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Phil Rhodes

How much MTF is enough?

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I've been looking at documentation recently on comparative resolution figures. Inevitably, most of them eventually start talking about MTF.

 

Various people cite various percentages of MTF as a cutoff point for "effective resolution."

 

Has there ever been an MTF number that would be widely accepted as enough modulation to be considered a reasonable reproduction of an image - that is, a cutoff point for resolution? My impression is that there isn't any such widely-agreed metric for what constitutes resolution and what doesn't, perhaps because that would be an oversimplification.

 

P

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I wish it was as easy as explaining the acronym :rolleyes: I always stumbled wondering what the minimum contrast is that the human eye can discern. Who's eye. We're all different. Some people have extreme acuity with seeing. There is probably a definition explaining that, somewhere, in very, very fine print, at very low contrast, so we can almost read it.

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Eh, sorry, my bad.

 

Shorthand definition:

 

When we're testing lenses, codecs, digital storage, systems, and so on, we often use charts with black and white stripes at various pitches and evaluate how much they're reduced in contrast by the system we're testing. As lenses, sensors etc approach their resolution limits, we start to notice that the black and white stripes start to merge into grey, effectively having their contrast reduced; we might say that at 1000 line-pairs per millimetre, contrast is reduced 50%. At some point the degree of contrast reduction in fine detail exceeds our personal tolerance and we consider that a resolution limit for the system.

 

More formal definition:

 

That 10%-at-1000 figure can be seen as a point on a graph which would show the frequency response of the system, like a spectrum analyser in audio.

 

1920px-Illustration_of_the_optical_trans

 

Modulation transfer function is (formally) a subset of optical transfer function and is defined as the Fourier transform of the point spread function. This effectively means it's a frequency-domain expression of the behaviour of the optical system.

 

P

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I think at the beginning of the digital era it was common to measure film mtf with approx 30% or 20% response and digital with 0%.

that is because the largest film grains mask the fine details sooner (finest details drawn by the smallest thus least sensitive grains) so there is no point to try to see the 0% details on film whereas with digital it is somewhat possible

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