Lenses - Focal length and angle of view
Camera lenses are normally referred to by their focal length (in millimetres). Most people understand that the shorter the focal length (lower number) the wider the field (or angle) of view. But focal length in itself does not determine angle of view. Lets start at the beginning.
Focal length is simply the distance from the optical centre of the lens to the image when focused at infinity, so that the optical centre of a 50mm lens is 50mm form the film or digital sensor when the focussing scale is on the infinity mark. But the reason why this is not the whole story regarding angle of view is because we have several camera formats. Actually, lets really go back to the beginning. Some of the first cameras (at least loosely) were in fact rooms (camera obscura meaning darkened room or chamber) with a hole facing outside - in essence a giant pinhole camera. Now imagine that you were in this darkened room. The focal length would be the distance from the hole to the opposite wall onto which the image was projected (upside down). Now lets say that you had a choice of different sized sheets of light sensitive material. If you pinned a big sheet to the wall you would record a wider field of view than if you chose to pin a smaller sheet to the wall. So it is with different camera formats.
Digital compact cameras have tiny sensors and therefore record from a tiny image area. Cropped frame SLRs have sensors similar in size to APS film (so smaller than 35mm). Full frame digital SLRs and all 35mm film cameras have an image area of 24mm x 36mm. There are both film and digital formats above this, and these are commonly used where high resolution imaging is needed. For the reasons mentioned above digital compact cameras with very small sensors need lenses of much shorter focal length to get an equivalent field of view to larger format cameras. As another example, a 90mm lens on a 35mm film camera is on the long side of normal, on a medium format film camera (say 6cm x 6cm) it is near to normal, but on a 5" x 4" large format film camera a 90mm lens is a wide angle. Same focal length. Different image area hence different field of view.
For a more in-depth description of angle of view you may wish to read this article aptly entitled "Angle of View" and for more on the implications of using different lenses how about reading "How to choose the best lens for a specific composition". More on lenses coming up soon.
Image taken in High Wycombe, December 2005 (the day after the Buncefield explosion)