Going full circle (image circle and choosing a lens)
With PMA (the Photo Marketing Association exhibition in Las Vegas) just a couple of weeks away there are going to be quite a few new lenses announced (Canon and Pentax have already started the ball rolling), so it seems like a good time for a little piece here on the relevance of image circle to the format (sensor size) of the camera. Images produced by digital cameras are rectangular, yet the image formed by the lens is round. To get an image without any information missing at the corners you need a lens that produces an image circle at least as big as the diagonal or the rectangular image recorded by your camera. In fact since the image quality normally falls off towards the edges you need a lens to produce a circle that is a bit bigger than the format.
Now why should you care? You fit lenses to your camera and you get an image, right? That's fine until the time comes to buy a new lens. If you have a camera with a cropped frame sensor (currently all except a couple of Canon cameras) you have a choice of lenses that cover the full frame and those designed exclusively for the smaller format of most digital cameras. The lenses with the smaller image circle are no problem and are usually smaller and lighter (oh, and cheaper) than those designed to cover the 35mm format. It sounds like a good idea, and it may be, but you had better polish up your crystal ball, because you are making an investment in the future. What you need to decide is if you are likely to own a full frame camera, at least within the life of your lens, or be prepared to by a similar lens with a wider image circle when the time comes. A lens with a larger image circle will cover all formats up to it's maximum (the format for which it was designed). The smaller format lenses are restricted to that format or smaller.
Now you may wonder if this is such a big deal, as most cameras have cropped frame sensors. Go back to the crystal ball and find out if the maker of your camera system is going to introduce full frame cameras in the foreseeable future, and ask yourself if you are likely to be tempted by one. Larger sensors will get cheaper to make as the process of making them become still more reliable, and they may well find their way into cheaper cameras, and as we have seen, all cameras are tending to work their way down in price anyway. But cameras with cropped frame sensors have some very real advantages. Reduced size, weight and cost are all in their favour, and are likely always to be so, however far prices fall.
Photo: Lake district 2005