Canon Crop Factor

Canon digital SLR cameras are fitted with one of three different sensor sizes, depending on the model:

1. Full-frame sensor -- same size as a 35mm negative or transparency (36 x 24mm); found on Canon 1Ds and Canon 5D digital cameras.
2. APS-H sensor -- slightly smaller than full-frame (28.7mm x 19.1mm); found only on Canon EOS 1D digital cameras.
3. APS-C sensor -- smallest of the three (22.5 x 15mm); found on Canon Digital Rebel cameras (400D, 450D, 500D models) and Canon 30D, 40D, 50D models, plus the new 7D.

However, if you replace the full-size sensor with a smaller one (sub full-frame), some of the image will "overflow" as the size of the image will be bigger than the sensor. So a portion of the image, along the top, bottom, and sides, will "spill" over the edges of the sensor and be lost.

See diagram below, where the black rectangle represents a full-frame sensor (36 x 24mm in size), the blue rectangle represents an APS-H sensor (28.5 x 19mm in size), and the red rectangle represents an APS-C sensor (22.5 x 15mm in size) (Note: sensor sizes not to scale).
In this example - using a 300mm Canon EF lens on a Canon 5D body - note how the eagle fits snugly into the area of the full-frame sensor (black lines). But if you were to switch bodies and attach a Canon 1D Mark III (with APS-H sensor) to the lens, the image of the eagle is now too big for the sensor, and parts of the bird overlap or fall outside the edges of the sensor (blue rectangle).

Now attach a Canon 450D (Rebel XSi) to the lens and even more of the eagle falls outside the sensor (red rectangle), cropping off significant bits of the bird.

Okay, let's take a second scenario, with the same eagle (amazing how it holds its pose!), but this time using a 200mm lens. When attached to the 5D body, notice how much space there is around the eagle (black rectangle). Switch to the 1D Mk III body, and it looks like we've zoomed in tighter (blue rectangle).
Lastly, we attach the 450D to the lens, and now the eagle is filling the frame (red lines), with little space around the subject. Notice how similar the composition is to that in the previous example where the 5D with 300mm lens filled a similar amount of the frame.

A Canon 5D produces an image 4368 x 2912 pixels in size from a 12.8 megapixel sensor. The 450D produces a very similar sized image -- 4272 x 2848 pixels -- from its 12.2 megapixel sensor.

Can you see the implications? The consumer-level 450D, with a 200mm lens, can produce an image in which the subject is virtually the same size in pixels as that produced with the full-frame 5D using a 300mm lens. (Note: the full-frame image will nevertheless be of higher quality because the pixels of the full-frame sensor are larger, with a greater surface area available for gathering light).

Crop Factor

This cropping of the image caused by the smaller, sub full-frame sensor is referred to variously as "crop factor", "lens magnification factor" or "focal length conversion factor".

Call it what you will, in practice when you fit a regular Canon EF lens to a digital camera with either an APS-C or APS-H sensor, your subject will fill more of the sensor -- mimicking what would happen if you used a tele converter or switched to a longer lens on a full-frame camera.

The crop factor on Digital Rebels and the Canon 30D/40D/50D range is 1.6x, while that for the EOS 1D cameras is 1.3x. So if you're using a 200mm lens on a Canon 450D, you'll get the same magnification as you would using a 320mm lens on a Canon 5D (200 x 1.6). On the Canon 1D, it's like using a 260mm lens (200 x 1.3).

The crop factor is a huge bonus for anyone using telephoto lenses such as wildlife and sports photographers. Fitting a lens like Canon's EOS 70-200mm f2.8L IS on your 450D or 50D body is like shooting with a 112-320mm f2.8 image-stabilized zoom. Such a lens, if produced by Canon, would cost about the same as a small car!

But it's just the opposite for landscape and architectural photographers -- or anyone who shoots predominantly with wide angle lenses. Now that incredible 16-35mm ultra wide angle zoom that produces such striking images has been "lengthened" by 60% -- and now is equivalent to a rather mundane 25-56mm wide angle zoom.

It's for this reason that Canon has recently added a number of EF-S lenses to its range. These are specifically designed for Canon digital SLRs with the APS-C sensor and are aimed at countering the magnification factor on the wide angle side. A good examples is the Canon EF-S 10-22mm f/3.5-4.5 USM, which equates to a 16-35mm ultra wide on a full-frame body.

See also Impact of crop factor on Canon EOS 50D.

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