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

Thankyou for your help

John Ventura

-- John Ventura (johnnybmail@yahoo.com), October 10, 2001

Answers

Hi John

Its about the diagonal of the negativ format! Good light and peace for all!

-- Armin Seeholzer (armin.seeholzer@smile.ch), October 10, 2001.

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

-- John Ventura (johnnybmail@yahoo.com), October 10, 2001.

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........

-- Phil Brammer (filsta@goconnect.net), October 11, 2001.

if X is the larger side of your film, and Y the smaller (in mm!), and N the diagonal 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 depends 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 in vertical with your 80mm , then you want to make "the same image with a 6x9 camera (without moving yourself), if you shoot in horizontal 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 have 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 ration, vertical or horizontal...)!

-- dg (sacripant@online.fr), October 11, 2001.