Lenses and sensor sizes

In “When one inch is not one inch” I discussed how to interpret quoted sensor sizes and hinted that this links to lens selection. Let’s take a minute to explain how.

As you’ll have noticed, lenses are cylindrical, which means they actually project a circular image. If the image is smaller than the CCD sensor you’ll get dark corners. (This is termed vignetting.) On the other hand, if the image diameter is larger than the sensor diagonal you’ll lose some of the image (and your field of view calculations will be wrong.)

Fortunately, the lens manufacturers have made our lives easier by specifying the sensor size for each of their lenses. Thus you’ll see a lens described as 2/3” format, for example.

So, when you’re figuring out what lens you need, you’ll need to know the specification of the sensor in your camera. Armed with that info, you’ll then be able to make use of the various lens calculator programs on the web.