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# Thread: Recalculating MF focal length to 35mm equivalent?

1. Think of the image circle you have to capture. On 35mm you have - 35mm (OK, 36mm), with 645 and 6x6 you have 60mm (or 56mm or so), 6x7 is larger again (almost 70mm), 6x9 (almost 90mm), and 4x5 (about 110-115mm). Divide the image circle numbers, and you have your conversion factor. There's some slop built into my method, but it works OK.

- Thomas

2. Originally Posted by Thomas Bertilsson
Think of the image circle you have to capture. On 35mm you have - 35mm (OK, 36mm), with 645 and 6x6 you have 60mm (or 56mm or so), 6x7 is larger again (almost 70mm), 6x9 (almost 90mm), and 4x5 (about 110-115mm). Divide the image circle numbers, and you have your conversion factor. There's some slop built into my method, but it works OK.
"Some slop"?

A lens projecting an image circle of 36 mm would produce severe vignetting on 35 mm format.
The same for your other circles.

What you are doing here is compare the size of the sides of the different formats, not that of the image circle.
And not consistently too: long side for all formats smaller than 4x5", but short side for 4x5"?

A good way to compare formats is the one that compares horizontal angles of view. We tend to compose using the horizontal angle a lot; the vertical less. But not always. And who knows the horizontal angle of view of his or her lenses?
But probably the best way - which you are using - is indeed to use the long side of the format. The ratio between those is also the ratio between focal lengths.

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