Hi all! I'm after some general advice about buying and selling second hand lenses, and lens depreciation.

How quickly do lenses depreciate?

I bought a Tamron 17-50mm f2.8 VC for the mid $700's in June this year. By late September it was selling new for around $500, and I sold it for $445. That's quick depreciation! Maybe this one depreciated so quickly because I bought it when it was recently released, and then it got a bunch of bad reviews...

Another example: Over the last few months I've been eyeing the Canon 24-70 f2.8 L. I haven't been looking real closely, but I think a few months ago good 2nd hand one would sell for around $1600. I think I could get one for $1100 or $1200 now. Does that sound about right to you?

So... is this depreciation normal, or just specific to these lenses, or am I imagining things? How does depreciation compare between Canon / Canon L series / 3rd party lenses?

I'm asking because I'm likely to be buying (and maybe selling) some 2nd hand lenses soon. I'm particularly interested in getting a 70-200mm. I think, eventually, I'm going to want to go all the way and get the 70-200 f/2.8 L USM IS MkII, but I'm trying to hold off for a while till I can actually afford it. If I were to buy a second hand 70-200 f/4 IS or f/2.8 non-IS, would it have lost much value in, say, 6-12 months?