Well, I was careful to say "more like".Cyclops wrote:I'm intrigued tho how a 50 mm on a digi goes back to being a 50 but with a higher magnification!
If you can stand just a few numbers...
Imagine that the 50 mm is a "thin lens". Then when it's at 1:1 on the small sensor, the working distance is 100 mm and the field width is 22.7 mm.
Suppose we ask the question: what length lens would be required, to give 100 mm working distance and a 22.7 mm field on a full-frame 35 mm camera?
To get 22.7 mm field on a full-frame 35 mm camera requires 1.6:1 (36:22.7 = 1.6:1).
And to get 1.6:1 with 100 mm working distance requires a lens whose focal length is 61 mm.
So, the 50 mm at 1:1 on the small sensor would be equivalent to a 61 mm at 1.6:1 on a full frame, in terms of working distance and field size.
61 mm is a lot closer to 50 mm than it is to 80 mm, hence my comment "more like" a 50 than an 80.
The difference between equivalent focal lengths becomes less and less as the magnification rises. At 5:1 on the digi, the working distance would be 60 mm and the field width would be 4.54 mm. The equivalent magnification on full frame would be 7.9:1, and the equivalent focal length (7.9:1 at 60 mm working distance) would be only 53 mm, not much different from 50 at all!