Understanding camera lenses?

When comes to first time buying a digital SLR camera, I am not sure what kind lens I should buy. there are too much choices out there. what are the lens specific # mean, like mm and f #? I know that I'd like a zoom lens, maybe a wide angle lens. but what the mm# and f# i should buy?

I am thinking to buy the Nikon D40x digital SLR camera. Any comment or suggestion would be great appreciated.

Lets take one for example:

Nikon 28-70mm f/3.5-4.5 AF-D Lens

The numbers 28 - 70 tell you what the widest ranges of the cameras focal length are. In practical terms, if you and your subject (I'll assume a person) stand in the same positions but you take the picture at 28mm and then at 70mm they will be different, even though the two of you didn't move. The smaller the number the more of the background around your subject will be in view. The larger the number the closer you will appear to your subject.

The f/stop (f/3.5-4.5 in this case) tells you the maximum aperture for this lens. Aperture is basically how wide the diaphragm in the lens can open. The smaller the number = more light. Pretty much all lenses will close down to f/22 so the high end is what is important. Aperture also affects depth of field in a picture. The reason there are two numbers for the lens above is because the lens can open to 3.5 at the lower focal length (28mm) but only to 4.5 at the higher focal length (70mm). Basically more light that comes in this way means you can have a faster shutter speed.

The lens above is a good general purpose lens. If you are shooting wildlife or sports you would want a longer focal length. If you're going to shoot weddings you might like an image stabilizer and a larger maximum aperture. It just depends on what you are using it for but the lens above is a great lens for all around use.

if you're just starting out and have the money to spend, I would suggest buying a 15mm-150mm+ lens. It will give you the freedom to work on close-up, portrait and telephoto photography without having to remove lens. that way, you can adjust with the camera.

personally, I think the Canon 360xi Rebel is a far more superior camera than the Nikon. Just my opinion, though.

In optics, the f-number (sometimes called focal ratio, f-ratio, or relative aperture[1]) of an optical system expresses the diameter of the entrance pupil in terms of the effective focal length of the lens; in simpler terms, the f-number is the focal length divided by the aperture diameter. It is a dimensionless number that is a quantitative measure of lens speed, an important concept in photography.
There are three general types of lenses for a camera: normal or standard, wide angle, and telephoto. The focal length of a normal lens for a 35 mm SLR camera is approximately 50 mm. A standard or normal lens produces a picture with a perspective similar to the human eye. The focal length of a wide angle lens is any measurement less than 50 mm, but is typically 28 mm. A wide angle lens makes things appear smaller and distorts the view if the object is too close to the camera. The focal length of a telephoto lens ranges from 60 to 1000 mm. A telephoto lens magnifies the subject while at the same time narrowing the field of vision.

DX cameras like the D40x have a cropped sensor. What you need to do is take whatever "number" you see on the lenses and multiply it by 1.5X. So a 28-70 lens that you would use on a film camera or a full frame digital camera becomes a 42-105 lens.

A wide angle lens is considered to be in the range of 24-35.
An ultra wide angle lens is in the range of anywhere from 10-24
A normal lens is considered to be 50mm (full frame)
A short telephoto is around 85 or 105mm
A longer telephoto is 200mm and 300mm
Really long telephotos are 400+mm

The smaller the number the greater your field-of-view. The smaller the number, the shorter your field-of-view.

F numbers relate to the aperture. 2.8 or smaller gives you a "bigger opening." A number larger than 2.8 gives you a "smaller" opening. The smaller the opening the less light that hits the sensor. 2.8 lenses are always bigger, heavier, and more expensive. You would have to read a wikipedia article on f-stops to understand how the numbers relate to DOF, bokeh, speed, etc.