Ok,I just aqquired a digital rebel with the included lens. Now I am used to point and shoot digicams. What is all this 100mmx500mm stuff. Since I know what 10x zoom looks like in a normal camera, what does that equate out to in the algebra like values of slr lenses. I am looking for a somewhat affordable zoom lens to take out places, and am looking at getting the most bang for the buck. Are there any good zoom lenses that dont cost freaky prices???

Also with digital slrs there is a conversion factor that involves the size of the image chip/sensor.  My 20D Canon EOS Digital has a 1.6 factor.  What that means is that in "film camera terms" a 28mm lens would be 1.6x that aperture, or about 45mm.  A 70-200mm telephoto would translate to 112-320mm.  Then, as Paul said you can convert the magnification power and every 50mm equals a power of magnification, thus a 320mm lens is 6x optical.  Other Digital SLRs may have different size sensors and larger or smaller factors.  A point and shoot like my Nikon 885 with a 8-24 mm zoom would be equivalent to an slr lens of 38-114 mm.

ok, so If I want to count  the hairs in a rhino's nose at the zoo, what lens do I need???

sorry, but I am really new to all this lens stuff.

Take your camera to Wolf Camera or similar and have them let you test-drive several telephotos in the store. Good glass is very expensive.

A 500mm Mirror would be good - but that takes some skill to use.

On just your regular point and shoot cameras, 10x, could be different on two different cameras.  Because it is based on the widest focal length on that camera, for example of the widest focal length is 28mm, a 10x zoom would take it out to 280mm, but on another camera at 38mm wide, 10x zoom would be 380mm.  

So, 10x zoom can vary from camera to camera.  However, the XXmm focal length will be the same on any camera.  Except in the case of most digital SLRs where there is usually a crop factor, the Digital Rebel has a crop factor of 1.6, so basically multiply the mm on your lens by 1.6 and thats the actual focal length you are looking at through your camera.

you can learn a lot about the Canon lenses here:
Canon Advantage

If you are looking for an affordable telephoto lens, just stay away from the "L" lenses.

The larger ther mm, the farther you can zoom. The lower the aperture "f-stop", the tighter focus you can get. You should be able to find good zoom lenses under a couple hundred $$$ but the aperture will be in the 5 range.

Any EOS lens should work with your Rebel. I'd do a search on "Zoom Lens EOS" on Ebay.

Probably one of the best consumer grade all-around Canon lenses is the 28-105mm f/3.5-4.5 II USM.  BHVideo has it for $ 229.00.  The USM in the name stands for Ultrasonic Motor...allows for fast, quiet autofocus.  I had one and loved it.  Sold it off when I got the 28-135 USM IS (Image Stabilizer).  I also had the Canon 100-300mm lens before I got my 70-200 2.8L.  That lens only set me back $1400.00.  

Do some research before you plunk down money on lenses.  Some of the cheaper lenses are just that.  Remember, for the most part, the camera body is just a light tight film holder.  The quality of photos hinges largely on what type of lenses and techniques you use.  

how about this one

Quantaray - 135-400MM F4.5-5.6 Lens for Canon AF

and what the heck is this???


Quantaray - 600-1000mm Zoom Lens (T-mt.)

Quoted: Are there any good zoom lenses that dont cost freaky prices???

Depends on what you consider freaky, and that probably depends on how much you paid for your camera.

A good all-purpose zoom telephoto is a 70-200mm

The cheapest "good" one is about $500

the aperture (f-stop) dictates the amout of light the camera lets in.

here's a range of  f stops: f/1 f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32 (keeps on going)

f/1 allows in twice as much light as f/1.4 and f/1.4 allows in twice as much as f/2.8 (and so on)

Lenses with large apertures are preferable because they allow you to use faster shutter speeds. (slow shutter speeds = blur and shake)

freaky to me is anything over 2,000, and thats still way high. I am looking at staying around 5-600 range.

ok, here you go:

www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=148317&is=USA



You can find it for cheaper than that if you surf around.

Been playing with my 300D Rebel and the 28-135 mm.  Nice lens.  I like the lens.

I am looking and reading about telephoto lenses.  Better quality sharpness with a 70-200L mm than the 75-300mm USM.  Cost of the 70-200 is $570.  Still looking.  

Adorama and B&H Photo are good places to find lenses, both used and new. You can save alot of money buying used lenses. A 35-80 and a 70-210 would be two good lenses for the average consumer. Don't get the cheap lenses. Canon for Canon, Nikon with Nikon. There is a difference in aftermarket lenses. Pickup a Shutterbug magazine to see what's available.

Get the Canon 50mm f1.8.  It should be around $70 and you won't believe how sharp the images are.  On your Rebel that should work out to about 80mm, but you can't beat that lens for the price.

I would avoid the NY places like the plague--gray market stuff, missing accessories, bait and switch, etc.

The ONLY places I would do business with are KEH (out of Atlanta) and B&H out of New York.  You may find cheaper prices for new stuff, but you will get a much worse deal in the end if you go somewhere else.    KEH has excellent prices for used stuff and represents it fairly.

Digital Cameras are a bit different than traditional film cameras.  

As people have covered before, the focal length of the lens (50mm) indicates the magnification of the lens.  For Digitals,  1x magnification is about 35mm, so a 300mm lens is about 9x magnification and a 15mm lens is 1/2  x magnification.    Too Much magnification can be bad, as you get camera shake (rule of thumb is to use no SLOWER than 1 over the focal length of the lens (so a 300mm lens needsshutter speed of 1/250 or 1/500 or 1/1000, etc.  If you use 1/125 or 1/60 or less, you will start to get blurring from camera shake).

The other thing that is important is the aperture of the lens--the 'f' number.  This is how much light will get into the lens.   It is expressed as f stops, but they are basically a fraction.  If you have a lens that has  f 2.8, then it transmits (very basic explaination) 1 over 2.8 of the light that comes in the front of the lens (or about 1/3 of the light makes it through the lens to the film).   An f4 lets 1/4 of the light to the film,  an f1.2 lets almost all the light to the film.  The downside to low F numbers is cost.  An f1.8 lens might be $100, but an f1.2 in the same focal length might run $600 (they need a bigger body, better coatings on the lenses, etc to get all that light through with no loss).

Exposure is based on a combination of shutter speed and aperture--a 'correct' exposure for your camera in a given situation might be 1/500 shutter speed at f8 aperture.  You can vary this combination by say closing the aperture to f16 and changing the shutter to 1/250, or opening the aperture to f4 and increasing the shutter speed to 1/1000.  This gives some creative control (the first modification gives more depth of field to the picture--more is in focus, and the second modification would freeze the action better).  

So, you need to figure out what focal lengths (as in a Zoom Camera) you need for your photography, and the lowest f # you can comfortably afford.  

That being said, if you are new to the field, I would go with something like this:   Tamron 28-300mm lens
Use it for a while, and once you start getting comfortable and realize--"hey, all my pictures are in the 100-200mm (or 28-75mm, or 200-300mm, etc.) range, you can start to look for a lens more suited to your photography.  This lens has a different f stop rating for each focal length--at 28mm focal length, it has an f3.5 rating, but at 300mm it has an f6.3 rating (more light is 'lost' at the longer focal lengths due to the design of the lens.

AFARR

Well a lot of closes but no cigars.

50-58mm lenses are considered "normal" lenses for a 35mm camera because they most closely approximate the perspective as seen by a "normal" eye.  "Normal" lenses  for other size film vary.

The focal length of a lens is determined by the distance of the focal point in front of the film plane.  Think of a "cone" of light coming into the lens and being projected on to the film.That cone comes to a point in the lens and then cones out again to the film.  The picture on 35mm film is (IIRC) 24mm high by  36mm wide.  So as the focal point moves further out the lens moves from a "wide-angle" lens to a  "normal" lens to a short telephot and so on.  A "zoom" lens optically moves the focal point in and out from the film plane.

The f-stop is a measurement of the volume of light coming through the lens.  As noted the lower the value the more light, and each f-stop is usually about twice or half the adjacent setting.  In most slr lenses the f-stop is controlled by the mechanical expansion and contraction of an iris similar to the iris in your eye.  (Also slower films (needing more light for a given situation) are usually more color and detail accurate.)

Depth of field is the range of front to back/far to near in focus.  The lower the f-stop or wider open iris, the shallower the depth of field.

In slr lenses all of these thing are determined optically by the lenses and controlled mechanically by either the operator or in auto-focus and auto-exposure cameras by the electronics of the camera.  

On digital cameras many of the optical effects are produced by electronically manipulating the basic light pattern on the sensor. with minimal optical or mechanical manipulation of the light hitting the sensor.

BTW lenses are one of those areas where you generally get what you pay for.

Higher priced lenses have more elements (lenses/prisms) in the lens.  The lens element coatings promote transmission and minimize glare.  You know how a prism splits light into a rainbow?  Well lenses do that also,  so higher priced lenses take that phenomena into account so they are designed to take that splitting and condequential widening of the light beam and minimizes it or optimally brings eveyting all together right at the surface of the film.  If the lens is made with cheaper glass, fewer lenses, poorer coatings etc, then the image that finally hits the film isn't in as sharp focus, the colors can be slightly shifted off, etc.

Also better lenses have better designed mechanics.  the movement of one or more internal elements in and out  to focus and/or zoom is smoother, the alignment of all elements is precise, etc etc etc.

Oh and if you really want to get a really good count all the rhino nose hairs lens, you might consider a 1:1 macro.  

Quoted: The f-stop is a measurement of the volume of light coming through the lens.

Well, if we're playing close but no cigar....

the f/stop is the focal length of a lens divided by the diameter of the glass. It doesn't measure the amount of light, it shows a ratio.

So a 50mm lens with a diameter of 50mm would be an f/1.

This is why you don't see 300mm f/1 lenses....they'd be too damn big.

t/stops (transmission) measure the amount of light but nobody uses them much.

Avoid Quantaray like the plague.
For a good lens that won't cost you an arm and a leg, try the Canon 70-200 f4 L lens.  You can find them online for around $500 and they are incredibly sharp and not that heavy...no tripod required most of the time.