Subject:	You mean the focus issues? (+)
Author:	drdread
Date:	2/8/2002 9:45:13 AM
Message:

If so, "no." Modern teleconverters connect to the camera's AF circuitry and allow all autofocus operations you get with the single lens. There are 'costs,' however, as a 1.4x teleconverter costs you an entire stop of max aperture (2 stops for a 2x tele)...no biggie most of the time, but something to keep in mind.

Otherwise, all 'normal' rules of optics apply. The DOF issue isn't as bad as what you might think. DOF can be calculated from a single number, namely the magnification or reproduction ratio of the scene. That, in turn, is related to both the lens focal length and the distance of the subject. So teleconverters don't have an immediate effect on the DOF; you have to take into account the distance of your subject.

Teleconverts are not all gravy. They typically do not change the allowable repro ratio. That is, if your lens could do a 1:1 repro at 6 inches, adding a 2x tele will still do 1:1 except now it will do it at 12 inches. To get higher repro ratios, you have to add that diopter in order to decrease the minimum focus distance. That's what Rooster (and others) are doing with these extreme macro setups. You can get 1.5:1 or even 2 or 2.5:1 with various diopters on a 60mm lens, except that your lens will be an inch (that's about two centimeters to you metric types) from the subject. Using a 200mm lens and a 2x teleconverter will let you shoot the same 2:1 from six inches away (15cm). Not bad! FYI -- I'm making up those subject distances; I don't know the actual numbers offhand.

Anyway, the first place to start if you want to do extreme macro is with the longest macro lens you can afford. In my case, I own a Nikon 105mm macro lens, which will do 1:1 from about 8-10 inches. Then get the diopter (I have one, but have never used it in the housing), then start thinking about the teleconverter.

Did you ever buy that digital camera you were looking at?

D

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