Crop Factor and how this affects the Canon EOS 50D digital SLR camera

Impact of Crop Factor on Canon EOS 50D Digital SLR

The Canon EOS 50D digital SLR camera is one of Canon's "prosumer" models that is affected by the crop factor (also known as lens magnification factor or focal length conversion factor).

Note that the other prosumer models (the Canon EOS 40D, 30D, and 20D), the entry-level "Rebel" models (300D, 350D, 400D, 450D, 500D, 550D and 1000D), plus the new Canon EOS 7D, are also affected by the crop factor, in the exact same way as outlined below.

The impact of the crop factor on the 50D (and the other Canon models mentioned above) is that all Canon EF and EF-S lenses act as if their focal length has been increased by 1.6x or 60%.

This apparent increase in focal length of 60% also applies to any Canon-compatible lenses such as those produced by Sigma, Tamron, or Tokina when fitted to a 50D.

We talk of "apparent" increase in focal length, because the actual focal length of the lens obviously hasn't changed -- a 50mm lens remains a 50mm lens, no matter the camera body.

What's happening is that the sensor on the 50D is smaller in size (22.5 x 15mm) than a 35mm negative or full-frame sensor (36 x 24mm). The smaller sensor is referred to as an APS-C format sensor or sub full-frame sensor -- see size comparison below: sensor and film size comparison for Canon 50D
So in practise the image cast by a particular lens fills more of the APS-C sensor than it does of the full-frame sensor -- see diagram below, where the cute critters (dwarf mongooses) easily fit into the full-frame sensor (left), but "overflow" the smaller sensor of the 50D (right). As a result some of the image has been cropped, hence the term "crop factor".

full-frame sensor for Canon Canon aps-c sensor for 50d

To get all the critters to fit in the frame of your 50D, you either have to move back, or fit a lens with shorter focal length. using 50d with 200mm lens to fill the frame

If the picture above left was taken at a focal length of 320mm on a full-frame digital camera, you can get all the same elements in the frame of your 50D using a 200mm lens -- see right.

This gives you a huge advantage if you're shooting sport or wildlife with your 50D, as you can use shorter focal length lenses and still get your subject to fill the frame. This means that a fast zoom lens like the Canon EF 70-200mm f/2.8L IS USM on the 50D is similar to using a 112-320mm f/2.8 zoom lens on a 35mm film or full-frame digital camera.

The downside is that wide angle lenses are effectively "lengthened" by 60% when fitted to your 50D. So the outstanding Canon EF 16-35mm f/2.8L II USM ultra wide angle zoom now mimicks a somewhat mundane 25.6-56mm standard wide angle zoom when used with a 50D, losing the ultra wide impact and "wow" effect.

It's important to remember that the crop factor still applies to Canon's EF-S lenses, introduced specifically for EOS digital SLR cameras. However, these lenses do provide owners of Canon 50D cameras with additional wide angle options.

Below is a line-up of EF-S lenses for use with the 50D, together with the equivalent focal length in brackets:
* Canon EF-S 10-22mm f/3.5-4.5 USM (16-35.2mm)
* Canon EF-S 15-85mm f/3.5-5.6 IS USM (24-136mm)
* Canon EF-S 17-55mm f/2.8 IS USM (27.2-88mm)
* Canon EF-S 17-85mm f/4-5.6 IS USM (27.2-136mm)
* Canon EF-S 18-55mm f/3.5-5.6 IS (28.8-88mm)
* Canon EF-S 18-135mm f/3.5-5.6 IS (28.8-216mm)
* Canon EF-S 18-55mm f/3.5-5.6 IS (28.8-88mm)
* Canon EF-S 18-200mm f/3.5-5.6 IS (28.8-320mm)
* Canon EF-S 55-250mm f/4-5.6 IS (88-400mm)
* Canon EF-S 60mm f/2.8 Macro USM (96mm)

You'll notice that all but the 55-250mm and the 60mm macro are between 10mm and 18mm on the wide side of the zoom range and are clearly intended to counter the crop factor and provide more wide angle options for users of Canon 50D bodies.

Even though both the EF-S 10-22mm f/3.5-4.5 USM and EF-S 17-55mm f/2.8 IS USM are highly rated by users, there are no professional class L-series lenses within the EF-S range. For professional photographers using 50D cameras this is a disadvantage, particularly if their work involves a good deal of wide angle shooting.

The opposite holds true for those who use mainly telephoto lenses, as all the L-series tele zooms and prime lenses when used on the 50D have the advantage of a 60% boost in equivalent focal length.

Below are popular Canon telephoto zoom and prime lenses with their equivalent focal length when used on the EOS 50D:
* Canon EF 70-200mm f/2.8L IS USM (112-320mm)
* Canon EF 100-400mm f/4.5-5.6L IS USM (160-640mm)
* Canon EF 300mm f/2.8L IS USM (480mm)
* Canon EF 400mm f/2.8L IS USM (640mm)
* Canon EF 600mm f/4.0L IS USM (960mm)