In article <4655ff3e$0$19455$4c368faf@[EMAIL PROTECTED]
>,
"joe mama" <dmoss74@[EMAIL PROTECTED]
> 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.
The biggest problem is that you're comparing a 4:5 ratio film with a 2:3
ratio film, so the focal length equivalent isn't quite so easy as if the
ratios were the same.
Typically, people compare along the diagonal. 4x5 is c. 96x121mm of
usable film, making for a 154mm diagonal. 6x9cm is around 56x84mm (it
may vary by manufacturer) for a 101mm diagonal for about a 1.53
conversion factor, if you use the diagonal. Your lenses on 6x9 work as
if they were 352, 195 and 138, so the 90 is a short normal, and the 127
is a long-ish normal and the 203 is a moderately long lens, if that's
what you're wondering about.
People don't often look along the diagonal, some are happier comparing
using the horizontal or vertical dimensions.
Personally, after doing lots of math and using 3 different formats, I
find I compare different formats using the horizontal. When I look for a
lens that's like a 35mm in 24x36mm, I generally look for a lens that's
close to the horizontal, or a bit shorter, in focal length. I find a
55mm lens in 41.5x56 (6x4.5) and a 120mm lens in 96x121 gives me a
similar feel as the 35mm in 24x36, so I do my comparisons based on the
horizontal. Even when I shoot verticals, I tend to think about the long
dimension when I'm looking for a particular angle of view. If I were
comparing 4x5 and 6x9, I'd compare 121 to 84 and get a 1.44x conversion,
which is awfully close to 1.5 or 1.53 anyway, and I'd still treat the
90mm as a short normal, I'd probably prefer the 127 as a longer normal,
and find the 203 as a pretty good ****trait lens.
Drew
--
Drew W. Saunders
dru (at) stanford (dot) eee dee you
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