Varying minimum apertures

[Stein K S]

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After doing photography for 35 years now, it suddenly struck me... why do different lenses have different minimum apertures... 16 or 22. I could make some guesses linked to max aperture... or even more "logical"... linked to if its a wide or tele lens... But on my 8 different M lenses I still see no clear logic? Could it be that different designs handle different small apertures differently and that Leica then choose not to go too small on certain lenses...?

 

Possibly stupid but still curious!

 

 

Thanks for all input on this one!

 

Stein

[plewislambert]

Diffraction occurs where the diameter of the stop in use is very small. A 135mm lens at f22 has an actual opening 3 times greater than a 45mm lens at f22. Loss of sharpness caused by diffraction at f22 is much less likely on longer focal length lenses than on short-focus lenses.

This is without addressing any manufacturing reasons why iris diaphragms vary as to minimum apertures.

One of my cameras is a very small Canon S95. I don't think it will stop down smaller than f8. In bright conditions the meter adjusts the ISO and shutter speed to suit f8. Such a camera has no need of f22!

Philip

[01af]

Diffraction occurs where the diameter of the stop in use is very small. A 135 mm lens at f/22 has an actual opening three times greater than a 45 mm lens at f/22.

That's right.

 

 

Loss of sharpness caused by diffraction at f/22 is much less likely on longer focal length lenses than on short-focal-length lenses.

That's wrong.

 

At the same (relative) aperture, like, say, f/22, the longer lens will have less diffraction, but, for some given degree of diffraction, will lose more sharpness. These two effects will cancel each other out. So while the diffraction is different due to different absolute apertures, the diffraction blur will be the same due to the relative apertures being the same.

 

In general, slower lenses and longer lenses tend to have a smaller smallest aperture. In a slow lens, it makes the row of aperture numbers on the aperture ring look better. And in a long lens where depth-of-field generally is narrow, the user might be willing to accept a little more diffraction blur in exchange for at least some increase of depth-of-field.

 

One might suspect that a smaller smallest aperture was an indication that the respective lens was less prone to diffraction blur so the designer allows to stop it down more. Not so! At f/16, for example, the diffraction blur will always be the same—no matter if that's the lens' smallest aperture or not (assuming always the same image format, of course).

[IkarusJohn]

Is the format relevant?

 

I recall that field cameras and other large format cameras (eg, used by Ansel Adams) had f stops down to 34 - did they suffer the same diffraction at and after f22 as any other camera regardless of format?

[John McMaster]

Is the format relevant?

 

Yes (or pixel size) as is output size - Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks

 

john

[01af]

Is the format relevant?

For diffraction—no. For diffraction blur—yes, sure.

 

 

I recall that field cameras and other large format cameras had f-stops down to 34 ...

In fact, they stop down to f/64, f/90, or even f/128.

 

 

... did they suffer the same diffraction at and after f/22 as any other camera regardless of format?

Yes, they do. But that's not the relevant question.

 

Diffraction isn't relevant; diffraction blur is. So in terms of diffraction blur—as well as in terms of depth-of-field, by the way—f/64 on an 8×10" large-format camera is about equivalent to f/8 on a 35-mm-format camera.

[01af]

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Yes (or pixel size) ...

Contrary to common belief, pixel size has nothing to do with it.

[jankap]

The wavelength of the frequencies of the light is the primary factor, because it is given by nature as a standard in nanometers.

The enlargement of the picture follows, large format is more forgiving than "full frame".

Jan

[pico]

That's right.At the same (relative) aperture, like, say, f/22, the longer lens will have less diffraction, but, for some given degree of diffraction, will lose more sharpness. These two effects will cancel each other out. So while the diffraction is different due to different absolute apertures, the diffraction blur will be the same due to the relative apertures being the same.[...]

 

True. Thank you for a most concise explanation.

 

It took me too long to realize that congruence years ago.

[Stein K S]

Thank you all

 

Impressed by the technical knowledge you keep! I myself do need to read through this a couple of times to get it all.