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You are here: Home / The Last Word / Diffraction, aberrations, & fill-factor in the object field

Diffraction, aberrations, & fill-factor in the object field

June 16, 2016 By JimK Leave a Comment

This is a continuation of a report on new ways to look at depth of field. The series starts here:

A new way to look at depth of field

Yesterday, we looked at object field MTF50 behavior with the lens focused at infinity, and found that with our simulated top drawer 55mm lens, a simulated Sony a7RII, the middle-distance curves were not flat, as object-field theory says they should be. The gross reason is not a mystery: lens aberrations, diffraction,  and fill-factor considerations all conspire to provide other sources of blur than just geometrical optics.

But which one is the most important?

I ran a series of curves at f/4 at pixel pitches varying from 3 micrometers (um) to 8.5 um, with each pitch being 1.414 times the previous one.

First in the image plane:

3-9p 55mm infinity focus

And then in the object field:

 

3-9p 55mm infinity focus obj

The blue line is for the 3um pitch. You can see that decreasing the pitch makes a big difference.

Zooming in:

3-9p 55mm infinity focus mag obj

OK, now let’s back out the lens aberrations:

3-9p 55mm infinity focus no aberr mag obj1

That helps. The 3 um line is now pretty flat until 40 or 50 meters.

Now I’m going to take out diffusion. You can’t do this in real life, but you can with a sim.

3-9p 55mm infinity focus no diff no aberr mag obj1

Diffraction doesn’t make all that big a difference; that’s because we’re at f/4, and because we took it out last. Diffraction is a slightly larger effect than the lens aberration measure that we’re using.

{The material below was added later.]

I’ve been asked to show what the above vurves would look like at f/8, Here goes, first in the image plane:

3-9p 55mm infinity focus f8

And now in the object plane with sources of blur successively removed:

3-9p 55mm infinity focus obj f8

3-9p 55mm infinity focus no aberr mag obj f8

3-9p 55mm infinity focus no diff no aberr mag obj f8

 

← Object field DOF methods and MTF50 A more accurate defocusing algorithm →

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