The "Sigma 17-70mm f/2.8-4.5 DC Macro AutoFocus Super Wide Angle Zoom Lens for Canon EOS Digital Cameras" you linked is not a macro lens. It might work for your general day to day but NOT macro. With a maximum magnification of 1:2.3 it isn't even close. It is IMPOSSIBLE to crop to 1:1. Cropping is a vaguely-related but non-relevant topic. It will always be 1:2.3 without heavy modifications to the lens itself. In your camera you have an image sensor which is effectively digital film. Think of a classic roll of 35mm film. Those yellow kodak tubes with the brown squares that are ruined if light touches them. A 1:1 image states that the coral polyp you are taking a picture of is the same size projected onto that little square as it is in real life. When you develop the film and make a 4x6 print, the data on that film strip is blown up and now it looks much larger than it does in real life. This is the definition of macro photography. 1:1 is either achieved or not achieved when you press the button that goes ::click::. No amount of work on your computer after the fact can change that.
"How well will this lens work to take good up close macro shots of nano tanks, corals, inverts etc?"
This lens will not take "good up close macro shots of corals", but it will take a picture. You can take the soccer van to a local race track and drive it around the course, but you can't expect to complete with the guys who have cars designed to race on a track.
"Now mind you, this lens isnt going to be soley just for macro shots & I can only afford 1 lens right now."
Perhaps you should re-consider buying a DSLR. Using multiple lenses for different effects is the name of the game. There are many nice point and shoot cameras on the market capable of regular shots and macro. They won't do it as well as a properly equipped DSLR, but many will do it better than a poorly equipped DSLR. Check out the Canon G-11 for instance. I might also recommend a cheap prime, such as the 50 f/1.8 to get you buy until you can afford more lenses (it's super cheap). My 24-70 f/2.8 was stolen (grrrr) and I am getting by okay with a 50 f/1.4. There are shots I miss without the 24-70 and I miss it dearly but I am getting by. I am shooting a space shuttle launch in a few hours for instance and wish I could go wider, but many day to day shots can be tackled by a 50mm lens.
If you go the DSLR route, you are going to have to make a choice:
Day to day shots...or macro shots. The 17-70 "macro" lens you posted would work alright for day to day shots. The 90mm macro amorealex mentioned will work for macro shooting. Either can do the other's job but neither does it particularly well.
If you choose to go the Macro route I would recommend the Canon 100mm f/2.8:
http://www.adorama.com/CA10028AFN.html?searchinfo=canon+100mm+f/2.8
This is what the great majority of Canon DSLR users on Reef Central use.
The only advantage I see with the Sigma is the available f/2.8 at its widest focal length. Although I am trying to think how often I shoot wide open at 17mm... I think I shoot mostly around 50mm in available light... In that case spend another $50 or so and get the Tamron 17-50mm f/2.8 across the range. :thumbsup: But now were just going off tangent here, just get the 28-135mm with the body and save up for a macro. Unless you want to spend an extra couple hundred bucks and get a lens you can keep for a long time, then get the Tamron 17-50mm f/2.8 or the 28-75mm f/2.8 which I personally own but sometimes think I should have gone with the 17-50... lol.
