[Leica] The myths of crop factor

[Mark Rabiner]

Most people I knew in the 90's and before filed out their negative carriers
of their enlargers so they'd both get black borders from the clear edges of
the film before the sprocket holes; and also insure the people looking at it
that not cropping had been done. We did this with our medium format negs as
well. And our 4x5 sheet film. Full frame black borders. Soon at least the
Omega company started making the neg carriers already wide enough so we'd
not have to file them.
On the paper we'd leave ample white borders. Minor White said if you don't
leave am inch border you're print was not archival.
So no your image size or magnification was not determined so much by paper
shape. We printed the entire neg. if we were shooting with a Nikon F and
other flagship cameras which gave us 100% viewing then we were seeing
exactly what we'd be getting at the edge.


On 1/5/15 6:06 PM, "Ken Iisaka" <ken at iisaka.com> wrote:

> 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.
> 




-- 
Mark William Rabiner
Photographer
http://gallery.leica-users.org/v/lugalrabs/