I talked about crop factor in the previous installment, it’s the reason why an identical lens mounted on a camera with a smaller sensor will appear to have a local focal length. I thought that a diagram might help clarify things further, so let’s try this. Imagine you’re looking straight down the lens onto the sensor, mounted at the back of your camera.
On the left you see the sensor, represented by the yellow box. You can see that the image projected onto it by the lens is a little larger than the sensor – this is quite normal. Whatever does hit the sensor becomes your final image, shown on the right. Now imagine the sensor is smaller – the lens is exactly the same, so the projected image is the same, but less of it will hit the sensor. The final image is also the same size (we’re assuming that both sensors have the same resolution – the same megapixel count). End result? The final image looks like it’s taken with a longer lens, it’s been magnified. The crop factor is a measure of how much smaller the sensor is, and thus how much longer the lens appears to be. It’s called the crop factor because it has the effect of cropping (trimming) the image.
Hopefully that makes things a little clearer, if you have any questions feel free to leave them in the comments!