I said last week that sometimes angle of view (and in lenses this is the same thing as saying focal length) can be as important as image circle and optical resolving power when choosing a lens. To demonstrate this, I need nothing more than a smiley face and some lines.

Above is a poor artistic rendering of a background, a subject, and a camera. In this example the subject and photographer are close to each other, forcing the photographer to shoot at a wide angle. When this is done the line of sight extends beyond the subject and views more of the background.

Please note that this means a focal length is, in practice, an angle of view. If the photographer steps back several paces she has to zoom to more telephoto focal length, which means a smaller angle of view.

The subject might be the same size within the frame, but the photos will still look different. The telephoto lens will render less background despite being placed further away. It will also make the background appear like it is closer to the subject. In the world of optics we call this effect perspective distortion and it has a very real effect on your photography.

Here’s the key: while the crop factor we discussed previously gave a full frame lens on an APS-C camera a more telephoto effective focal length, this background rendering relates to actual focal length. In other words, the way background renders with a 50mm lens on a full frame camera is the same way it will render when that 50mm lens goes on an APS-C camera. So it’ll have more zoom, but the way it grabs and renders background will remain that of a 50mm (in particular here, I am talking about background defocus). So when we discuss how focal length changes depth of field, we mean actual focal length - not effective focal length on an APS-C camera.

This is why in the image above you see almost no background and what you do see is the same plant the flower came off of and is very out of focus. This was shot at 90mm. Some photographers will shoot with a wider angle to record more background within the frame. Others will shoot more telephoto to see less and have it more blurred.

Because of the way the crop factor works, the macro lenses that are full frame compliant (have full frame image circles) are the longer focal lengths. They tend to be 60mm and on up. So they have shallower depth of field anyway. If you shoot with an APS-C camera, these might have your better optical quality, but they might render background in a way you don’t like. Or you may want an APS-C lens so you use all of the available glass.

Next week we'll come back to the image circle and sensor size and examine how much detail is lost by putting full frame lenses on APS-C cameras.