http://www.adammillerrealtor.com/camera-lenses-explained/
How does the numbering system for CAMERA LENSES work?
I’ve been shopping for a film camera, and I don’t understand the numbering system for the lenses. They have two numbers separated by a dash, I’ve seen everything from 28-40 to 70-200, and all kinds of stuff in between. Can someone explain?
Also, if anyone knows, does this same numbering apply to film lenses as well?
These numbers are focal lenghts in millimeters. When everybody shot 35mm film these numbers had a kind of universal meaning regarding how much magnification a lens has. But with digital, that changes as dfferent cameras ave different sensor sizes (as opposed to all cameras having the same image area – a 35mm film negative). The result is that the focal lenght in millimeters does not tell precisely what the magnification of the lens is (unless you know your sensor size and can do the math)
Be that as it may…
When there are 2 numbers it is zoom lens.
The lower number represents the shortest focal lenght (widest filed of view) and the higher number represent the longest focal lenght or narrowest field of view).
A 28-40 lens is a zoom lens from wide angle to ‘normal’ (for a 35mm film camera)
a 70-200 is a zoom lens from low-power telephoto to medium power telephoto.
I like to have at least 2 lenses, but I don’t like large gaps in my range. So if you had a 28-40 lens and a 70-200, you are missing most of the ‘normal’ focal lenghts 40-70. But a lot of people have a high quality fixed focal length 50mm lens to fill the gap.
Again, the descriptions here are based on 35mm film cameras and lenses. When using the same lenses with a digial camera, things can be very different.
Photography Techniques : Selecting a Camera Lens