The 20D has, by comparison to the 5D, essentially a 1.28x TC built-in because of its smaller pixel pitch. So that means, if I put a 1.28x TC (if it existed) on the 5D, they'd equal out.
Before you say anything about sharpness, let me tell you the main reason TCs have a reputation for degrading image quality.
If you have a lens capable of 100lp/mm at 50% MTF and you install a 1.4x TC, the TC magnifies that image effectively reducing the lens to 71lp/mm at the sensor but at 1.4x the magnification. Likewise, a 2x would reduce it to 50lp/mm with twice the magnification.
Now the sensor has a certain pixel density and the final sharpness is related to both the MTF of the lens and the resolution of the sensor. A sensor with higher pixel density ends up requiring a lens with higher lp/mm at the same MTF to be just as sharp at the pixel level. So the built-in 1.28x "TC" of the 20D stresses the lens at the pixel level just as much as a real 1.28x TC would on the same lens the 5D. In other words, the 5D's larger pixels make it effectively sharper at the pixel level with the same lens and a TC that equalizes out the pixel pitch will equalize the sharpness at the pixel level and equalize the effective reach.
So all is equal, right? Nope! The 20D is still better.
Did that help?
--
Lee Jay
(see profile for equipment)
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