Re: Focal lengths

----- Original Message -----
From: "Andrew Amundsen"
Subject: [pinhole-discussion] Focal lengths

> Can someone tell me what the 12mm lens (in 35mm format) equivalent
would
> be in 4x5" format?
>

I use quite a range of formats and to convert back to 35mm format
equivalent, I use the following formula (nothing different from what
Chris or Guillermo provided you with earlier):

F(35mm) = F(format) * D(35mm) / D(format)

where:
F(35mm) is the focal length in 35mm format
F(format) is the focal length in the new format you are using
               (e.g. 4x5, 8x10, 20x24, etc...)
D(35mm) is the diagonal of the negative in 35mm format (48mm)
D(format) is the diagonal in the new format

Suppose you are using a 4x5 with a 50mm focal, then this will convert
into 35mm format as follows:

50 * (48/180) = 13.3mm

If you want to go from 35mm format to 4x5 format, as you are asking,
then use the reverse formula:

F(format) = F(35mm) * D(format)/D(35mm)

In your case, then, the results would look like this:

12 * (180/48) = 45 mm

These are approximations, of course, which work when using formats that
are relatively similar to the 35mm format. If you were using a large
panoramic format, then you would need to work through comparisions with
angle of view.

Hope this helps,

Guy
Received on Thu Jun 13 05:51:49 2002