crop factor on older lenses

Started Jun 6, 2014 | Questions thread
Brian D. Schneider
Brian D. Schneider Forum Pro • Posts: 18,225
Re: crop factor on older lenses
3

Sean Nelson wrote:

mbower wrote:

What is the general crop factor that should be applied when one uses older lenses?

First of all, remember that the actual focal length of the lens does not change, nor does it's f/ratio. Using the crop factor to calculate a different focal length is just a shorthand way of expressing the lens' field of view in terms of what you'd get on a 35mm full frame camera.

The crop factor doesn't change for different lenses. A 45mm lens mounted on an M43 camera will always give the same field of view as a 90mm lens on a 35mm full frame camera. And your 35mm prime will always give the same field of view on your M43 camera as a 70mm lens would on a full frame 35mm camera. Both have a crop factor of 2.

The only time the "crop factor" would change is if you're comparing a lens intended for use on a camera other than 35mm. For example a 75mm "normal" lens for a 6x6cm medium format camera mounted on an M43 camera would have a crop factor of about 3, which means that if you mount it on a M43 camera you'd get the same field of view as if you used a 225mm lens on a medium format camera.

It's 75mm and would still give the field of view of a 150mm on a 35mm camera.

It doesn't matter what format it was originally used on.

Since very few people use medium format cameras, and since fields of view are almost always expressed in 35mm-equivalent terms, you can pretty much forget about all that and just use "2" as the crop factor for any lens mounted on a M43 camera. Just remember that all it really means is "the field of view you'd get with an x-mm lens mounted on a 35mm camera".

I'm sure someone's going to jump in and start talking about depth of field, and then someone else will jump in and start talking about "total light gathering ability", and then all kinds of hijinks will ensue. But the fact that you're asking this question suggests to me that getting into those topics might be a bit beyond what you're ready to absorb right now. First things first.

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