joe mama wrote:
> hi,
>
> just as is the case with digital, i am trying to work with a crop
factor,
> but can't figure the math. i have a 4x5 camera that has a 6x9 reducing
back.
> my lenses are 203mm, 127mm and 90mm. what i am trying to fiure out is
what
> "effectice" focal length they become when using the 6x9 back. i'm
guessing
> somewhere around 1.5, but i don't know what to divide, or multiply. i'm
> assuming you take the overall square footage, teh divide the 4x5 into
6x9
> and get the CF. does that make snse???
>
> any help would be great.
>
>
Take the diagonal of the frame, so pythagoras says: √(x²+y²).
5"x 4" = 127mm x 101.6mm which has a diagonal of 162.64mm.
6 x 9 has a diagonal of 108.17mm.
Crop factor is therefore 1.5x or ⅔x. Your guess was spot on.
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