Lenses and confusion

jackiex_x · Aug 28, 2011

argh help please.

I'm trying to get my head around the numbers on my lens and what they mean.

mine says 18mm-55

am i right in saying the 18mm is the focal length and thus a wide-angle lense? what then does the 55 mm mean please? is it right that you divide the 18 into 55 and therefore it's practically a 3x zoom? or does the 55 say something about the focal length also?

and... if my aperture is 3.5-5.6 on the lens even tho my camera body will do a lot more than that will my lens not allow that?

i am confused... please help thanks

analog.universe · Aug 28, 2011

18-55mm means it's a zoom lens, and yes, divide 55 by 18 and you get about 3x. 18mm is the focal length at the wide end, and 55mm is at the long end when you zoom in all the way. Aperture is determined only by the lens, your body can set the aperture to anything allowed by the lens. 3.5-5.6 indicates a variable aperture zoom, where the widest aperture is 3.5 at the short end and 5.6 at the long end. Some zoom lenses maintain a constant aperture throughout the range, and these are generally more expensive to produce.

clanthar · Aug 28, 2011

18mm is the lens focal length and so is 55mm. Your lens is a zoom; it can change its focal length over the range from 18mm to 55mm. And yes 18 x 3 = 54 so you have a 3X zoom lens.

In practice the values 18mm and 55mm don't tell you all that much until you pair them up with the size of the sensor in your camera. Your camera has an APS-C sensor. Now we know that the angle of view of your lens/sensor combination covers a range between 63 and 23 degrees. If you're standing in the street you have a maximum potential angle of view of 360 degrees -- a full circle. On any given camera we've customarily settled on 45 degrees as "standard" or normal (a lens focal length equal to the diagonal of the sensor). So 63 degrees is mildly wide angle and 23 degrees is mildly telephoto. You have what we call a normal zoom that encompasses the normal angle of view and reaches out about the same on either end.

f/3.5 - 5.6 is the maximum aperture of the lens. Zoom lenses come two ways 1. fixed aperture and 2. variable aperture. You have version 2 which is less costly to make. When the lens is at it's widest it can open to f/3.5. On the other end it can only open to f/5.6. On either end it is capable of stopping down to smaller apertures like f/11 or f/16.

The aperture is in the lens not the camera body. So the lens determines the range of possible f/stops. It's convenient to be able to have a linkage between the lens and body so the f/stop can be controlled from a wheel in the back.

Joe