 Emmett S. Speelman |
Extension Tubes for Close-Up Work Has anyone used extension tubes for autofocus lenses? And, if so, what are the pros and cons of them? I am interested in purchasing a set but needed some advice. Thanks.
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robert G. Fately |
By extending the distance from the lens to the image plane you do a number of things: 1) You lose the ability to focus to infinity, but gain the ability to focus on objects very close to the lens. Exactly how close depends on both the focal length of the lens and the physical length of the tube (or bellows). 2) Depth of field becomes paper thin - if your subject is a flower, then you might see the stamen in focus and the petals in front of and behind the stamen blurry (or whatever). DOF is measured in fractions of millimeters when you do macro work - though smaller apertures still help by extending the depth. 3) Due to the inverse square rule, less light hits the imager than it will with the lens directly on the body. With through-the-lens metering, however, this should not be an issue - it just means that between wanting to use slower apertures (for greater DOF) and the additional light loss you will be forced to use pretty slow shutter speeds. That said, you will want to use a support (tripod or table tripod) - for sharpness' sake as well as the fact that you simply won't be able to hand-hold the setup and keep what you want in focus. You'll also want to use manual focusing - auto-focus is notoriously useless in the macro world.
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Michael H. Cothran |
Extension tubes serve only one purpose - to physically move your lens farther from the film plane or digital sensor. What this does, in turn, is enable your lens to focus closer. The farther away your lens is from the film plane/digital sensor, the closer it can focus. To my knowledge, there are not any 'autofocus' extension tubes - you will need to manually focus your AF lenses when tubes are attached. I would try to buy extension tubes that have all the necessary contacts on the back end, so that they couple to your camera's electronics (read - metering). You can buy extension tubes one at a time, or in sets of threes, and mount as many of them in any configuration you desire on your camera. Note, after a point, your rig may become a little wobbly. Also, the longer the lens, the less effect the tubes have on close focusing, so you will need a lot more tubes on a 200mm lens than you will on a 50mm lens (4 times the amount to be exact) in order to get the same focusing capability. Even more for a 300 or 400mm lens. FYI - in order to achieve 1:1 focusing, the length of your extension tube(s) must match the focal length of your lens. Tubes WILL cause light loss, since the farther away your lens is, the less light actually hits the film/digital sensor. This is no problem as long as the tubes will couple to your in-camera metering system. At 1:2 you will lose 1 stop of light, and at 1:1 you will lose 2 stops. I have/own all three types of close focusing capability - prime macro lenses, extension tubes, and high-end close-up diopters. I use them all, and, in my opinion, you can't go wrong with any. Michael H. Cothran www.mhcphoto.net
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Emmett S. Speelman |
Thanks Guys that helped alot. I knew thier must have been some things to think about. I did find one site that as Tubes for some Auto focus systems http://www.camerastore.com/dl_cat_A/-A11_exttubes.html here is the link to that site.
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Bob Cammarata |
Another thing worth mentioning when using 'tubes or bellows in macro work: It's better to move your camera and its support back and forth to manually lock in on critical focus than to turn the focusing ring. (A bellows set-up is actually designed to do this.) I use extension tubes quite often with macro lenses and telephotos and will usually set my lens to a little less than the minimum focusing distance. Then while looking through the viewfinder, I'll get into position and move the camera and tripod forward until the subject pops into focus. Then, I'll fine-tune critical focus on a key element with a minor adjustment of the camera and tripod by moving either forward or backward (just a hair).
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Robert Sanford |
There's a gadget called a Macro Slide-- Velbon makes one-- that will help in critical close-focus situations. It's very hard to move your tripod 3/8 of an inch to focus or re-compose . . . . Think of the two rails a bellows moves on. Picture the camera sitting on bellows rails (but no bellows) which attach to the top of your tripod. See how easy it is to move your camera closer to or farther back from the subject to focus and compose? Now picture a second pair of bellows rails at right angles to the front-to-back pair. These allow you to move your camera side to side for composition, and leave your tripod where it is. These gadgets cost $125 or so. If you do a lot of really close work, a Macro Slide will allow you to focus on the shot and not on the equipment. Robert Sanford
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- Shirley D. Cross-Taylor Contact Shirley D. Cross-Taylor Shirley D. Cross-Taylor's Gallery |
Many years ago, when I purchased my Nikon N90, I also bought the autofocus extension tubes to go with it, but they were spendy. So, yes, there are AF extension tubes. However, I usually prefer to manually focus anyway, because I have more control with such shallow depth of field.
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David Robinson |
I agree with the prevoius respondent, switch off the auto focus when you use tubes. Using the lens on autofocus in this situation can cause it to 'hunt' backwards and forwards and will come to focus (if at all) at an enlargement that you probably don't want.
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Sharon Day |
The only thing I have to add is I have the Kenko AF extension tubes and use them with a Nikon 105mm macro lens. It seems to me when it's in auto focus that the lens labors harder to focus. I don't mean "hunting" but more of a strain of the mechanism. When I use mine I manual focus. You about have to do it that way anyway.
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