The formula is to look at the ratio of the format diagonals. For
example:
---
Question: What lens on the DS will give me the same field of view as
a 50mm lens on my MX?
Pentax DS has the format 16x24mm, diagonal 28.8mm. 35mm format is
24x36mm, diagonal 43.3mm. To find the focal length of a lens that has
the same diagonal field of view on a DS that you're familiar with on
a 35mm SLR, calculate the factor as
28.8 / 43.3 = .67x ... then multiply by the focal length you're
familiar with:
50mm * .67 = 33.3mm
Answer: 33.3mm lens on the DS provides the same diagonal field of
view as a 50mm lens on your MX.
---
So now you have a conversion factor from 35mm to *ist Dx. A
conversion factor from *ist Dx to 35mm is calculated the other way:
43.3 / 28.8 = 1.5x
to answer the reciprocal question
"What lens on the MX will give me the same field of view as a 35mm
lens on the DS?"
You calculate the conversion factors for all the other formats in
similar manner, using the format dimensions:
645 :: 43 x 57mm, diagonal 71.4mm
6x7 :: 57 x 68mm, diagonal 88.7mm
The conversions are only accurate for diagonal field of view, as the
format proportions vary somewhat. You can also build an accurate
table of equivalent focal lengths by using Rui Salgueiro's Field of
View calculator:
http://www.mat.uc.pt/~rps/photos/angles.html
It would do a good job by giving you horizontal, vertical and
diagonal field of view values.
Godfrey
On Apr 20, 2006, at 11:40 PM, Thibouille wrote:
I'm really lost now. Could someone tell me how to convert between
those formats and 35mm?
35mm -> *istDx is 1.5x
35mm -> 645 ??
645 -> 645D ?? (I think it is 1.3x but not sure).
35mm -> 645D ??
35mm -> 67 ??
Thanks
----------------------
Thibouille
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*ist-D,Z1,SFXn,SuperA,KX,MX, P30t and KR-10x ...
