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Crop factors, why do they exist?


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I will bite.

A crop factor is a way of estimating how a picture with a lens from one size sensor will look on the other.

Example. 35mm camera film has been arbitrarily defined as "Full Frame" a sensor with the same size as a film camera. Micro 4/3 has a smaller sensor, this means that a picture taken with a full frame camera and a m4/3 camera will look very different with the m4/3 picture having on the center portion of the photo.

The crop factor tells you what size lens you will need for an equivalent picture. So if using a 50mm lens with a full frame camera, so have a picture look the same from m4/3 you need a 25mm lens. This is a 2x crop factor, because the picture is zoomed in by 2x with the lens.

There are some advantages with smaller sensors including greater depth of field (more of the picture is in focus) and smaller lighter lenses. Larger sensors have better image quality and a 3d quality that is hard to define.

Hope this is somewhat useful, have to run

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[quote author=christianhubbard link=topic=675.msg4962#msg4962 date=1336004698]
None of that was what I was looking for, sorry  :P

I am fully aware of all of that but I wanted to know WHY, scientifically speaking.

It's a norm. Your question is as if you asked, why is it 50 cent, why not one euro/dollar? They made smaller sensors because they could make the cameras smaller.
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The "crop factor" is really just a way of helping us figure out the angle of view of a lens of a given focal length when it's used on cameras with different sensor sizes.

35mm film is taken as the reference, partly because it's something familiar to most people (we can visualize what a photo taken with a 50mm lens on a film or full-frame digital camera will look like), and also because commonly people fit 35mm lenses to other cameras, and knowing the crop factor makes it easy to work out what the angle of view will be on that different camera.

So, if you take a Canon 50mm lens, it gives the same angle of view on a 7D (1.6 crop factor) as an 80mm lens would on the 5D. And on a GH2 (2x crop factor) it behaves like a 100mm would on the 5D.

Using full-frame as the"standard", and knowing the crop factor for any given camera, means we can translate the marked focal length of any lens and figure out what the shots from that lens and camera combo will look like.

One more example: the Panasonic 20mm F1.7 lens, mounted on a 2x GH2, gives approximately the same angle of view as a 40mm lens would on a Canon 5D or other 35mm full-frame camera.
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It's arbitrary. There is, for example, no scientific reason why t-shirts should be labelled S,M,L,XL. But you know which size could be the right for you. Crop factors are even more exact.

EDIT: Better yet, if all people were born in XL size, it had been so forever, and suddenly some mutants were only half as big, you would call them half-sized (relative to the standard). There would be no specially tailored t-shirts for the mutants, the "shrink-factor" of 2 only describes how funny the folks will look in a standard t-shirt.
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The imaging circle laid down by a lens designed for full frame or 35mm onto the film plane or sensor falls off the edge of a smaller sensor, hence the idea of crop. But forget crop factors its camera manufactueres speak for trying to dumb down for consumers easier for them to understand than field of view, focal length and snsor size relationship.

If you put an EF-S lens on a T2i there is no dumb ass crop factor to consider as the lens system is designed for the APS C sensor. That's what the EF-S system exists for particularly important at wide end.
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