I am looking to do a little macro work, but am not ready to invest in a macro lens. For hobby work like photographing flowers and such would it be better to invest in a set of extension tubes or in a good close up filter? I would like to utilize my Canon 40D with either the 24-70mm f/2.8L or the 50mm f/1.2L lens I already own.
I've used extension tubes in the past, with success, but what I did find was that you'll have to manual focus because the AF sensor gets really crazy with tubes. For most macro work you'll want to focus manually anyway, so use your camera's live view.
I would stay away from close up filters. They're bulky and the quality is very poor.
In general, if you don't already own a portrait lens, a macro lens with a short focal length (say, the EF-s 60 or EF 100 macro lenses) can double portrait lenses.
The two lens you own might work, but be aware that their minimum focusing distance might not fit what you need. Macro lenses are special in that they have a much shorter MFD than regular lenses - this means that you can get closer to your subject and still focus, giving you a much larger subject for the same resolution. This is important when photographing bugs and flower stamen, etc.
As for the rings / filter debate, a couple things to keep in mind:
- rings have no glass, so they don't reduce light, change color quality, or reduce sharpness
- rings can stack, multiplying the effect
- rings without AF support can be extremely inexpensive
- filters can sometimes not carry additional filters on top of them - this can make it difficult if you want to attach a polarizer or some other filter
- filters cannot be used on all cameras, unless you buy a larger one (77mm for your 24-70) and also buy a step down ring.
You might also try and older toy called a focusing bellows - its similar to a ring, but its in accordion style, and lets you incrementally change your focus, allowing you to do focus stack macros. These are like the HDR of macro shots. Check out Lord V. on flickr for examples.
I agree that a set of macro tubes is the way to go, if you're interested in quality. On the other hand, they take up more room and are less convenient to use.
I also second @reuscam's suggestion to get a bellows. This is basically an infinitely-variable extension tube.
If you go that route, look into a "reversing ring" to use in combination with macro tubes or bellows.
This allows you to mount your lens backwards, with the objective lens facing the camera sensor. The theory is that the part that normally faces the sensor is optimized for close focusing.
Finally, once you have your bellows, you can enjoy experimenting with all manner of cheap manual focus lenses on it, since auto-focus is of limited use in macro, anyway. Whatever you put on the bellows or extension tubes should be set to its minimum focus, as many lenses have "floating elements" that move as a separate group when focusing close — you want that floating group fully engaged, if you're shooting closer than the lens would shoot without extension.