Converting large format lens focal lengths

[Kraemer]

Can anyone recommend a conversion chart for using large format lenses with a medium format digital back?
Or at least a rule of thumb?

Trial and error is getting quite frustrating.

Thanks

[Ron Marshall]

The factor by which the percieved focal length of the lens will change is the diagonal of a 4x5 (155mm) divided by the diagonal of the digital sensor.

[Leonard Evens]

If the aspect ratios (short to long dimnsion ratios) are the same, you can use either dimension to compute the multiplier. The 4 x 5 frame is just about 96 x 120 mm (depending on the film holder). If the digital frame were full size medium format, which is about 56 x 70 mm, the aspect ratio would be close enough to the 4 x 5 one, so you could use a multiplier of 120/70 ~ 1.7. Of course when you change formats the focal length doesn't change, but the angle of view does because you in effect crop the image. To see what focal length lens would give you the same angle of view, divide the focal length by the factor, in this case 1.7. So to obtain the same angle of view as a 150 mm lens in 4 x 5, you would have to use an 87.5 mm lens, or 90 mm which would be close enough.

But the medium format sensor array may not have the 4:5 aspect ratio. In that case, there is no one single answer. Possible choices are the short dimension, the long dimension, or the diagonal. Each will give you an angle of view for that dimension, but they will all be a bit different. If you use the diagonal, that will probably be the best compromise. To calculate the diagonal, you add the squares of the sides and take the square root of the result.

[Kraemer]

This is great information, thank you.
I still am left with one question...once I arrive at this "factor" or mulitplier", how do I apply it to the original focal length of the lens to get it's altered focal length.

Again, thanks!

Kraemer

[Kraemer]

BTW - I am converting large format to a 645 digital back.
If that's at all helpful.