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 Extension tubes are designed to enable a lens to
focus closer than its normal set minimum focusing distance. Getting closer has the effect of magnifying your subject (making it appear larger in the
viewfinder and in your
pictures). They are exceptionally useful for
macro photography, enabling
you to convert almost any lens into a macro lens at a fraction of the cost while maintaining its original optical quality.
The extension tubes have no optics. They are mounted in between the camera body and lens to create more distance between the lens and
film plane. By moving the lens farther away from the film or
CCD sensor in the camera, the lens is forced to focus much closer than normal. The greater the length of the extension tube, the closer the lens can focus. Extension tubes extend the focal length of the lens, increasing magnification. Total extension/Focal length used = Magnification. The longer the focal length the more extension you need to make a significant
difference in magnification. Extension tubes use results in a loss of light. Most tubes, but not all, are meter coupled allowing your meter to work as normal. You need to check your cameras manual for
compatibility. Some will result in loss of
autofocus, which may not be that big a deal
with macro photography.
Extension tubes are not the same as an extender (also
called Teleconverters). Extenders are for shooting things a long distance away and making them look closer. Extension tubes are about allowing lenses to get physically closer to your subject by reducing the minimum focusing distance. They fit between the lens and the body just like an extender but have one other importance difference - extension tubes are just a tube - they have no optics in them.... as a result you finger goes right through from one side to another if you stick it in the hole.
The amount that they allow you to get closer and the magnification increase on the subject as a result of that is governed by the focal length of the lens and how close the lens will focus on its own. Extension tubes can be fitted onto any lens. There is a small light loss but with lenses over 50mm this is negligible. These are not normally used with wide angle lenses as the extremely close focusing distances they give makes it impractical in use.
Without electrical contacts
Extension tubes without electrical contacts will not allow an electronic automatic camera to control the lens, As a result disabling
autofocus and in some cases forcing a user to shoot wide open unless the lens offers manual
aperture control. More expensive extension tubes contain electrical contacts allowing the user to use autofocus and electronically control the aperture of the attached lens. A great advantage to the non-electrical tubes is their low price. Since no electronics or optics is involved, it is usual to find rings cheap.
With electrical contacts
The KENKO AUTO EXTENSION TUBE SET contains three tubes of different length, a 12 mm, 20 mm, and 36 mm, which can be used individually or in any combination to obtain the desired magnification. Kenko also makes a UNITUBE 12 mm or 25 mm which can be purchased individually.
Kenko's Auto Focus extension tubes are designed with all the circuitry and mechanical coupling to maintain auto focus and TTL auto exposure with most Nikon lenses given there is enough light to activate the cameras AF system properly.
KENKO UNIPLUS
TUBE (extention ring) is an accessory exclusively designed for use with standard
AF lenses to shoot subject at closer distance with higher magnification.
Magnification ratio for TUBE 25 is about 1/2 with 50mm
standard lens. Perfect
coupling with automatic aperture, TTL metering and AE mechanism.
Shop Extension Tubes
Extension Tubes vs Close-Up Filters vs Macro Lens
 A close-up filter
is an additional lens that attaches to the front of
your camera lens to allow it to focus close-up. Basically, it decreases the focusing distance allowing you to get closer to
your subject. Since they do not increase focal length there is no
light loss. Advantages are low cost and small size. Disadvantage is loss of optical quality. especially with the lens wide open, and around the edges. They can be used with
zooms, but the weaker
zooms perform better. The convenience they offer make them very attractive and useful. See also
Circumvent brand loyalty without guilt
by owning them all.
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