[Leica] The myths of crop factor

[ken at iisaka.com (Ken Iisaka)]

When comparing sensors with different aspect ratios, the use of the
diagonal to calculate the factor is overly simplified.

With the diagonal measurement of roughly 21.6mm, the FourThirds sensor is
often said to have a crop factor of 2.0, compared to a "full frame" 35mm,
which has a diagonal measurement of 43mm. This implies that a 25mm lens on
FourThirds is thought to work like a 50mm lens on a "full frame" 35mm.

But it's not quite that way.

What you should be comparing is NOT the diagonal measurement of the
sensors, but the dimensions of the actual area of the sensor that will be
used for the final image.

So, if you are creating an image with an aspect ratio of 4x5, you are not
using the full area of the sensor, but only a smaller area of 24x30mm, and
13x16.25mm. When doing so, the "crop factor" is not 2.0, but is merely
1.846. The different is not all that big, but is not insignificant either.
A 25mm lens on FourThirds works more like a 46mm lens, not 50mm. There are
some of us, who split hair between smaller than that.

If you're creating an image with an aspect ratio of 5x7, the situation
changes. You'd be using an area of 24x33.6mm from a 35mm sensor, but
12.38x17.33mm. Here, the effective "crop factor" is 1.9386.

Want to make it more complicated? Sure, I can do that.

We didn't really use the full 24x36mm area of the 35mm camera. The slide
mount or enlarger negative carrier usually cropped the image to 23x35mm or
so. So, if you are creating an aspect 4x5 image, the effective crop factor
was only about 23/13 = 1.77, since you only used an area that measures
23x28.75 to create your image.

So, a 12mm lens on FourThirds behaves more like a 21mm lens instead of a
24mm on a 35mm film camera (12*1.77 = 21.24)

With hidden complexities such as this, I find all the discussions about the
crop factor, etc. to be absolutely absurd. Sorry, Mark.


-- 
Ken Iisaka
first name at last name dot org or com