Will a higher ISO camera compensate for a "light pig" lens?

[buckeye42]

I'm a good amateur photographer (from an artistic perspective), but haven't invested a lot of time learning the finer points of digital cameras.

I use a Canon XTi + Tamron 28-300 (f/3.5-6.3), and want to upgrade the body and lens.

I'm trying to choose between the Canon XSi and D50. 

I'll probably buy the new Tamron 18-250 lens, but am willing to consider the Canon 18-200 instead.  Both are f/3.5-6.3 - poor performers in low light - but I like having only one lens on the camera.

Here's my question - will the D50's higher ISO capabilities compensate more for the relatively poor low light performance of these lenses than the XSi?

'Sorry if this is a dumb question, but I'd appreciate your help.

I'd also appreciate any comments on the Tamron lens versus the Canon.  I'd prefer the Tamron because of the additional zoom range, but not if it is optically inferior to the Canon.  I've been happy with the Tamron lenses I've owned in the past.

Thanks very much.

[Bob Atkins]

Actually the Canon EF-S 18-200/3.5-5.6 IS is f5.6 at the long end rather than f6.3, but that's only 1/3 stop so the difference isn't huge.

I'd be reluctant to buy a telephoto zoom without Image stabilization these days. If you use a tripod all the time you may not need it, but for general handheld use, image stabilization is such a big plus that it's worth paying extra for.

My alternative to the Canon 18-200 would be the new Tamron 18-270mm f/3.5-6.3 Di II VC . "VC" stands for "vibration control", which is Tamron's version of Image stabilization.

I can't really comment on either lens from personal experience. The Canon has received reasonable reviews and the IS system is said to be very effective. The Tamron has a significantly larger zoom range and costs about the same. I don't know how their VC compares with IS, but the very few comments I've read on the Tamron have been positive, suggesting effective stabilization and good image quality. It hasn't been available very long and I haven't managed to get my hands on one yet (but it's on my list of lenses to look at when I can get a copy).

My inclination here would be to go with the Tamron lens based on my previous experince with their lenses (which has been positive). They have a 6 year warranty vs. Canon's 1 year. The Canon lens has a shorter zoom range and neither the Canon doesn't have the advantage of a USM motor, so the Tamron seems to give you more for your money.

When you mention the D50, I assume you mean the new EOS 50D (Nikon have a D40, D60, D80 and D90 but no D50 yet!). The expanded ISO range is certainly useful with slower lenses, but don't forget that as the ISO goes up, the noise goes up and the image quality goes down. The 50D does give you a usable ISO 3200 wheras the XSi tops out at ISO 1600, but I'd be wary of expecting to shoot at ISO 6400 or 12,800 and getting good results. They are really more of "if you really need it, it's there" setting than setting you'd chose to use very often (which one reason they are only accessible via a custom function setting rather than being a "regular" speed).

I'd get the EOS 50D over the Digital Rebel XSi mainly for its additional features and the ability to shoot at higher ISO setting when all else fails. Of course it's more expensive and the XSi is certainly great value.