I am in the market for a good Macro lens - however someone told me that you can just use a spacer, and your normal lenses will work as macros...is this correct?
That is correct but I wouldn't recommend it. People also take a coupler ring and screw another lens into the front of the first. You can get really close macro shots this way, but again, I wouldn't recommend it (huge light loss). There are even diopters and close up filters which attach to the front of your lens, but those can cause distortion and other unwanted problems.
Using a 2nd coupled lens on the front of your first lens will:
1) Look ridiculous and could render follow focus and rail systems inoperative
2) Double the glass elements which light will have to travel
3) Significant Lightloss
Using a spacer tube will:
1) Push your lens further from the camera causing a possible follow focus and rail issue.
2) Lose the ability to focus to infinity (You would have to take the spacer off when not macro focusing
3) Light loss (more loss the longer the spacer is)
Diopter and cu filters:
1) I've noticed image quality suffered, but have not ever used a superior quality double filter.
2) I don't know much about these, but they seem to not be used very often from my personal opinion.
If you want hassle free good macro photos buying a micro/macro lens is the way to go!
EX: Nikon makes a really good 55mm and 105mm f/2.8. They are pretty fast and can focus down to a 2:1 ratio. They do not have spacer tubes, so they are normal in length, and have the close focusing marks on the barrel of the lens. Nikon even sells spacers made for each of the lenses,(pk12 and pn11).
In summary... YES, you can us a generic spacer tube on other lenses or attach a second lens to your first for the "cheap mans" macro focus.
But if you want ease of use and a lens that was engineered and designed for macro/micro photo's, go with the macro lenses. If you need closer then 2:1 macro, you can by the spacer tubes for the macro lenses, which recommend the exact tube to use to ensure a good picture.