Quote Originally Posted by Dr Croubie View Post
Take a 4x5 frame, actual image size is 94x120mm (numbers I got from a bit of googling) and a 35mm frame size of 24x36 (may be optomistic for film, but it's the size of digital FF at least).
The 4x5 has a ratio of 1.27:1 (near enough a 5:4 ratio), the 35mm has a 1.33:1 (6:4 or 3:2 ratio).

So what do you want your output image ratio to be?

A different way to look at 'equivalent FL' is to merely compare the FL to the frame dimension. Using the short dimension of the frame as the baseline, then 24mm FL on 135 format would be equivalent to 94mm FL on 4x5 format. After all, you fit the short dimension of the frame into the short dimension of any print size, so the print is the great format ratio equalizer!

Comparing the 'normal', 50mm FL is 2.1x the short dimension of 135 format, and 200mm FL is about 2.1x the short dimension of the 4x5 format. Or, to put it differently, 150mm FL is 1.6x the short dimension of 4x5, so you really need 38mm FL on 135 to frame the same amount of vertical area.

So in 'really wide angle' land, 75mm FL on 4x5 is like using 20mm on 135 format.