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I'm trying to measure sharpness through a script in order to measure sharpness across different devices.

My methodology:

  • I have a pattern with pure white and black stripes like this: enter image description here
  • I use HD printing, matte, no reflection or texture
  • I take a picture in exactly the same conditions

When I take the picture with a device, I then analyse the picture pixel by pixel. Of course, the picture will never be perfect but as testing conditions will always be the same, I should be able to compare sharpness.

So what I measure:

  • the amount of pixels in pure white (usually close to 0)
  • the amount of pixels in pure black (usually close to 0)
  • the amount of pixels in grey
  • the amount of pixels for everything else For each of those measurements, I have the amount of unique colors (in LAB format) as well as the sum of all pixels for each type of color.

From what I see with my eyes and what the pixels are saying, I see some common trends but I also see different directions I could take.

Some discoveries:

  • the number of distinct greys seems to give a good indication about sharpness (less distinct greys = more sharpness)
  • the difference of light greys (when L from lab color is >50) and dark greys (when L from lab color is <50) also seems to give a hint about sharpness/contrast as when the difference between both is big, sharpness is better

Do you have ideas of the criterias I should use to measure sharpness?

Thanks

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    \$\begingroup\$ Do you measure both tangentially and sagittally? Most lenses will differ slightly in each direction. Some lenses differ significantly between the two, especially near the edges and corners of the frame. \$\endgroup\$
    – Michael C
    Commented Jun 6, 2021 at 13:31
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    \$\begingroup\$ If you really want to do it right \$\endgroup\$
    – Michael C
    Commented Jun 6, 2021 at 14:02
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    \$\begingroup\$ Are you measuring lenses in the abstract without a camera? If not, the reality is that the camera will also affect your measurements to some degree. If you are using an auto focus camera and lens combination, you can save yourself a lot of wheel re-inventing by using existing products that already do this. One relatively inexpensive product I've used is Reikan FoCal focal-iq.com . If you're still determined to do it yourself, take a look at some of their analysis results like focal-iq.com/lenses/… \$\endgroup\$
    – user10216038
    Commented Jun 6, 2021 at 15:57
  • \$\begingroup\$ @MichaelC I have horizontal & vertical bars but as I'm looking at pixels values programmatically I take the easy route by measuring any deviation from the original white and black stripes \$\endgroup\$
    – Laurent
    Commented Jun 6, 2021 at 18:43
  • \$\begingroup\$ @user10216038 Thanks for the suggestion, I should have pointed out that I'm testing smartphone cameras which means that I can't run any software like for dslr? I need somehow to reverse engineer what the camera is measuring, compare with real life observation until the scripts produces something ok. \$\endgroup\$
    – Laurent
    Commented Jun 6, 2021 at 18:46

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