Quote Originally Posted by johnnywalker View Post
I believe a rule of thumb is to go with the length of the hypotenuse for "normal". The hypotenuse of 4" by 5" inches is 6.4 inches, or 163 mm. By that measure, 180 mm would be a trifle on the long side.
I realize why people settled on this idea, about the hypotenuse. They had to pick something.

But I find the better rule of thumb, for me at least, is to think of a normal lens as the length of the long side of the film. Thus for 4x5 I think of 127mm (or 135, depending on which camera body - I don't have all lenses for all bodies!) as normal, and for 135 I think of 35mm as normal. So my "standard" is a bit wider than the common usage.

I don't shoot wild sizes like 12x20.