Comparing various measures of lens sharpness

[JimKasson]

Thanks Jim, I think that one of the things that was confusing me most was that some review sites don’t include any units when they publish MTF graphs.

Makes them not very useful, huh? I've noticed that some manufacturers don't provide all the units when they publish MTF curves, too.

Here's a more or less typical Imatest plot.

Hi Jim

Without wishing to colour the answer from my likely lack of knowledge what if any are the potential issues when reading these charts?

You could start with the Burns papers, and also look at the explanatory material on the Imatest website.

Off the top of my head, you need to worry about the sharpness of the target, the white and black levels of the test images, the methods of converting to CIE color spaces, if that is done. I'm with Jack Hogan on this: do the analysis on raw color planes is the high road. That's not what's posted above, though, because Imatest gets the image size wrong when you do that.

They do appear to be very much used in the photographic world rather than other industries that use similar systems. I don't know why but would appreciate your knowledge.

--

the last word the last word - Photography meets digital computer technology. Photography wins -- most of the time.

Photography meets digital computer technology. Photography wins -- most of the time.

blog.kasson.com

 

[Ephemeris]

Thanks Jim, I think that one of the things that was confusing me most was that some review sites don’t include any units when they publish MTF graphs.

Makes them not very useful, huh? I've noticed that some manufacturers don't provide all the units when they publish MTF curves, too.

Here's a more or less typical Imatest plot.

Hi Jim

Without wishing to colour the answer from my likely lack of knowledge what if any are the potential issues when reading these charts?

You could start with the Burns papers, and also look at the explanatory material on the Imatest website.

Off the top of my head, you need to worry about the sharpness of the target, the white and black levels of the test images, the methods of converting to CIE color spaces, if that is done. I'm with Jack Hogan on this: do the analysis on raw color planes is the high road. That's not what's posted above, though, because Imatest gets the image size wrong when you do that.

Thank you Jim.

From my world the lack of a solid test plan is a little difficult for my mind. I will do my best to take your direction.

When you say the image size is wrong could you perhaps point me in the right direction as I don't think I understand.

Thanks again.

They do appear to be very much used in the photographic world rather than other industries that use similar systems. I don't know why but would appreciate your knowledge.

--
https://blog.kasson.com

 

[JimKasson]

When you say the image size is wrong could you perhaps point me in the right direction as I don't think I understand.

With a Bayer CFA sensor, if you tell Imatest to look at one of the raw planes, it gets an image that is half as tall and half as wide (in pixels) as the original image. When Imatest computes MTF in cy/ph, it uses the picture height of the plane that you specified instead of that of the original image, so the numbers are wonky and must be manually corrected.

 

[Ephemeris]

When you say the image size is wrong could you perhaps point me in the right direction as I don't think I understand.

With a Bayer CFA sensor, if you tell Imatest to look at one of the raw planes, it gets an image that is half as tall and half as wide (in pixels) as the original image. When Imatest computes MTF in cy/ph, it uses the picture height of the plane that you specified instead of that of the original image, so the numbers are wonky and must be manually corrected.