Feel like going back in time and taking an SAT test? Ok, this is a techie question that is tough for me to frame.... so here goes...

If I crop the image from my 5D Mk II taken with a 200mm lens, what is the focal length equivalent? This question is like the crop sensor vs full sensor with the same lens length issue.

My 5D Mk II put out a JPG with these dimensions: 5616x3744 pixels. If I crop it down (same proportions and not resizing) to where the final image is 960x640 pixels, that is as if I had a ???mm lens.

Some more background..... a friend asked me if they should jump to a 500mm lens, a very expensive proposition, and it got me thinking.... with my 5D Mk II and my 200mm lens, I get fantastic results cropping down a bird, small in the frame, down to 100% assuming I get the exposure just right. That is just like having a bigger lens. What would this effective lens be?
OK, for additional points, if I did the same using a 500mm lens and a 105mm lens, what would the effective focal lengths be? I am assuming this a single formula I could use... no?

Thanx so much.
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