Can anyone let me know the basics of camera focal maths?

Question

In a lens camera, there are some writings inscribed on the lens like: 1.2:1 and x....
What does this mean? Can someone inform me fairly in detail?

Answer 1

There are usually 3 (sets of) numbers on a lens: the aperture, the focal length, and the diameter. These are technical specifications. Now, not only is reading about specifications tedious - it rarely clarifies anything important for day to day photography. But here goes:

Aperture tells you something about the quality of a lens and also how good it is for low light situations. Aperture values are written on the lens like 1:2.8 (also written as f/2.8), or for a zoom lens perhaps something like f/3.5-5.6.
The first bit, the '1:' or the 'f/' is just to let you know that what follows are the aperture values. The next bit, '2.8' or '3.5 to 5.6' tells you exactly what the aperture values are for that lens.
If a lens has just one number for the aperture, that's the maximum aperture for that lens. The lower the number, the better (and more expensive) the lens. If a lens has a maximum aperture of 1.4, it's insanely good for low light photography, portraits, etc. A maximum aperture of 2.8 is still very good. A maximum aperture of 4 is about average, and a maximum aperture of 5 (or more) usually indicates a pretty cheap lens that you should only use on sunny days.
If the aperture has two numbers (like 3.5 and 5.6) it's always a zoom lens. The first number (in this case 3.5) represents the maximum aperture when you're fully zoomed to wide angle, and the second number (5.6) represents the maximum aperture when you're completely zoomed to tele. With most zoom lenses, the maximum aperture changes when you zoom in. With expensive zoom lenses, you can get a constant f/2.8 however.

The next set of numbers are the focal length (the zoom range). The focal length is usually expressed in mm. A lens might be 17-85mm, for example. And again, a lens can have one or two numbers. All zoom lenses have two numbers (a starting point and an end point for the zoom range).

The third number tells you the diameter of the screw thread at the front of the lens (so you can attach filters). Point & shoot cameras usually don't have this option, so they might not state a lens diameter.

I hope this clears thing up a bit - if not just ask for more details in your question (and perhaps include the exact numbers on your lens).