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Why does stopping down a camera lens increase resolution?

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#1 cam1936

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Posted 04 December 2021 - 10:51 PM

This has baffled me for a while and I can't seems to figure it out.

I know that, all else being equal, you gain resolution with a larger objective diameter. This is obvious to see when going from say a 75mm APO to a 120mm APO refractor.

I also know that if you stop down a telescope, you will generally reduce aberrations, but lose resolution. For example if you put an aperature mask and turn a 150mm f6 achromat into a 75mm f12, you'll see a lot less CA, but you'll also lose resolution.

However, when I stop down a camera lens, I see a reduction in aberrations and an increase in resolution. I have a 300mm f2.8 lens, wide open the objective diameter will be 107mm. If I stop the lens down so it's now a 300mm f8 (essentially the objective diameter is now 37.5mm) I see a reduction in CA and coma, but also an increase in resolution when looking at a test chart. How is it that the 37.5mm refractor can see more resolution than a 107mm? Does it have to do with where in the optical path the aperature blades are located?
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#2 JamesMStephens

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Posted 04 December 2021 - 11:15 PM

I think you've sort of answered your own question in your second paragraph.  Camera lenses are an optical compromise to cover a much larger FOV than telescope objectives, so they suffer from aberrations to a greater degree (and the aberrations are probably usually less apparent because of the shorter focal lengths.)  Stopping the camera lens down reduces the aberrations, and when the lens is wide open the greater resolving power that should be afforded by the larger aperture is degraed.  Aberrations increase at a greater than linear rate with aperture.

 

Jim


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#3 Rustler46

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Posted 04 December 2021 - 11:30 PM

This may be why, according to my reasoning. It has to do with image scale.

 

With the short focal length of most camera lenses, the image scale is too small to display the actual resolution capability, when paired with a given pixel size on a sensor. Thus when wide open, the potential resolution would be given by the astronomer's empirical formula

  • Resolution = 4.56 arc-seconds ÷ aperture in inches

A 50 mm f/2 camera lens would have a 1 inch aperture (50 ÷ 2 = 25 mm = 1 inch). So it would have a potential resolution of 4.56 arc-seconds. At just 50 mm focal length, 4-1/2 arc seconds is far smaller than the pixel size (or film grain for that matter). So image quality is not governed by potential resolution, but wide open lens aberrations are the overriding factor. 

 

Stopping down the lens does reduce potential resolution. But the result is still far smaller than the pixel size. So reduced aberration contributes to better image quality. At least this seems to be the case with a 50 mm f/2 lens. You can calculate the resolution of your 300 mm lens at different apertures. Then see what linear size that angular resolution entails in the image plane. Then compare that with your pixel size.

 

That's how I see it. But I'm open to be corrected by others more knowledgeable than myself.


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#4 RichA

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Posted 05 December 2021 - 12:00 AM

This has baffled me for a while and I can't seems to figure it out.

I know that, all else being equal, you gain resolution with a larger objective diameter. This is obvious to see when going from say a 75mm APO to a 120mm APO refractor.

I also know that if you stop down a telescope, you will generally reduce aberrations, but lose resolution. For example if you put an aperature mask and turn a 150mm f6 achromat into a 75mm f12, you'll see a lot less CA, but you'll also lose resolution.

However, when I stop down a camera lens, I see a reduction in aberrations and an increase in resolution. I have a 300mm f2.8 lens, wide open the objective diameter will be 107mm. If I stop the lens down so it's now a 300mm f8 (essentially the objective diameter is now 37.5mm) I see a reduction in CA and coma, but also an increase in resolution when looking at a test chart. How is it that the 37.5mm refractor can see more resolution than a 107mm? Does it have to do with where in the optical path the aperature blades are located?

There are a handful of camera lenses which this does not apply to, but most are slower like Leica's 50mm Apo-Summicron which has an f2.0 focal ratio.  Fast, larger lenses are never really diffraction-limited and even if they are,

 there are other factors which at speed prevent them from reaching best MTF.  The best lenses (pro-grade Nikon/Canon telephotos, etc.,) generally will render best images down one stop (e.g., f/2.8 to f/4.0) so in comparison with most telescopes, they are still very fast.



#5 niaz

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Posted 05 December 2021 - 07:58 PM

also the big difference is you are comparing entry diameter (aperture mask) with exit diameter (aperture blades). the Aperture on a camera lens is the exit diameter and doesnt really affect the number of photons entering the lens, it just helps focus them a bit better giving sharper images (this also decreases overall brightness, but for a different reason than smaller entry-diameters, but that is a diff conversation).


Edited by niaz, 05 December 2021 - 07:59 PM.



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