What is "detail"?

Started 3 months ago | Discussions thread
mike earussi Veteran Member • Posts: 9,127
Re: What is "detail"?

xpatUSA wrote:

mike earussi wrote:

xpatUSA wrote:

In this and other fora, we often read that item A captures better detail than item B. But detail is rarely qualified or even measured (except in my posts ).

It turns out that the subject has been explained by none other than Don Williams of Kodak in a paper published here .

A precis transcript.

two items are required for defining the MTF, (1) a measure of spatial detail, called frequency, and (2) a fundamental measure for determining how that detail is preserved, called modulation transfer [contrast ratio].

1) Spatial detail can be measured by the spatial frequency content of a given feature. ... The higher the frequency, the greater the detail, the greater the number of cycles per unit distance, and the more closely spaced the lines become.

2) ... how well the input modulation [contrast] is preserved after being imaged or in some way acted upon ...

Note that "greater" detail is not necessarily better. Because there are two, not one, parameters involved, it is necessary to consider both MTF and spatial frequency when judging the goodness of detail.

It is no coincidence that one measure of image quality is the so-called MTF50 which fixes one of the two parameters at a 50% contrast ratio, a.k.a. MTF. It tells us, somewhat arbitrarily, that a contrast in the image that is half of that of the scene is "good enough". Therefore, we could say that if the spatial frequency at which MTF50 occurs is higher than that of item B, then item A is "better at capturing detail".

However, fixing one parameter and considering only one value of spatial frequency in the continuum from zero to the max sampling rate of the sensor does not and can not tell the whole story.

I continue to believe that the shape of an MTF vs. frequency curve contributes greatly to the appearance of detail in an image and that an MTF50 up there close to the Nyquist frequency of the sensor is not necessarily a Good Thing.

This is fine for B&W but doesn't work for color sensors like Bayer since they interpolate color.

Mike, which part of the original post doesn't work for color sensors like Bayer?

Some time ago I tested the MTF of the SD9 and Bayer G9 where the color MTFs were shown in addition to the grayscale Y' response:

Best viewed original size.

Does anything there relate to MTF somehow "failing to work" for the Bayer?

Just know what my tests revealed. Try it yourself and see.

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