Understanding Canon’s EF-S Lenses

I’ve been meaning to write this post for over a year now and finally got around to it after reading so much misinformation on the subject.

What is this Field of View Crop Factor (1.6x FOVCF) everyone keeps talking about and how does this affect my lens choices for the 50D?

As you know the sensor in an EOS 50D is much smaller than the full frame sensor in the new EOS 5D Mark II camera. The physical focal length is a optical measurement of a lens and does not change just because you mount it on a 1.6x FOVCF camera like the 50D, but the field of view the lens exhibits certainly does.

For example, if you are looking for a field of view that a 50mm lens provides on a full-frame DSLR body like the 5D Mark II, you’ll probably want a 35mm lens on your 50D since 1.6 x 35mm = 56mm. The lens is still a 35mm lens, but the final image captured by your 50D will only include a crop of the lens’ complete image.

Bird photographers like me really love the benefit of using high crop factor (1.6x) DSLRs like the 50D since we can achieve tight subject framing from a greater distance or from the same distance with a shorter, less expensive lens. Using an EF 500mm f/4 L IS USM telephoto lens on a 50D yields the same field of view as an 800mm f/4 IS USM lens would on a 5D Mark II.

So where does the EF-S lens fit in this?

Canon developed the EF-S series lenses (the “S” stands for short back focus) with the rear element of the lens closer to the image sensor than on the EF series lenses. They also matched the image circle of these lenses to the APS-C sensor size. This design enables EF-S lenses to be made smaller, lighter and less expensive. A perfect match for their consumer and prosumer grade DSLR cameras.


Canon EF-S lenses are designed specifically for the 1.6x FOVCF DSLR bodies but still require the same 1.6x crop factor to be applied as the standard Canon EF Lenses to get the equivalent field of view comparison. Again, this is because the physical focal length of the lens is the same, regardless of which camera it’s mounted on.

The EF-S 10-22mm f/3.5-4.5 USM is a great example of a well designed EF-S series lens. It provides a field of view similar to what Canon’s popular EF 16-35mm f/2.8 L II USM and EF 17-40mm f/4.0 L USM do on a full frame camera like the 5D Mark II.