I suggest you get a real macro lens rather than a macro zoom lens. You will be much happier with the results. I would also suggest you get a 90mm, 100mm, or 105mm macro lens as they are good herp photography lenses.
You choices for the D50 include the
- Nikon Micro Nikkor 105mm $659
- Nikon Micro Nikkor ED-IF VR $829
- Sigma 105 EX Macro $399
- Tamron 90mm Di Macro $489
- Tokina 100 ATX Macro $399
If money is no object, I would buy the ED VR Nikkor. I haven't seen any reviews of it, but the ED lenses are usually tack sharp and the VR would be a great bonus.
Comparing the other four is less clear.
If you look at photodo.com which rates lenses based on their optical performance you see that the Nikkor and Tokina lenses score 3.9, the Sigma gets 4.1, and the Tamron scores 4.3 (higher is better). Few lenses ever score more than 4.5 on this test so 4.3 is an excellent score. (The low score of the Nikkor 105 Macro is quite surprising since there are usually the sharpest lenses in most companies lines. A low score is atypical for Nikon who normally make sharp lenses).
Somewhere else online there is a site that compares these lenses that also picks the Tamron over the Nikkor and Sigma lenses.
I have the Tamron and can say without a doubt it is TACK sharp and a pleasure to use. Of course, people with the Sigma say the same thing. I don't think you would be unhappy with either of these lenses (or the Nikon). But if I was going to shell out $650 for the Nikon, I might be tempted to shell out the extra $125 and get the ED version.
Here's a shot with the Tamron -

If you want to take head shots of hots without glass in the way, I suggest you get a good 200mm macro, but now you are talking about significantly more money.
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Chris Harrison
San Antonio, Texas