Quote Originally Posted by ham
The review mentions "stopping down" more than once, but I assume that this was a mistake and that the fixed aperture literally means you're "stuck" at f/2.8 and have to adjust light via shutterspeed alone.

Not at all. You can always stop down the lens. A 'fixedaperture' ('constant aperture' would be a better term) means the aperture doesn't change as you zoom the lens - the 17-55mm has an f/2.8 max aperture at 17mm, and it has an f/2.8 max aperture at 55mm. Conversely, the 15-85mm f/3.5-5.6 is a variable aperture lens (that's why the f/number is a range, not a single number) - it has an f/3.5 max aperture at 15mm, but an f/5.6 max aperture at 85mm (with stepwise reductions), A variable aperture is a compromise that allows zoom lenses to be produced more cheaply (because less glass is required for smaller apertures).


Avariable aperture has two consequences: one, the longer your selected focal length, the less light the lens allows in. So, with the 15-85mm lens at 25mm, you're max aperture is f/4 which may be enough light for your shot. But if you zoom in to 61mm or longer, you're at f/5.6 and only getting half as much light as you got at 25mm - mayne not enough. The other consequence applies ifyou shoot in manual (where you set both the aperture and shutter speed) - using a variable aperture lens in manual means you need to adjust the exposure every time you change focal length, whereas with a constant aperture lens, you don't.


Quote Originally Posted by ham
I suppose the question is if I go with the kit lens and the "nifty" to start with, how long will it be until I want to upgrade to a better all-round lens. What are the re-sale values of the kit lenses like?

It all depends on your impression of how well those lenses perform. If you're heppy with them, they may last you a long time... Resale value for them will be pretty low - despite the standalone price of the kit lens, anyone buying one used would know it's a kit lens, and not pay anywhere close to the high individual price of the lens. The high-end glass (which includes EF-S lenses like the 17-55mm and 10-22mm) holds value pretty well, the low end glass, not so much.


Quote Originally Posted by ham
My Dad's lenses are both non-IS, and I had read that the IS (II especially) is an improved lens from the non-IS equivalents

For the 18-55mm lens, the IS version is definitely an improvement. There's no optical difference between the EF-S 18-55mm f/3.5-5.6 IS original and MkII (cospetic changes only).


OTOH, for the Tamron lens, the VC version (VC is what Tamron calls IS) is optically substantially worse than the non-VC version, thus the recommendation for the non-VC version. If you want IS in that focal range (personally, I find IS very helpful!), the Canon 17-55mm is the way to go.