Printable View
I am considering buying an extension tube for my 70-200 2.8 IS and or my 24-105 to do more macro stuff. I heard that for a given aperture, DOF becomes less with these tubes. Is this true? If so how much and does moving from the 12mm tube to the 25mm cut the DOF in half (since size is doubled)?
As I understand it, the reduction in DoF is not a function of the extension tube, per se (although light loss is). Rather, DoF is reduced because that is determined in part by distance from lens to subject, and the closer the subject to the lens, the shallower the DoF - i.e. the reduced DoF is a phenomenon of macro photography, regardless of how you get to macro (dedicated lens, extension tube, or close-up filter).
The other factor to consider is the amount of magnification you'll need/want.
Generally, extension tubes work best with short lenses, and close-up filters work best with long lenses, in terms of providing the most magnification. So, for example, extension tubes can get you to ~0.7x on the 24-105mm lens, with the greatest magnification zoomed wide at 24mm. Note that with that combo zoomed wide for max mag, your working distance will very (very!!) short - your subject would be less than an inch from the front element of the lens. Zoomed to the tele end, your working distance goes up to about 12", and your magnification drops to ~0.4x. The extension tube on the 70-200mm will be less effective, getting you to only ~0.4x at the most even with the 25mm tube at the 70mm end (where the tube is most effective).
On the other hand, a 500D close-up filter will get you to a magnification of ~0.55x when used on the 70-200mm @ 200mm (less mag with shorter focal lengths), and with that combo the working distance will be fixed at about 20" regardless of zoom. Close-up lenses do not cost you any light, but they do reduce off-center IQ a little bit. Also note that you can use an extender/teleconverter (1.4x or 2x) behind the lens in combination with a close-up filter on the front for even higher magnification.
What will you be shooting? If you're going after insects, etc., then you want more working distance. If you'll be shooting flowers with from a tripod, you can move in close more easily.
The other thing to keep in mind is angle of view. With extension tubes, you're getting max mag at wider focal lengths - even though you can get to 0.7x with the 24mm end of your 24-105mm lens, it's still a 24mm lens. That means you'll be seeing wide swath of the background behind whatever you're macro subject is. The telephoto + close-up lens will give you a much narrower angle of view and thus include less of the background.
This article by Bob Atkins might help.
Overall, I think the clear winner would be a true macro lens. Short of that, I think you'd probably be better off with a 500D close-up lens on your 70-200mm than with an extension tube on your 24-105mm lens.