop, that will determine how many photons will pass through it in a given length of exposure, no? And that in any case, the numbers of photons involved may well reach into the billions/trillions/who knows?

Something that I personally cannot understand wrt diffraction, is that surely (given the extremely small size of those photon wavelengths) the actual percentage of photons that _strike_ the edge of the diaphragm blades (out of the total number that pass during the exposure time) must be quite tiny, and not significant in proportion to all the other image-forming light rays that flow through the "open" area of the aperture. Even at small apertures, relative to the size of a photon's path, that is one HUGE hole.

I know there is some obvious explanation to this, but I haven't figured it out yet. Having said which, I still don't use f45 to f280, even though they're available...... I know very well that diffraction occurs, of course.

[aduncanson]

Wouldn't that number depend on the focal length AND aperture of the lens, rather than just the aperture?

Sure does, good catch! That 20,000 estimate was for a 210mm lens at f/22.

It seems to me that it's the area of the aperture rather than the f stop, that will determine how many photons will pass through it in a given length of exposure, no? And that in any case, the numbers of photons involved may well reach into the billions/trillions/who knows?

Not sure that I believe in photons myself.

Something that I personally cannot understand wrt diffraction, is that surely (given the extremely small size of those photon wavelengths) the actual percentage of photons that _strike_ the edge of the diaphragm blades (out of the total number that pass during the exposure time) must be quite tiny, and not significant in proportion to all the other image-forming light rays that flow through the "open" area of the aperture. Even at small apertures, relative to the size of a photon's path, that is one HUGE hole.

Here we go again. Try not to think of the cause as waves (or photons) grazing the edge, but rather as the summed contributions of all of those infinitesimal wavelets on DJ's pond. To achieve a diffraction free image requires an infinitely large aperture. The airy disk arises due to the missing contributions from the wavelets not present in the infinite plane surrounding the aperture.

[pocketfulladoubles]

...and to make it more complicated, the ripple in the pond represents only one wavelength of the fundamental propagation for an unbounded half-space with a pure point source (although higher orders may travel as well, but keep it simple for just the first order). Light consists of a continuum of wavelengths that will diffract accordingly. So, the question is then, and I don't know myself, is the blurring coming from the separation of frequencies, or maybe due to total internal reflection within the glass due to large angles from the diffraction? Also, WRT to Fraunhofer diffraction - isn't this assuming that the source is sufficiently far away to estimate that the incident spherical wave is essentially planar. What about focusing on close objects, and where is the cutoff when Fresnel diffraction comes back into play?

[JoeV]

I recall reading in a Photo Techniques magazine from years ago that diffraction is an artifact of the physical size of the aperture, not necessarily of the focal ratio (f-number), thus f/16 on a short lens (like a 50mm lens for a 35mm camera system) is about a 3.125mm aperture, whereas a 3.125mm aperture on a 150mm lens (for 4"x5") would yield a focal ratio of around F/48. Hence the "Group F/64" dictum isn't a proscription for diffraction, since a lens like a 300mm (for 8"x10") at F/64 would yield an aperture size of around 4.7mm in diameter, larger still than the F/16 on the 50mm lens.

Conversely, I've thought recently about diffraction and aperture sizes on subminiature digital sensor formats; for instance with the micro-4/3'rds format the 20mm f/1.7 Panasonic Lumix lens would already be at a 3.125mm aperture at just f/6.4, so it makes me wonder how much image quality is degraded by what would have been mere moderate apertures in larger formats.

~Joe

[rdenney]

Rather than delve into the details already being discussed, I would like to go back to the original question and make a statement of principle:

Diffraction t'ain't no big thang.

I would never choose an aperture based on diffraction.

1. When I want the background to be fuzzy, I use a large aperture.

2. When I want the background to be sharp, I use a small aperture.

3. If the background isn't sharp when it should be, the enlargability of the image, even if it is acceptable in a contact print (which it might not be), will be much more limited than it would be because of diffraction.

Thus,

4. Choose the aperture that provides the required depth of field, after adjusting the tilts to establish the proper focus plane. Get this step wrong, and the image is probably unusable at any enlargement. Get it right, and at worst diffraction will limit print size slightly. If the diffraction effects are intolerable by the time acceptable depth of field has been reached, then the image desired is photographically impossible and a different visualization (or print size expectation) is needed.

Further obsession about diffraction is motivated either by scientific curiosity (which is, of course, wholly acceptable) or by those who confuse precision with accuracy.

Rick "preferring accuracy" Denney

[N Dhananjay]

I recall reading in a Photo Techniques magazine from years ago that diffraction is an artifact of the physical size of the aperture, not necessarily of the focal ratio (f-number), thus f/16 on a short lens (like a 50mm lens for a 35mm camera system) is about a 3.125mm aperture, whereas a 3.125mm aperture on a 150mm lens (for 4"x5") would yield a focal ratio of around F/48. Hence the "Group F/64" dictum isn't a proscription for diffraction, since a lens like a 300mm (for 8"x10") at F/64 would yield an aperture size of around 4.7mm in diameter, larger still than the F/16 on the 50mm lens.

Conversely, I've thought recently about diffraction and aperture sizes on subminiature digital sensor formats; for instance with the micro-4/3'rds format the 20mm f/1.7 Panasonic Lumix lens would already be at a 3.125mm aperture at just f/6.4, so it makes me wonder how much image quality is degraded by what would have been mere moderate apertures in larger formats.

~Joe

This is the way physics textbooks typically describe diffraction - as the physical size of an aperture. But diffraction patterns are angular projections and so, in photography, it depends upon both the physical size of the aperture as well as the distance between the aperture and viewing screen (or film), which in photography is the f stop. In other words, while the physical aperture of f/8 is larger (lower diffraction) than the f/8 on a 50mm lens, there is a greater distance travelled by the less diffracted light from the 300mm lens (compared to the 50mm lens) and the resultant circle of confusion is thus as bad.

In other words, larger formats need smaller f stops for the same depth of field. This means more diffraction. However, large formats are enlarged less, so the blur circles are enlarged less. Thus, in reality, the only cost for larger formats is speed.

Cheers, DJ

[Brian Ellis]

Clearly I have not been successful in convincing many people that the size of the aperture, not the length or character of its perimeter, is responsible for the limits to resolution imposed by diffraction. . . .

I haven't read all the responses but anyone who doubts that it's the size of the aperture that affects the degree of diffraction is wrong. It clearly is the size of the aperture because the size of the aperture affects the proportion of bent light to "direct" light and it's that proportion that controls the degree of diffraction visible in the print. I'm not aware of any issue or dispute about that, it's like Photography 101.

[sanking]

I haven't read all the responses but anyone who doubts that it's the size of the aperture that affects the degree of diffraction is wrong. It clearly is the size of the aperture because the size of the aperture affects the proportion of bent light to "direct" light and it's that proportion that controls the degree of diffraction visible in the print. I'm not aware of any issue or dispute about that, it's like Photography 101.

Leslie Stroebel in View Camera Technique, 5th edition 1986, gives a formula for diffraction limited resolution as R = 1800/f–N, where R is the resolution in lines/millimeter, 1800 is a constant, and f-N is the f-number.

This is an approximation and would be different for Red, Green and Blue light. Other formulas exist for specific light wavelength.

However, from the Stroebel formula it is clear that it is not the absolute size of the aperture that determines diffraction limited resolution, but the focal length of the lens and the aperture.

For example, if we apply the Stroebel formula lenses of 200mm and 400mm would both have diffraction limited resolution of 112 lines/millimeter at f/16, but at f/16 the diameter of the aperture of the 400mm lens would be 25mm, that of the 200mm lens 12.5mm.

Sandy King

[Kirk Fry]

I did an unscientific test. I made pictures in my back yard on Tmax 100 with a 19in Artar at f22, f32, f45 and f64 and looked at the results with a microscope. I could only see minor diffraction effects at f64. I figure you would need to optically print at the size of a bill board and examine it with a loupe to see it even at f64. After that I decide not to worry about. KFry

[Rodney Polden]

I did an unscientific test. I made pictures in my back yard on Tmax 100 with a 19in Artar at f22, f32, f45 and f64 and looked at the results with a microscope. I could only see minor diffraction effects at f64. I figure you would need to optically print at the size of a bill board and examine it with a loupe to see it even at f64. After that I decide not to worry about. KFry

............so maybe the f/64 Group were for real, and not some wry comment about diffraction at all!

The curious thing about this discussion is that, having read it all and thought about it some, I find myself back at exactly the same state of mind that I encountered at the end of the previous discussion regarding diffraction on this forum.

Confused about diffraction? You bet (at least in the area of practical results vs. theory).