On 21 Jan 2003 22:27:44 -0800, rinfl0@yahoo.com (florin) wrote:

>I've been trying without success to find some *basic* information

>on camera lenses -- like, what do those numbers on a camera lens mean?

>(for instance, what is "2-2,4/9,7-48,5" ? Is it better

>than "35~70mm, 1:3.5~4.8"?).

>

>All you can find on the web is so-called tutorials going on and on

>about "long lenses" or "short lenses", or "zoom lenses" whatever the hell

>these things mean.

>

>Now, I understand the basics of thin lenses and geometrical optics and

>my math is not bad.

>

>I'm not looking necessarily for free, web-available information, I am

>ready to spend money on books, but for God's sake, are there any books

>or web sites that do *not* assume that you already know everything about

>cameras, and also do not assume that you are scientifically illiterate?

>

>Many thanks in advance for any help!

Try www.nikonlinks.com/ruether/posts - but to

get you started, "short lenses" are "wide angles",

and have a focal length shorter than the diagonal

of the format frame (for 35mm, the format frame

is 24mm x 36mm, with the diagonal being *about*

45mm) - so 20mm, 28mm, 35mm, etc. lenses are

shorter than the frame diagonal, and therefore

they are "wider" in coverage (you can work out

the simple geometries involved, assuming the lens

is a pinhole of no diameter...) than a "normal"

lens of about 45-55mm; a "longer" lens has a focal

length greater than the diagonal of the format

(same units, of course...) - so 85mm, 105mm, etc.

lenses are "long", and are often called "telephoto"

(though this is often incorrect, since this word

applies to a specific optical design); a "zoom"

can change FL, as with 28-70mm, 35-200mm, etc.

The other number is the lens "relative aperture",

which indicates its "speed" (f2 lets in more light

than f4.5, etc.), and is determined by dividing the

FL (measured at the lens optical center, with the

lens focused at infinity) by the aperture diameter

(as in, a 50mm f2 lens has a maximum aperture

diameter of 25mm...). Bottom line: the "f-stop"

tells you the relative light-passing ability of

the lens (smaller number passes more light), and

the "FL" tells you how wide or narrow the field

of view is (smaller number is wider); neither tells

you anything about the image-quality of the lens...