You know those numbers on the side of that lens you just bought for your digital SLR? 70-200mm … 50mm … etc? Those numbers are a lot less absolute then they used to be?

The reason? Not all image sensors on digital SLRs are built the same, and the sensor you have has a large effect on exactly what focal length your lens actually is. Here’s why.


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Back in the era of film cameras, SLR cameras had a standard, full-frame sensor. But with the advent of digital cameras, sensor sizes began to change. Sensors on the first generations of digital cameras were expensive to produce, so camera manufacturers simply made them smaller. They created the APS-C sensor, which has become the standard on most digital SLRs.

What was the effect? An APS-C digital camera situates the lens closer to the sensor and introduces a crop factor - typically around 1.6x. What this means is that the operative length of the lens is altered as well. A 200mm lens becomes the equivalent of a 320mm lens, a 20mm becomes a 32mm and so forth.

What types of cameras have an APS-C sensor? All entry level SLRs like the Canon 30D, the Canon Rebel XTi and the Nikon D40 have an APS-C sensor. Only a few, select high-end consumer cameras like the Canon 5D use a full-frame sensor now, although Canon has said that they plan to re-adopt full-frame sensors in most of their digital cameras in time.

What are the advantages and disadvantages to a APS-C sensor? For the photographer who loves shooting telephoto, a cropped sensor can be a good value. For example, the Canon 300mm F/2.8L lens costs upwards of $3000, while a photographer with a APS-C sensor can use the 200mm f/2.8 prime and get the same focal length equivalence for around $700.

Where the APS-C sensor really hurts is in wide-angle photography. It’s tough to get that sort of dramatic wide perspective from a cropped sensor - or it will cost you a lot to do so.

From a technical standpoint, images from a full-frame sensor are usually less noisy and suffer from less chromatic aberration. However, full-frame sensors - especially on the Canon 5D - often fall victim to vignetting, as the sensor does not receive enough light on the corners of the frame.

Is an APS-C sensor a bad thing? No, and many photographers find it preferable. But the full-frame sensor carries with it many benefits as well, benefits that hopefully won’t be limited to high-end cameras in the coming years.


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