crop factor help

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.

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.


--
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This is an inviolable rule.

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(http://users.erols.com/mwhite28/wikiwoo.htm)

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 â…”x. Your guess was spot on.

David Nebenzahl wrote:

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.


That's not really an issue as he's trying to work out the crop factor
against his existing LF lenses, which presumably have a covering power
for at least 4x5" plus room for movements, I'd guess at 200mm.

"Richard Polhill" wrote in message
.. .

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 ?x. Your guess was spot on.

thanks for the answers, and a small pat on the back--pesonally--for the
guess ;=)

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.

Easy...
The 203mm becomes - ready? - exactly 203mm. Et cetera for any others


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"dadiOH" wrote in message
news:LIo5i.16253$qp5.2447@trnddc03...
Easy...
The 203mm becomes - ready? - exactly 203mm. Et cetera for any others

I know what you are saying, but that wasn't the question. Crop factors have
nothing to do with focal length. Ask any Nikon hump running around with a
D200, and a 200mm lens, saying he has a 300mm.

dadiOH wrote:
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.

Easy...
The 203mm becomes - ready? - exactly 203mm. Et cetera for any others


Yes but he didn't ask what the focal length became, but what the
"effective" focal length was. The subject said "crop factor".

Smartass! ;-)

joe mama ha scritto:

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.

Hi from Italy.
Before all sorry for my poor english!

I think you are used to 4x5 format and you are figuring how your lens
will "behave" using them on 6x9 format, compared to 4x5 behaviour
naturally!
Well, the crop factor is a linear expression and not a square
expresion. The simplest way, though a bit rough, is to compare the
normal lenses in both formats (105mm for 6x9 and 150mm for 4x5) and
divide the larger by the smaller.

In this case is 150/105=1.43 which we can consider 1.5.

The lens behaviour on 6x9 will be the same of 1.5 longer lens on 4x5.

For your lens:

- the 203mm will behave on 6x9 like a 303mm on 4x5

- the 127mm will behave on 6x9 like a 190mm on 4x5

- the 90mm will behave on 6x9 like a 135mm on 4x5

As usual in such cases you will gain in long focal and you will loose
wide angle.

Gully

In article ,
"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.

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 portrait lens.

Drew

--
Drew W. Saunders

dru (at) stanford (dot) eee dee you