"Arca" type plates and clamps
Arca Swiss introduced many years ago a system of plates and clamps to attach medium- and small-format cameras to tripod heads. This system consists of a 1.5 " wide (38.1 mm) plate with V-grooves on opposite sides, typically attached to the tripod socket on the underside of camera equipment via a 1/4 " bolt built into the plate. The plate fits into a matching clamp, traditionally closed by tightening a thumbscrew. Modern examples, as an alternative, use a tightening lever instead of, or in addition to the thumbscrew. I use only models with a thumbscrew because I find them more reliable, and securely tightened by using a lesser amount of force.
Modern plates usually have two small screws on the underside, which fit into corresponding, wider and longer slots of the plates. When the thumbscrew of a clamp is slightly loosened, the plate slides within the jaws of the clamp but is stopped by either end screw. Further loosening the thumbscrew by turning it one turn or more completely frees the plate. This is a useful safety feature that prevents the camera/lens from accidentally falling off a tripod.
Both plates and clamps are available in a variety of lengths, but the width is constant. This makes all plates and clamps (in theory) compatible with each other. In practice, the width of the plate/clamp and the inclination of its grooves/jaws vary somewhat among manufacturers. Some combinations are truly compatible, others are only partly compatible, and yet others simply don't work.
Several manufacturers claim to produce "Arca-compatible" plates, clamps and other accessories, including Arca Swiss, Wimberley, Hejnar Photo, Kirk, Markins, RRS, Sunwayfoto, FotoPanda, Desmond, Jobu and others (if any patent was originally involved, it has either expired or is being largely ignored). In practice, items of different brands may or may not work together. Possibly in order to prevent being sued by Arca Swiss, some manufacturers make slightly wider or slightly narrower plates and clamps. In some cases, it is necessary to remove the safety screws at the bottom of a plate to mount it on especially tight clamps (where they can only be slid in and out sideways, not upwards). Some clamps are just a tiny bit too wide to clamp onto some plates. Another feature that differs among brands is the construction of the clamp jaw, which in some cases is built with two pins, sliding into matching holes, that guarantee that the clamp jaws remain parallel to each other (which is especially useful with long clamps but may damage the clamp if a short plate is strongly tightened at one end of a long clamp), in other cases with a slit-and-tongue mechanism that does not prevent the clamp jaws to open to a different extent on either end of the clamp (which makes it more difficult to insert and extract plates).
Typical plates and clamps have a length between 40 and 60 mm, but both shorter and (much) longer ones exist. A relatively recent addition to the Arca system consists of plates with a double-sided groove (or two separate grooves in very thick plates), which accept a clamp on each side of the plate. The second clamp may be used, for instance, to attach a flash bracket or other accessory. An extension of this idea has led to the creation of very long, extra-thick plates (the above example, from Sunwayfoto, is 30 cm long) with a double groove. In practice, this type of plate functions like the rail of an optical bench. It can be attached to a tripod head or stand by one clamp, and the camera equipment is attached to a second clamp riding along the opposite side of the rail.
The small plate pictured near the top of the page is another variant on the double-faced plate. It accepts a clamp on each side, although the rubber pads make this difficult on the upper side of the plate.
In practice, this arrangement allows a total travel of the equipment mounted on the top plate approaching twice the length of the rail. As a result, the above rail, properly attached to a vertical stand, allows a vertical travel of over 55 cm, which is enough for most macrophotography and close-up photography subjects and can replace a traditional copy stand (see below).
Further additions to this system (also by Sunwayfoto) are double clamps, which consist of a pair of back-to-back clamps either parallel to each other or offset by 90 degrees (an offset double clamp is shown above). Although a double clamp can be built by screwing together two ordinary clamps, and some clamps feature anti-twist sockets for drop-in pins at their bottom that help prevent shearing movements, a double clamp with its body built in a single piece is much more precise.
Plates attached at the bottom of camera bodies are typically oriented with the V-grooves perpendicular to the lens axis, in order to use most of the plate surface and achieve a more rigid connection to the camera. Plates attached at the bottom of lens shoes, instead, typically have the V-grooves parallel to the lens axis to use the whole length of the tripod shoe. This is where double clamps can come in handy to connect either type of equipment to a rail. The above example combines a single clamp, a 30 cm rail and a double clamp to make a focusing rail with a total travel of over 55 cm.
Above is a combination rail-and-clamp that can be used as a focusing rail (albeit the camera is always placed at one end, so the total amount of travel is less than the length of the rail). A double clamp can be attached at the top of this double-gooved rail, turning it into an extended focusing rail like the one shown earlier on.
The figure shows also a round clamp, which is sometimes more useful than a square or rectangular clamp when mounted on top of a tripod head. Unlike other examples of Arca-type clamps, the bubble level is in the right place and at the right orientation to be useful in practice. Note also the long "neck" of the locking knob of the round clamp. This is often necessary to clear the top of a tripod head without adding a spacer between clamp and head.
Several models of replacement tripod shoes, with built-in Arca plates, are available for the tripod shoes of telephoto and supertelephoto lenses (especially Nikon and Canon).
The first clamp above at the left is reasonably well-built, although the water level mounted on its thumbscrew is of questionable usefulness (it works only with the thumbscrew pointing straight upward). Sunwayfoto places a water level on the body of some clamps, in a more reasonable orientation for landscape photography (see above).
The second clamp and plate above, although advertised as Arca-compatible, simply do not fit truly Arca-compatible items, are manufactured to loose tolerances, and are made from extruded aluminium instead of being machined. The usefulness of a compass on the thumbscrew is also highly questionable (it seems they stuck it on without wondering why a user might need to know where due North is while shooting, and ignoring the fact that the compass, like the bubble level of the preceding example, is oriented the wrong way when the clamp is used in landscape photography). A typical example of "added value" with no real value.
L-shaped plates that surround a camera body on its bottom and left side are used to quickly shift a tripod-mounted camera between landscape and portrait orientation. Most of these plates have an inherent weakness: the portion that joins the vertical and horizontal portions may be flexible, and sometimes looks obviously too thin to be rigid (in order to allow the passage of cables connected to the left side of the camera body, or to bend around a battery compartment). Plates made specifically for one camera model and used for studio photography of large subjects may get away with a modest amount of flexibility, because the left side of the camera rests directly on the corresponding portion of the plate and transmits its weight to the latter, even though the camera is attached to the plate only by its bottom 1/4" socket. So-called "universal" L-plates, as well as ordinary L-plates used in different orientations that with the lens axis horizontal, may display clear flexibility and vibration problems, e.g. in macrophotography, when attached to a clamp along the left camera side. I recommend against the use of L-plates in applications where even small amounts of vibration and sagging are likely to be a problem.
In the past, I used the much larger Manfrotto 357 clamps and plates for most of my needs. I recently replaced most of them with Arca-compatible items (with a combination of no-name and Sunwayfoto items). Of course, a 357 plate on a Micro 4/3 camera would look like a mouse with elephant feet. On the other hand, an Arca plate looks tiny when attached at the bottom of a super-telephoto lens, and a 50 mm Arca clamp can hardly be expected to hold a 500 mm lens steady (or to hold it at all, except perhaps when placed directly under its center of gravity). The metal-against-metal locking action of Arca plates and clamps may prove sturdier than it looks, but I do not wish to find out by personal experience the load limits of the Arca system, and for the time being the Manfrotto 357 plates will remain attached to my heaviest pieces of equipment.