meaning of the labelling of lenses ?

I am trying to understand what the numbers on the lens actually mean : if you take a lens you have the size and the f-stops... I do know what a f-stop is but why do they specify it...
For instance, what's the difference between a 100mm - F4 lens and a 100mm - F2.8 lens ? Is that because on the lenses the f-stop ranges in the first case from f4 - 22 and in the second case f2.8 - 22

I know it's maybe a silly question, but it's something I don't get.

Rosa.

Hi Rosa,
The f stop relates to the maximum aperture of the lens. In the case you qoute the F2.8 lens will let in twice as much light. This means you can use twice the shutter speed or a slower film. This is particulary useful in low light situations. On a "normal" well lit day, there would be no difference in the exposure. However, the faster the lens the more expensive it is to make and quite often the wider aperture lenses are better performers as a result.

The scale is logrithmic and goes 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32. each time you go up one of these numbers, the max amout of light halves. Some lenses will have half stops and these will be between these figures. The aperture will also have an effect on the depth of field. the wider the aperture, the shallower the depth of field. For portraits, a shallow depth of field,ie wide angle will tend to isolate the subject whilst a narrow aperture may be more useful in landscapes to make sure everything is in focus.

Hope this helps

Kim

Thanks kim : )