MikeScone
Mike - Camera Corner Mod
The numbers - like 18-55mm or 70-300mm - are the focal length of the lens - and, to be honest, I can't explain just what the phrase "focal length" really means in scientific terms. It doesn't matter, just know that the focal length is an indication of how the lens views the world. The longer the focal length, the more the lens magnifies an image.I feel silly asking, but those numbers mean nothing to me. What do they mean?
A "normal" lens is one which sees more or less what your eye sees, neither magnified nor (whatever the opposite of magnified is). If you look through the camera viewfinder when you have a normal lens, you should see exactly what you see when you take your eye away and look directly at the subject.
A "wide angle" lens has a focal length shorter than that of the normal lens, and therefore has a wider angle of view than a normal lens. It's like looking through a telescope backwards.
A "telephoto" lens has a focal length longer than that of a normal lens, and sees a narrower angle of view. In other words, it magnifies the image. To convert the focal length of a telephoto lens on a DSLR to the "2x" notation used by point-and-shoots, just divide the focal length of the telephoto by the focal length of a normal lens.
No need to understand where the numbers, as numbers, come from - it's kind of a historical accident that for 35mm film the focal length of a "normal" lens was about 50mm. So, for 35mm film, any lens with a focal length greater than 50mm is like a telescope ("telephoto"), and any lens shorter than 50mm has a wider view ("wide angle").
In digital cameras the actual focal length number for a "normal" lens isn't usually 50mm, because digital camera sensors aren't the same size as 35mm film (except on some very expensive DSLRs like the Nikon D700 or D3 or the Canon equivalent). Most DSLR's have "APS" sensors which are about 2/3 the size of 35mm film, so the effective focal length of a lens is multiplied by 1.5.
Therefore, the same 50mm lens which would be a normal lens on a film camera is actually about a 1.5x telephoto on a DSLR. For a point-and-shoot with a very small sensor, the same lens might be a 10x or more telephoto. That's why many people refer to "35mm equivalent" when they're evaluating lenses on digital cameras. It allows comparing apples to apples - a normal lens is 50mm equivalent, even if it's actually 35mm or 5mm.
A zoom lens has two focal length numbers, and it is capable of changing focal length ("zooming") continuously from one to the other.
OK, so what does that really mean? Let's look at some examples. For each, I'll give the actual focal length and the equivalent for film. All of these pictures were taken from the same spot, about 50' from the mailbox.
Wide Angles:
This picture was taken with a 10mm-24mm zoom, set at its shortest focal length of 10mm (15mm equivalent). That's a really wide angle, about as wide as you can get before you start getting distortions (think "fisheye"), and lenses shorter than 10mm tend to be very expensive:
(picture taken several years later than the following ones, with a new lens, so it might not have been from exactly the same spot - see my Ultra-Wide Zooms thread for comparisons of this picture with 12mm and 18mm shot at the same time)
This picture was taken with a 12mm-24mm zoom, set at its shortest focal length of 12mm (18mm equivalent):
There's a bunny in that picture - can you see it? Probably not. 12mm is not a good focal length for wildlife.
This picture was taken at 18mm (28mm equivalent), the lower end of most "kit" zooms:
Either of these wide angle lenses would be good for taking landscapes, or pictures of groups without having to back away too far.
Telephoto:
Telephoto lenses are good for bringing the subject closer, to isolate an element or let you get a frame-filling shot of an animal who is too skittish to approach closely.
This picture was taken at 50mm, roughly the upper end of the typical 18-55mm "kit" zoom (70mm equivalent). It would be considered a "mild" telephoto, 1.5x in point-and-shot terms:
Here's 100mm (150mm equivalent), the upper end of the more expensive 18-105mm zoom. It's a reasonably long zoom, about 3x:
At 300mm (450mm equivalent), the upper end of a 70-300mm zoom, you're really pulling wildlife close in - you can think of this as a 9x zoom in point-and-shoot terms. You can find zooms with longer upper ends, but they tend to be very expensive. Also, at much above 300mm you really should be thinking about using a tripod:
Finally, this is a 600mm mirror telephoto (900mm equivalent) - you really need a tripod for this!
The other number you'll see on lenses is the "f" number. The f-stop number is a measure of how much light the lens can gather. The lower the number, the more light it will let in (and hence the lower the light you can take pictures in). A typical "kit" zoom lens might say "f3.5-5.6" or something like that. A good normal fixed lens might be as wide as f1.4. But that's a topic for another time...
Last edited: