Extension rings, or extension tubes, are used to decrease the minimum focusing distance of lenses. Therefore, they are used primarily in close-up, macro photography and photomacrography. However, a short extension ring may also be useful to decrease the minimum focusing distance of a long telephoto lens. These lenses often have rather long minimum distances (sometimes, several metres), and an extension ring may be the only practical way to focus on a closer subject, like a small bird.
Basically, an extension ring is a hollow tube inserted between the lens and the camera body. An extension ring increases the distance between the lens and the focal plane of the camera. Modern cameras have numerous mechanical and/or electronic connections between lens and camera body, and extension rings that lack such connections force the user to compromise on ease of use of the equipment. With several camera bodies and lenses, the use of extension rings may be impractical or outright impossible. This is especially true of consumer or semi-professional DSLRs, while professional ones usually have provisions for "manual" extension rings and lenses (i.e., equipment that lacks these connections).
Extension rings of good quality possess internal baffles and/or matte paint on internal surfaces to prevent reflections on the inner walls and mechanisms. These reflections would reduce contrast. Rings of good quality must also be mechanically sound. They should be rigid and have bayonet mounts of good quality, in order to eliminate mechanical play. Since each ring has two bayonets, and rings can be stacked onto each other, any mechanical play adds up.
Kenko extension rings are the only ones for Nikon DSLR cameras to provide mechanical and electronic couplings for diaphragm and autofocus. While diaphragm couplings are necessary, or at least useful, autofocus is not normally used in macro photography. Unfortunately, the body of Kenko rings is made of plastic, and is slightly flexible under load (especially with heavy cameras and lenses). There are reports of electric contacts of Kenko extension tubes working intermittently in these conditions.
Among legacy extension rings, Zeiss Ikon ones for the Ikarex 35 are interesting because their bayonet locks without play or wobble. They are discussed here.
Bellows can be used to achieve an extension distance greater than normally provided by extension rings. Although you can stack extension rings on top of each other, the small play present at each bayonet connection may add up to a significant wobble when more than 2-3 rings are used.
Computing the magnification of extension rings
Traditionally, camera lenses consisted of several optical elements that moved back and forth as a single unit in order to focus. Modern lenses often employ one or more groups of elements that move with respect to each other when focusing. This results in the focal length of the lens changing at different focus settings. In addition, camera lenses that consist of several optical elements do not behave in the same way as a lens made with a single element. Without taking into account these factors (which are often unknown at the outset), it is not possible to compute an exact value for the reproduction ratio provided by an extension ring. However, the value calculated for a single-element lens is indicative. It can be computed as:
where t is the focal length of the lens (which is also the distance between lens and focal plane when focused at infinity), t1 is the length of the extension ring (i.e., the effective distance it introduces between lens and camera body, regardless of internal projections like lens bayonet and mechanical connections) and R1 is the reproduction ratio attained with the extension ring mounted and the lens focus set to infinity. Thus, a stack of extension rings of length equal to the focal length of the lens provides a 1:1 reproduction ratio. An estimate of the effect of turning the focus ring of the lens can be obtained by measuring the amount by which the front element of the lens moves outward when focusing, and adding it to the length of the extension ring. Obviously, this provides only an approximate value with lenses that use "floating" element groups that move independently, and is not relevant for lenses that use internal focusing and have no moving external lenses.
Examples of extension rings
Nikon extension rings can be purchased separately. The current Nikon extension rings (PK-11A, PK-12 and PK-13) measure 8, 14 and 27.5 mm in extension length, respectively. None of these rings allow automatic light metering and exposure with current consumer-grade Nikon DSLRs (with certain limitations, they do work with professional-grade Nikon DSLRs, i.e., D200 and upwards). This can be partly corrected by modifying a ring by adding the electric contacts at the lens mount and the appropriate electronics, usually salvaged from a broken lens. I performed this operation on a PK-13 ring shown above.
If you want to obtain information about his operation, you may check this page on my site (which provides generic information), or make a web search about "chipping" a Nikon lens.
The Nikon PK-11/12/13 and PN-11 cannot be used with G-series lenses and equivalent (i.e., lenses without an aperture ring). For these lenses, and also if you want to keep the mechanical and electronic transmission for autofocus and exposure between body and lens when using extension rings, you can use extension rings made by Kenko.
Sometimes, chipping is done to allow the use of an old but expensive Nikkor lens with newer camera bodies (D80 and below). The same operation can be carried out with extension rings or bellows, in order to eliminate some (albeit not all) of the problems connected with being forced to use manual exposure.
There are several discontinued Nikon extension rings. Some of them have special features, like the E2 (above), which has a plunger for temporarily opening the diaphragm while focusing, when a lens is reversed or mounted onto accessories like bellows. Releasing the button closes the diaphragm to its preset aperture. This ring is also useful as a lens shade when using a reversed lens.
The Nikon M ring shown above is an early model, probably used in connection with some of the Micro Nikkor 55 mm models. It contains a rectangular baffle to reduce internal reflections.
The Nikon K1 ring (above) was part of a set of extension rings, and perhaps also available alone. It is remarkable for its 6 mm thickness, which is the lowest among Nikon extension rings, and probably third-party ones as well. A lens mounted on this ring becomes rotated roughly 60° clockwise with respect to its ordinary position. In some series, this ring is instead engraved A, but is otherwise identical.
The Nikon PN-11 ring has an extension length of 52.5 mm and a collar with a rotating tripod mount. It was originally made for an early Micro Nikkor 105 mm model.
Olympus used to make a highly unusual variable-length extension ring (shown above with the Olympus Macro 135 mm f/4.5 mounted at the bottom). This ring extends continuously between 64 and 116 mm. It does not contain a focus helicoid, but a set of sleeves that slide inside each other. Twisting a sculpted rubber ring locks and unlocks the extension mechanism. Magnification scales for use with Olympus macro lenses are printed on the sleeves. This ring also has a collar with a rotating tripod mount, similar to that of telephoto lenses.
This ring can be used with proprietary 20 mm, 35 mm, 80 mm and 135 mm macro lenses (also discontinued), which have a reduced focus helicoid or none, and cannot be mounted directly on a camera body. They are designed to be used on bellows, or on extension rings. Extension of the variable ring controls the magnification, while the lens helicoid can be used as a fine adjustment of focus or magnification. There seems at present to be no equivalent equipment for Nikon cameras.
The main advantages of this ring over bellows are its lesser weight and much higher sturdiness. Bellows are more vulnerable to damage, and unwieldy in the field, while this variable extension ring behaves essentially like a macro lens. Now that a broad variety of macro lenses of different focal lengths is available, of course, there is scarcely a need for this type of accessory. Nonetheless, this tube and the four special-purpose Olympus macro lenses mentioned above can be packed into a much smaller space than four general-purpose macro lenses, and also achieve a larger range of magnifications (from infinity to more than 9:1). With appropriate adapters, the variable extension tube can be used in the field also with other special-purpose lenses, like enlarger lenses or Luminar/Photar-type macro lenses devoid of focusing helicoids. It is, however, expensive and rather difficult to obtain.
For my collection of accessories for close-up, macro and photomacrography, I decided to standardise on two types of lens attachment. The first one (Nikon bayonet) is the obvious choice, given that I use Nikon lenses and bodies. A set of PK-11, PK-12, PK-13 and PN-11 satisfies these requirements. The second required some thought. M42 (also known as Pentax screw fittings) rings and accessories are common and inexpensive. This attachment is simple and connects solidly, with no play or wobble. This is a major concern if you need to stack several rings or accessories on top of each other. A ring to connect M42 accessories to Nikon bodies is also easy to find. However, I use no lenses with M42 screw attachment, which means I has to add yet another standard to my equipment, and needed a whole set of adapters for anything I needed to connect to M42 rings.
As an alternative, M39 screw fittings (also known as Leica screw fittings) are attractive in two respects: they are standard in enlarger lenses, of which I have several, and old-stock M39 extension rings are very cheaply available from ex-Soviet countries. It seems that inexhaustible supplies of these extension rings were made in the Soviet era, and stocked away for generations to come - or possibly the same production lines are still running unchecked since the fall of communism, churning out truckloads of products without a market. These rings are extremely simple and rugged. Each ring is made of a single machined piece of aluminium tube, without any of the diaphragm transmissions usually present in M42 tubes. In fact, I was able to order four unused sets of Soviet-made FED M39 rings from Ukraine, each consisting of four rings of different lengths, at a price equivalent to one set of M42 rings made in China. Actually, by mistake I was shipped three M39 and one M42 sets, which means now I do have also an M42 set, should I need it.
If you decide to buy this type of rings, be aware that the matte black paint used to cut reflections on the interior may need retouching (it feels more like soot than paint, and leaves a stain if you rub it with a finger), the anodizing on the exterior is not of good quality and can be different from ring to ring, they squeak loudly - and sometimes lock - when screwing and unscrewing them together, and as a whole their manufacturing quality is truly Soviet-style. However, they do the job they are supposed to, and usually require no more than a good wash in water with a little dish detergent before they can be used. Screwing and unscrewing the rings into each other a few times can also take care of small manufacturing defects of the threads. It is probably a good idea to order one or more extra sets, because occasionally a set contains a defective and unusable ring.
A focusing helicoid is, essentially, an extension tube of variable length. Focusing helicoids are discussed here.
A useful thing to remember is that M39 and Leica (L39) attachments are not exactly the same thing. Older Leica bodies and lenses use a metric M39 thread, while newer bodies and lenses use a slightly different and incompatible thread pitch, introduced to lessen competition by third-party manufacturers. Most Soviet-made M39 rings, however, are compatible with both types. Enlarger lenses always use metric M39 threads. M39-to-Nikon adapter rings (as well as M42-to-Nikon) are easy to find among the plethora of adapter rings manufactured in China. Look for rings made of machined brass, not aluminium. A Nikon bayonet machined from aluminium is simply not strong enough. M39 to M42 adapter ring sleeves are also available from third-part sources (those from ex-Soviet countries are shoddily made, while those made in China are usually better).