Here is a comparison of ISO6400 reach for a bright, well-lit, high contrast subject (the reach advantage for 7D will be less if the subject has dark tones or is in a darker part of the image or you underexpose in general, while it's still very clear, surprisingly, for a bright subject well lit it might be barely there for a black crow in a dark corner of an image most likely; I may do mid-tonality reach comparison later since many subjects might fit that scenario):
top image is the 7D re-scaled to match the bill size on the 5D2 - you notice that it delivers BOTH less noise, less artifacts and more detail than the 5D2 and slightly more detail and less artifacts than the 5D3 with maybe a touch more noise
second image from top is the 5D2 - you notice it retains a bit less detail than the 5D3 bill
third from top is the 5D3 bill - you notice that it and the 5D2 show a less detail than the 7D bill even at ISO6400, the 5D2 shows tons less
bottom is the native size 7D bill - bright subject, well-lit the 7D still has a sizeable reach advantage over the 5 series cams, especially compared to the 5D2
but again for a subject that is not so super contrasty and is made up of many tones much farther from white the reach advantage may prove to be much less
make sure to click the image to see it at full scale
If you use firefox make sure to click all of these images to make sure you are seeing the full 100% view
and note that the 5D3 looks brightest and the 7D darkest so at ISO6400 the bodies are probably not applying the same gain so the effective ISO compared here is highest for 5D3 and lowest for 7D most likely which would give 5D3 a slight disadvantage and the 7D a slight advantage.
top image is the 7D re-scaled to match the bill size on the 5D2 - you notice that it delivers BOTH less noise, less artifacts and more detail than the 5D2 and slightly more detail and less artifacts than the 5D3 with maybe a touch more noise
second image from top is the 5D2 - you notice it retains a bit less detail than the 5D3 bill
third from top is the 5D3 bill - you notice that it and the 5D2 show a less detail than the 7D bill even at ISO6400, the 5D2 shows tons less
bottom is the native size 7D bill - bright subject, well-lit the 7D still has a sizeable reach advantage over the 5 series cams, especially compared to the 5D2
but again for a subject that is not so super contrasty and is made up of many tones much farther from white the reach advantage may prove to be much less
make sure to click the image to see it at full scale
If you use firefox make sure to click all of these images to make sure you are seeing the full 100% view
and note that the 5D3 looks brightest and the 7D darkest so at ISO6400 the bodies are probably not applying the same gain so the effective ISO compared here is highest for 5D3 and lowest for 7D most likely which would give 5D3 a slight disadvantage and the 7D a slight advantage.
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