MTF Mapper target help needed

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bbobby
Posts: 67
Joined: Sat Jan 15, 2022 12:40 pm
Location: Indianapolis, IN

Re: MTF Mapper target help needed

Post by bbobby »

rjlittlefield wrote:
Wed Mar 29, 2023 3:24 pm
...That original thing will have one constant brightness value on one side of the edge, and a different constant brightness value on the other side of the edge, with a sudden transition between those two brightnesses.
... Note that the blur width is a constant value, despite that the first edge has high contrast and the second edge has low contrast.

Very roughly speaking, what MTF Mapper does is to look around and find the values of locally constant dark and light on opposite sides of edges, and then it measures the width of the blurred edge that transitions between those values. Sharp lenses will give narrow blurs, soft lenses will give wide blurs. The MTF50 value that is reported by MTF Mapper is an indication of the width of the blur.

So, the answer to your question is that all the information comes from the image. MTF50 is a measure of how quickly the pixel values change from A to B, not how much different the values of A and B are.

Make sense?

--Rik
I follow you here and yes, it does make sense... less width blur between light and dark areas - sharper lens, simple!
After about an hour mediation on this "width blur" I somehow am not convinced that width blur between the dark and light areas is the whole story...
In your example blur width is the same, but if I shoot backlit target and one lens is capable of image which is close to black and white (high contrast) as it should be with proper exposure without clipping (top figure in your example) and the other record some gray much (bottom one, low contrast) then I do not see how these lenses are the same.
If only the blur width matter there will be no need for black and white target or proper exposure, or evenly lit background...
I will think more about that and run some tests...

rjlittlefield
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Re: MTF Mapper target help needed

Post by rjlittlefield »

bbobby wrote:
Wed Mar 29, 2023 6:40 pm
After about an hour mediation on this "width blur" I somehow am not convinced that width blur between the dark and light areas is the whole story...
The width of the blur is certainly not the whole story in many ways.

But the blur width, or more precisely the whole MTF curve that the structure of the blur implies, is the only thing that is measured by MTF Mapper.

If, say, a lens is dirty or poorly coated, so that the image has a lot of veiling glare, then MTF Mapper will not properly diagnose that situation and may compute numbers that do not represent the lens's quality for actually taking photographs.
I will think more about that and run some tests...
I will be interested to see what tests you run, and what results you get.

If you plan to compute test images, then I suggest to proceed in the same way that I did, using Photoshop but with a workflow that is far from obvious. The key ingredient was to convert MTF Mapper's test image to 16-bit RGB with a custom profile using gamma=1, so as to get a linear relationship between illumination units and pixel values. Working in that mode, I then blurred the image and used a partially masked Levels adjustment layer to change the local contrasts, before saving the image as 16-bit TIFF. It was the resulting 16-bit TIFF file, with linear profile, that I fed to MTF Mapper to get the results shown at viewtopic.php?p=288324#p288324 .

For physical experiments, using a real target and a real camera, it is best to shoot in raw and give those raw files to MTF Mapper to analyze. All this will give the best shot at retaining a linear profile all through recording and processing.

--Rik

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