# Math for figuring out 35mm lenses to Mamiya lenses

Started Jan 11, 2013 | Questions thread
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 Re: conversion factor 6x7 to 35mm full frame In reply to Kelowna, Jan 13, 2013

Kelowna wrote:

cedy wrote:

I hope the math is linear like you say, but I'm not sure..... If I can make it work with a 2.1 like you say for a small sample, it would prove it to the point of being useful.

One minor quirk to keep in mind is the format shape - 35mm is a 3:2 (or 9:6) ratio compared to the 7:6 ratio of your Mamiya. Otherwise, the conversion factor is about 2x between the two formats.

Note that this conversion factor is for shooting 120 film; the digital backs for medium format all incorporate a varying crop factor.  You may find that the 180mm on a MF digital back is closer to a 105mm on a Nikon full-frame.

I don't think the f-stop would work with the crop. It's a ratio that works regardless of cameras and lenses. It may work for DOF though.

The f-stops will be consistent in terms of exposure levels for the same ISO, however you will need to apply the conversion factor to give you the equivalent DOF. Your 85mm lens on your Nikon full frame may give a very similar magnification as your 180mm Mamiya lens, but the depth of field will be twice as deep for the Nikon lens than it is with the 180mm Mamiya lens.

Applying the conversion factor of 2x means:

An image shot with a 180mm lens @ f8 on your Mamiya will be similar image for framing and depth of field using a 85mm lens @ f4 on your Nikon.

If you put a 85mm lens on both cameras, the depth of field would be the same - once you cropped the medium format image to the same image captured with your Nikon.

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Desmond Murray
www.KelownaPhotographer.com
I shoot to thrill

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