The site of Photozone.de
shows the resolution graphs (MTF) of the Canon EF 70-300mm f/4-5.6 L IS USM
for different focus settings (zoom settings), when mounted on a Canon 5D (FF-body).
The same site also shows the resolution graphs (MTF) of the Canon EF 70-300mm f/4-5.6 L IS USM
for different focus settings (zoom settings), when mounted on a Canon 50D (APS-C-body).
I am puzzled by the (huge) difference between these two setups.
And I cannot explain that difference, nor can I find (the theory of) the reason behind it.
With my limited knowledge of applied optics, my reasoning (thatclearly is faulty) was as follows.
1. A lens projects a sharp image on the sensor if mounted correctly on the camera body: that is implicit in the design of the camera body (its dimensions) and the lens (its optical properties in combination with its size). That is the same for both setups, so no reason for discrepancies here.
2. When, on a given focus setting, the lens performs with a certain number of lines/mm, this is not influenced by neither the qualities of the surface on which it projects the image, nor the size of the surface. The light falls on the lens and is projected on that surface.
3. I see that aberrations like an imperfect plane of the projected image affect the MTF. But comparing the same lens on FF vs APS-C, that would be visible in the results for the borders and corners. And the same, so I was thinking, goes for all other lens aberrations.
4. I do not see how a different size (either in millimeters or in number of pixels per millimeter) of the surface on which the image is projected, affects the number of lines/mm that the lens can discern. And therefore I cannot understand how a property of the camera body, i.e. FF vs APS-C, makes a difference.
But the difference is there, so much is clear.
So the question remains:
Why is the size of the surface of influence on the MTF of the Canon EF 70-300mm f/4-5.6 L IS USM at the center of the image, as indicated by the different results for FF versus APS-C?
Can anybody explain the why (the theory) behind this difference in MTF?