Yup. From WikiPedia: "The further the lens is from the sensor, the closer the focusing distance, the greater the magnification, and the darker the image for the same aperture."
So the only reason you pay so much for macro lenses is it's focusing distance, right?
Yeah, well... it takes a bit more engineering to get that short focusing distance...
A cheaper alternative is to use extension tubes on non-macro lens.
What are the disadvantages to that?
You can't focus to infinity. Using on a normal zoom lens the quality won't be as good as a prime lens. The cheap versions (£10) will not auto focus ... the "propper" versions are still quite expensive, eg the canon extension tubes start at about £60 and are not compatible with all lenses. They have no glass in so the extension tube does not effect the quality of the pic
You can't focus to infinity. Using on a normal zoom lens the quality won't be as good as a prime lens. The cheap versions (£10) will not auto focus ... the "propper" versions are still quite expensive, eg the canon extension tubes start at about £60 and are not compatible with all lenses. They have no glass in so the extension tube does not effect the quality of the pic
Extension tubes can work pretty well; I've had decent luck with them, but definitely prefer my dedicated macro lens now that I have it. The Kenko tubes work great and are more reasonable, though still not cheap. Mine were labeled as not working with hypersonic AF but they do (with Sigma HSM and Nikon AF-S).
Some examples of entries I've done using tubes- You can get a set of tubes and a cheap MF prime to mess around with if you're curious. My 50 1.8 E series was $30 in immaculate condition and was what I used for my initial macro shots.
As was noted before, you can't focus to infinity, so if you're working in the field with insects for example, sometimes its difficult to determine if you're too far away or too close to your subject since the DoF can be so very very thin. When AF searches on a dedicated macro lens you can figure out where you're at a lot easier, in my opinion.
Extension tubes work better with some lenses than others. Though the have no glass in them, they can affect the quality of the image produced by the lens, for the following reasons:
- Many lenses have increasing aberrations as you focus closer; putting extension tubes on a lens not designed for close-focus will inevitably lead to greater aberrations such as CA.
- Moving the lens away from the sensor means that you are effectively using less of the image circle. If the lens doesn't have much "headroom" in its resolving power, you can get soft images as a result.
True macro lenses also have other benefits, such as a flat field.
