A phylosophical question regarding lenses (so to speak...)

[Seb]

Hello,

As some of you may already know, I have been involved into photography for nearly two years now, using a simplistic Nikon F65/N65 with the kit lense (28mm=80mm, 1:3.3-5.6 G type lense).

I have now reached the point where I will switch to digital (the Nikon D70, D100 and Pentax D *ist being my 3 possible choices). I want to take advantage of the fact that I am going digital to ugrade to a better quality system rather than simply getting the digital counterpart of my current system. The 3 bodies I am looking at all represent a significant improvement over the F65 but things are more complicated when it comes to lenses.

Chances are that I will "stay true" to Nikon and I was looking at their digital specific DX lenses. The 17-55mm zoom with ED elements appeared to be the obvious choice until I found out that it was a genuine pro grade lense and that it was twice as expensive I assumed it would. On the other hand the 18-70mm zoom should be way cheaper but I doubt that it would offer a significant optical improvment over my current entry level G type lense. Finally, a third and less obvious choice could be the Tamron SP AF 28-75mm f2,8 XR DI. Choosing a third party lense doe's not naturally come to my mind, especially considering that I am fairly clueless about third party brands. However, I have read a rave review about this lense in a recent magazine and reviews right on this site are highly positive as well.

Now here's the ultimate question: consumer grade lenses against pro grade lenses, is there a signigicant difference in optic quality? It seems to me that there are two clans here: people who consider that pro grade lenses simply offer a beefier, more durable built (the difference in optics being marginal at best) and people to claim that pro grade lenses do offer a significantly better optical quality. I obvioulsy don't want to compromise on the optics yet I am not sure that I can justify spending big money on a top lense.

Any thought here would be much appreciated!

regards

Seb