Defocus sharpness falloff of a few GF lenses

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JimKasson
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Defocus sharpness falloff of a few GF lenses
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I've been beavering away at what will eventually be a test of the on-axis performance of all the GF lenses. But this afternoon, I took some time off from the drudgery of capturing and crunching data -- and recapturing and recrunching it when I make a mistake or learn something important -- to present some of the data I've already gathered in a way that is new for me.

I'm going to show you some plots of MTF50 in cycles per picture height versus focus error for several lenses at several f-stops. The particular MTF50 that I've chosen was invented by Jack Hogan, and it is the MTF50 of a white-balanced raw image. Because of that, longitudinal chromatic aberration (LoCA) can significantly decrease WB MTF50. Here's one image to show you all how that works.

The black curve is the white-balanced raw MTF50 curve. Note that, because of the ample LoCA of the 120/4 GF wide open, that at its peak it is lower than any of the raw channels.

Here's the first of the multi-lens plots:

It shows the WB MTF50 for the 45, 63, and 80 GF lenses at f.2,8. you can see that the 80 is the sharpest of the bunch, and that it remains sharper even as it is defocused, though the differences aren't as important when the image is defocused a lot.

Note the odd shape of the 63. In the fullness of time, we'll go back and see why that happens.

Now I'll add in a bunch of lenses that can't do f/2.8:

I wouldn't pay much attention to the far right side of this plot.

In general, as you stop down, the lenses get closer together in terms of performance, but you'd have to go further than f/11 to make them all essentially the same.

Someday, I'll do some work to show how these plots differ from those for ideal lenses.

All of the data for these curves was captured at the same magnification.

You can construct a measure of depth of field by looking at the width of the curves at some arbitrary MTF50, say 1500 cy/ph. Then you could project the image-field displacement shown here in micrometers into a scene with the subject at a known magnification and see the object field depths that result. By comparing that to calculated values for ideal lenses, you could get an idea of how the depth of field of the GF lenses various among the lenses in the lineup and how it compares to the values computed using geometric optics.

But that's a discussion for another time.

Another discussion for another time: is DOF best expressed as the distance over which sharpness is above some absolute value, or the distance over which sharpness is above some value dependent on the sharpness of the in-focus part of the image?

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