Is there a Formula to Determine"Normal Lenses"

[John Ventura]

I was wondering if there was a formula to determine what the correct- that is "N ormal" lens for a given format. 35mm, 6x6, 6x7, 6x9, 4x5, etc... Could this form ula be used to determine unusual film sizes as well?

Thankyou for your help

John Ventura

[John Ventura]

what exactly do you mean when you say "the diagonal"?

[Phil Brammer]

Sure, the focal length of lens you use is one of many artistic tools a photographer can use, but from a purely mathematical and technical point of view the "normal lens" term came about because apparently that is approximately what angle of view the human eye sees. (once again this is subject to people's artistic views on the world, many would not agree with this).

Never the less a "normal " lens for a format equals approxiamtely the measurement from corner to corner, diagonally, accross the negative size for the format in question. (ie 35mm format approx 45-50mm)

an 8"x10" LF camera may use a 300mm+ lens for the same artistic effect. It is not written in stone and never should be

The term "normal" therefore was some*ones* idea of what a "normal" field of view was at the time he or she invented the term.

These simple terms do however help out the point and shoot people and thats all they are designed for i think........

[giancatarina]

if X is the larger side of your film, and Y the smaller (in mm!), and N the diag onal and the "normal focal lenght in mm then:

( X x X ) + ( Y x Y ) = ( N x N) so N = square root of ( ( X x X ) + ( Y x Y ) ) for exemple for 6x6 (in fact 56x56mm) it's 80mm this is just in theory because all formats have different aspects ratio, it dep ends what you are shooting and if you are shooting vertical or horizontal. If you use a 6x6 to shoot a building, and if the building fit the entire image i n vertical with your 80mm , then you want to make "the same image with a 6x9 camera (without moving yourself), if you shoot in horizont al you need a 80mm as well (same 56mm), but if you shoot in vertical, something like a 120 should be ok, in every case you will hav e your entire building, so then what is the normal lens for 6x9 ? in theory it's 106mm, but in real life it depends on you (aspect ratio n, vertical or horizontal...)!