Why does a clear looking graph yield a low value
Status: Beta
Brought to you by:
fvdbergh
Menu
▾
▴
The edge of the graph looks good, but the value of the software analysis is only about 0.08, what caused this
I quickly ran MTF Mapper on the image you provided above (not ideal, but I also got 0.08, so it should be representative), and extracted the attached Edge Spread Function. A sharp edge should rise from black to white over a very short distance, perhaps a distance on the order of 1 pixel or less.
From the ESF extracted from your image the rise distance looks to be about 8 pixels, so the edge really does appear to be quite blurry. The SFR curve has a "bounce" in it, which suggests that the lens was defocused.
There are a lot of factors that influence your best achievable MTF50 value for a given camera system (lens performance, sensor pixel size, sensor AA filter, printed target quality, how well the lens is focused, software processing, etc.), but you should be able to get MTF50 values between 0.2 and 0.3 cycles/pixel unless your sensor has tiny pixels (< 2 micron).
yes,For the focus of the picture, it is indeed able to reach 0.2-0.3 cycles/pixel ,Take the picture below
Ok, I also see values of about 0.24 cycles/pixel in 150mm.png. Not sure if you still have a question, or if I have answered it already?
There are still some question, like the photo below, which is much vaguer than the first one mentioned in the topic, but the result of his edge mtf50 analysis is still 0.08.
Hmm, I plotted the full SFR curve for your initial image lowvalue.png, as well as an edge from 350mm.png that also happens to have an MTF50 value of 0.08. You can see that the SFR curves are very similar until the contrast drops below 0.1, so it is not surprising that the MTF50 values are similar.
By the way, you can get more precision out of MTF Mapper if you process the outputs with a script, but I don't know if you will necessarily get improved accuracy. Is the difference between 0.075 and 0.08 meaningful? You would have to have very good imaging conditions (good lighting, high contrast, good test chart quality) to obtain reliable MTF50 measurements in the third decimal digit.
So in my opinion the MTF measurements look consistent, even though the visual appearance of the edges can differ in your samples. If you look at the rise distance from black to white, I also see consistent plots between your samples.
Ok, thank you very much. I get it.