[Leica] The myths of crop factor

[Mark Rabiner]

People always spoke of the sanctity of never cropping their pictures. People
in Magnum even did that. But I suspect it was all about the precious thin
black borders. A print in the stack with no black border stood out as weak.


On 1/5/15 8:46 PM, "Ken Iisaka" <ken at iisaka.com> wrote:

> Yes, Mark, been there done that.
> 
> Also, when using shorter lenses, such as Zeiss Hologon, the actual exposed
> area on the film was slightly larger than the actual aperture of the
> shutter, since the rectangular aperture was a millimeter or two in front of
> the film surface itself.
> 
> On Mon, Jan 5, 2015 at 5:23 PM, Mark Rabiner <mark at rabinergroup.com> wrote:
> 
>> 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/
>> 
>> 
>> 
>> _______________________________________________
>> Leica Users Group.
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>> 
> 
> 




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