question about focal lengths

My canon P&S cameras (G2 and S410) say something like 7.2mm - 22mm on the front of the lens-they are both 3x zoom. Does that mean that is the focal length? So if I had a 18-55mm lens on my 350D with the zoom set to 50 it would be just over twice the "zoom" of the highest zoom of the p&s (not accounting for the crop factor).

Not necessarily -- P&S cameras work differently. I think the relationship to the 35mm world is more in the range of 35-105mm lenses. The lenses in P&S cameras are in a different relationship to the sensors than the SLRs, so the magnification changes a lot from what the same size lenses would give on a SLR.

Your 18-55mm lens set to 50mm is probably equivalent to the P&S lenses zoomed to about 1/3 or half.

-my A75 is 37mm to 110mm in 35mm equivalent (taken from exif) - so your about right (and that has 3x zoom)

Leo

A nation divided by a common language.

However you can't not account for the crop factor. The G2 lens is equivalent to 34-102 at full frame while the 350D's 18-55 is equivalent to 29-88. At 50mm it is equivalent to 80mm.
Elie

Thanks. The thing is, I want a zoom lens but want at least what I had with my p&s cameras as far as reach. I guess the thing to do is take a picture at full zoom with my G2 and then take the same pic as close to the same as possible with the XT and see what the zoom is. I was going to save for the 28-135 lens because I want more zoom than my 18-55, but don't want a 200mm zoom (I can barely handhold at fast shutter speeds without blur). But I saw a few things about the 28-135 that showed it might not be the one to get (especially at that price) so I was thinking of getting the 28-105 f/3.5-4.5 instead. But I want to make sure that 105 is at least what I'm getting with my G2 at 3x zoom. Like I said, I will try some experimenting and see.

OK I took a picture of an object with my G2 at full 3x zoom, and the same (from the same spot) with my XT at 55mm and it's very close. If I could have zoomed in a little tiny bit more with the XT it would have been the same. So if I get the 28-105 that I want, it'll still give me much more zoom than my p&s cameras.

Two lenses you might want to consider are the Canon 17-85 or the Tamron 28-75 f2.8, which is an excellent lens optically and has a faster max. aperture (35mm equivalence = 45-120). Also, keep in mind that the 350's 8 Mp will allow you to crop without much loss of quality.

The Powershot G2 has a zoom lens that has a focal length from 7mm to 21mm, and its maximum aperture at 7mm is f/2.0. The maximum aperture at 21mm is f.2.5.

The manual for the G2 says that the field of view is equivalent to a 34-102mm zoom on a 35mm camera. That would make the "crop factor" for the G2 approximately 4.9. Extrapolating, that would suggest that the size of the sensor in the G2 is about 4.9mm by 7.4mm - a very tiny sensor compared to our DSLR's.

The "3X" zoom means only one thing. That is the ratio of the longest focal length to the shortest. Divide the 21mm (longest focal length) by 7mm (shortest focal length) and you get the 3(X). The "3X" tells you NOTHING about the actual range of the lens' focal length, only the amount that the particular lens can zoom from its shortest to its longest focal lengths.

If you are looking for a lens for your 350D that will give approximately the same field of view as the lens on the G2, you can simply use the stated 35mm equivalent of 34-102mm for the G2 and apply the 1,6 "crop factor" for the 350D to determine the equivalent focal length range for the 350D. Divide 34-102mm by the 1.6, and you will arrive at about 21mm for the short end and 64mm for the long end. In other words, a 21-64mm zoom on the 350D is roughly equivalent to a 34-102mm zoom on a 35mm camera. This is also equivalent to the 7-21mm focal length on the G2.

To have a truly equivalent lens, the maximum apertures would have to be the same. Truth is, you aren't going to find a zoom lens for the DSLR's that is as fast as the lens on the G2. However, the ISO settings in the DSLR's will compensate for the difference. Remember that the G2 has a minimum ISO setting of 50, while all the Canon DSLR's start at 100. However, to get a truly equivalent lens (equivalent in focal length and speed) you'd need something like the EF 24-70mm f/2.8L lens on the 350D. You will need to compromise if your wallet can't handle the 24-70. The optical and build quality of the 24-70 is so much better than the lens on the G2, of course, that it's worth the price in my book for use on my 20D.

When originally looking into purchasing my DSLR I thought that I wanted the same zoom range as my P&S. After using the XT for about a month now, I don't recall what the zoom range is on my P&S. You will adjust to whatever lens you decide to get. I agree that you will want a fast lens so look at getting the 24/28-70/75 f/2.8 lenses that wither Canon, Sigma, or Tamron make. I chose the Tamron because I found a used one for a very good price. I have no regrets. Sometimes I would like a wider lens so I switch to the kit lens. Sometimes the tele end isn't long enough, but unfortunately I don't have anything longer at the moment. Fortunately with the 8MP of the XT I can crop and still get a useful image.