There's a lot to know about lenses but its definitely worthwhile learning about.

In a nutshell, the larger the diameter of the lens and the shorter it is lengthwise, the more flexible it will be allowing you to shoot in low light conditions.

However that's not the whole story... there is also the issue of optical construction quality inside the lens but thankfully there is an easy way to tell if a lens is good or not just by looking at it.

Lenses are always rated with numbers that are usually located on the end of the lens barrel near the glass. There is a focal length rating and on a typical zoom lens might say something like 33mm - 105mm. This is a wide angle to telephoto rating and indicates how wide on one end and how far on the other the lens can zoom in or out.

Here's what the numbers mean:

Less than 21 mm = Extreme Wide Angle
21-35 mm = Wide Angle
35-70 mm = Normal
70-135 mm = Medium Telephoto
135-300+ mm = Telephoto

eg. our mentioned 33mm - 105mm zoom lens will be able to cover everything from wide angle to medium telephoto photography.

The other rating a lens will have is it's F stop rating, usually printed on the end of the lens barrel near the focal length. On a typical zoom lens like the ones found on most digital cameras it will look something like this: F3.5 - F5.6

The F stop rating is a measure of the lenses ability to capture light. The lower the numbers the better the optical quality of the lens and the greater their ability to capture an image in low light. In the example of F3.5 - F5.6, the 3.5 part (the lowest of the two numbers) is the rating of the lens at wide angle and the 5.6 is the rating at full zoom. Typically F3.5 at wide angle is only an average sort of lens where something like F2.0 is brilliantly fast and something like F1.4 is a super expensive pro grade lens.

* Remember * ... Focal Length is rated in mm, Low Light ability rated in F-Stop, The lower the F-Stop number the better the lens is.

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