It may be easier to recommend a particular lens if I knew a bit more as to what your goal is or what you want to accomplish.

I have a Nikon D5000 and a D7000 and use both Nikon and off-brand lenses. The make is more personal preference as Canon does a good job with their Rebel line.

Here is an article you may find useful, otherwise let me know if you have further questions.

http://www.nikonusa.com/Learn-And-Explore/Nikon-Camera-Technology/gnhy8b3m/1/Macro-Lenses.html

*I've

What are you doing that you need micron measurement for? Is it for a specific project?

I use an old manual focus lens on my Nikon, which works for what I need it for.

I found this: http://eosdoc.com/jlcalc/, but I don't really find it intuitive to use.

I am trying to make a cheap microscope with the webcam and macro lens. I know there are USB webcams out there. But the lens quality and field of view are pretty limited. I thought a macro lens might provide an excellent solution.

I was thinking of using the webcam because they are so fast at data transfer, whereas modern cameras have image download rates that are pretty limited.

I guess it depends on what you are looking to accomplish. Webcam images will be small and not necessarily the highest resolution. If you need cheap, you probably know that already. I'm not sure that chasing a 35mm SLR lens will help you achieve "cheap." Some of the lenses alone for a dSLR like what Critical_d mentioned are (awesome! but) $400+ and only produce a 1:1 ratio image. There are plenty of actual microscopes on ebay under $100. Or you can even find digital "toy" microscopes that actually work pretty well new for not much more.

Is this the kind of thing you are thinking of building?
http://www.bhphotovideo.com/c/product/715240-REG/Avangard_Optics_AN_E500_AN_E500_eScope_500x_USB.html

Check this out...

http://www.instructables.com/id/Simple-Super-MacroMicroscope-Webcam-Conversion/

http://blizzard.rwic.und.edu/~nordlie/microscopy/