AlanF said:
Here is a way of calculating the effective extra reach or resolving power of a crop body versus FF, which will amuse the geeks among us...
MTF is not a measure of resolution, it is a measure of a lenses ability to transmit contrast of the original scene through a lens. That's all it measures.
If what you really want is true image resolution, then there are two correct ways of measuring it. The first is a measurement of the performance of the lens itself. This measurement can be found in using something like a USAF resolution test chart and a microscope to perform aerial inspections of the image of the chart after it has passed through the lens.
Problems with this approach include difficultly in setting up a test bench (it's not easy at all). Additionally, the final result will not include measurements of chromatic aberrations, image distortions, field curvature, etc. The most important missing element in this kind of test is the exclusion of the imaging system itself. However, if the question is about real optical resolution, this test will give you the right answer.
Which leads to the second way of measuring, well, actually calculating real image resolution. This is diagnostic and very simple to perform. Simply take the number of image points ("pixels") in your file, divide by the size of your sensor (in millimeters), and divide by two. This number will represent the number of Line Pair per Millimeter (the measure of the ability to go from one white line to one black line) that your sensor can resolve.
This is rather interesting in that you can quickly see that a Canon 7D 18 mpixel sensor is capable of resolving 116 line pair per mm. The Canon 5D MkII is capable of 78 lppmm. As a comparison, Sony's 36mpixel FF sensor is capable of resolving 102 lppmm and Phase One's monster 80mpixel IQ180 returns 97lppmm.
Taking this a step further, look carefully at the physical limits (as in optical physics) of optical resolution, as measured in lppmm. You see that at f/2, an optically correct lens will return 695lppmm in the center of the scene where the light's wavelength is 589.3mu (green). At f/11, an optically correct lens will return 123lppmm, dropping off to 92lppmm at 25 degrees off-axis tangential.
Looking at this over the years, I have come to realize there is seldom a lens in-capable of resolving so poorly that a sensor (or old film for that matter) could out-resolve the lens. Sure, there are other optical effects, but we are talking pure resolution here. Nothing more.
Think about this for a moment. MTF does _not_ measure optical resolution. While useful, it does _not_ tell the story of resolution, no matter how much "math" you throw at it. Secondly, and perhaps most interestingly, optical physics show diffraction limited resolution at f/11 EXCEEDS currently manufactured sensors ability to return that resolution in all cases.
Rather shocking, don't you think?
Back to the original poster's point: Canon's 7D sensor outresolves (using the correct application of the word "resolution") the FF sensors from any manufacturer. It does so, however, for reasons other than those that were brought up.
Correct application of rational thought and real world science can help us properly understand and identify the errors and misleading comments widely published by marketeers and critics of optical imaging systems.
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