joe mama spake thus:
> 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???
By "square footage" you mean area (which, in this case, is more like
"square inchage".) But that's the wrong quantity: you simply need to use
the diagonal of the format. The formula for that is:
diagonal = sqrt (x squared + y squared)
where "sqrt" means "square root of", and x and y are the two sides of
the format. (Example: for a format 3x4, the diagonal is sqtt (3 squared
+ 4 squared) = sqrt (25) = 5.)
After you get the diagonals, just divide them to get the conversion
factor.
Remember that the lens needs to cover at least the diagonal of the
format, meaning a circle which encloses the entire frame, and then some
to allow for movements.
--
Any system of knowledge that is capable of listing films in order
of use of the word "****" is incapable of writing a good summary
and analysis of the Philippine-American War. And vice-versa.
This is an inviolable rule.
- Matthew White, referring to Wikipedia on his WikiWatch site
(http://users.erols.com/mwhite28/wikiwoo.htm)
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