How does the numbering system for camera lenses work?

I've been shopping for a film camera, and I don't understand the numbering system for the lenses. They have two numbers separated by a dash, I've seen everything from 28-40 to 70-200, and all kinds of stuff in between. Can someone explain?

Also, if anyone knows, does this same numbering apply to film lenses as well?

These numbers are focal lenghts in millimeters. When everybody shot 35mm film these numbers had a kind of universal meaning regarding how much magnification a lens has. But with digital, that changes as dfferent cameras ave different sensor sizes (as opposed to all cameras having the same image area - a 35mm film negative). The result is that the focal lenght in millimeters does not tell precisely what the magnification of the lens is (unless you know your sensor size and can do the math)

Be that as it may...

When there are 2 numbers it is zoom lens.
The lower number represents the shortest focal lenght (widest filed of view) and the higher number represent the longest focal lenght or narrowest field of view).
A 28-40 lens is a zoom lens from wide angle to 'normal' (for a 35mm film camera)
a 70-200 is a zoom lens from low-power telephoto to medium power telephoto.

I like to have at least 2 lenses, but I don't like large gaps in my range. So if you had a 28-40 lens and a 70-200, you are missing most of the 'normal' focal lenghts 40-70. But a lot of people have a high quality fixed focal length 50mm lens to fill the gap.

Again, the descriptions here are based on 35mm film cameras and lenses. When using the same lenses with a digial camera, things can be very different.

I'm not going to repeat everyone else's answer as they have given you good answers. But I don't think they are really letting you understand just what wide angle and zoomed looks like to you.

In 35mm photography, a 50mm lens is considered a "normal" lens. This means it has a field of view much the same as you see with your eyes.

When you get below 50mm to say a 20mm lens, this is considered wide angle because the lens is actually "seeing" or taking in more landscape than you can see when you look at it the same scene with your eyes.

When you get into the longer focal lengths such as 100mm, this is called telephoto or zoom. I am sure you have looked through binoculars. You see things closer, but you see much less of the scene. Only a small part of the scene is viewed as compared to what your eyes would normally see, but, the object(s) is appears much closer than it really is.

So a lens such as 24-70 is a wide angle to moderate telephoto lens range.

Crop factor has been mentioned. Most digital cameras have a sensor that is smaller than the 35mm film frame size, so in effect it "crops" the field of view the lens "sees". This crop factor for a digital SLR is usually about 1.5. So a 24mm lens on a DSLR camera will give the field of view of a 36mm lens would on a 35mm film camera. (24 x 1.5).

Yes, there are some digital SLR cameras that have sensors the same size as 35mm cameras and so the lens functions "normally" on these cameras.

But since you state you are shopping for a film camera, you will not have to be concerned with sensor crop factors.

As for your "stuff in between", you are probably seeing aperture numbers. These often vary with zooming focal lengths. More expensive zoom lenses will have a constant aperture that will not change as you zoom the lens.

steve

The numbers are the focal length of the lens. The focal length controls how close and how wide things look. Numbers with a range (17-85mm, for example) are for zoom lenses - you can change how wide/far something looks on a zoom lens.

The numbers are the same for film lenses.

Film's expensive - get a cheap digital camera with replaceable lenses and you will learn a lot!

its the focal lenghts of the lenses

its in millimeters a meteric measurement, heres some conversions for you:

100mm = 4inchs

1inch = 25mm

look up focal lenghts at wikipedia or ask here.....what is a focal lenght

its the same on film or digi - however only the best digi SLRs are full frame so for anything else you will have a "conversion factor" or cropping

a

Simple answer... the lower the number the less "zoomed" it is.... very low numbers 12-28 are in the wide angle range.... high numbers such as anything over 200 would be long range... over 400 - super long zoom. (These are approximates abviously and this does not address the wide variety of lenses on the market).... this is a very rough definition.... consult competant salesperson for more details....

Basically, the larger the number the further away you can be from the object.

There you go.

http://farm4.static.flickr.com/3012/2349...

Here is a mini-tutorial I made myself to compare focal lengths. This is NOT a lens test or a camera test! It is merely intended to show the difference between various focal lengths. The lens was the Nikon 18-200 VR lens, which is (by definition) an 11X lens, but that 11X does not tell you what the final image will look like. I added one more frame taken with a 300 mm lens. The camera was a Nikon D200 so there is a 1.5X "crop factor," "lens factor," or "focal length multiplier." There is further explanation on the image itself. It would help if you click on "All Sizes" above the image.

http://www.flickr.com/photos/7189769@N04...

This was done with two different lenses: the Nikon 17-55 and 70-300:

http://www.flickr.com/photos/samfeinstei...

As far as film vs. digital, the FOCAL LENGTH is always exactly what the number says it is. That's physics. What happens to the image when it hits the film or the sensor depends on the size of the film or sensor. This is called the "crop" factor, but you will also see ti referred to as the lens factor or focal length multiplier. Here's a great little tutorial to explain that. There are a couple of samples that explain it almost without reading the text. http://www.dpreview.com/learn/?/Glossary...

In my samples, you will see that I have used two numbers. One is the true focal length of the lens and the other one, in parentheses, is the "35 mm equivalent," which takes into account this crop factor.

Pooky has posted his sample, which is excellent.