Focusing in close-up, macro photography and photomacrography
In close-up, macro photography and photomacrography, it is usually necessary to know exactly the magnification of a picture. For this purpose, focusing usually is performed by setting a fixed magnification first, and leaving it unchanged. This is done by changing the distance between lens and camera sensor/film plane, or by changing the optical properties of the lens if the latter has internal focusing and/or a variable focal length. At this point, the subject is brought into focus. This is best performed by moving the camera and lens, as a whole, toward or away from the subject. This procedure guarantees that the magnification will remain constant.
If it is not important to exactly know the magnification, or if the goal is finding a suitable framing and magnification by checking the viewfinder, the final focusing may be performed by changing the focus settings of the lens instead. This process is similar to the normal way of taking a picture: first frame by moving the camera toward or from the subject, and/or by changing the zoom setting of the lens, then focus by turning the focusing helicoid of the lens (or, more frequently, by letting the camera autofocus).
In close-up and macro photography, the lenses normally used for this purpose have a built-in helicoid that, in most cases, allows focusing down to a 1x magnification. Special-purpose lenses (e.g., for UV photography) and lenses used for photomacrography often don't have focusing helicoids, and usualy are mounted on extension rings or bellows. Extension rings can be quite short: the Nikon K1 ring (6 mm) is probably the shortest for a Nikon camera, and the Nikon PK-11and PK-11A (8 mm) are the shortest among those easy to find and compatible with most modern Nikon cameras. However, extension rings have a fixed length. They can be stacked to obtain a rather broad range of lengths, but don't allow the distance between camera and lens to change continuously. Adding and removing rings necessitates the lens to be dismounted, which is time-consuming, may cause dust to enter the camera, and may disturb a subject located near the camera.
Bellows allow the distance between camera and lens to change continuously. They are generally bulky and delicate, and are difficult and impractical to use hand-held. In addition, they rarely allow a distance between camera and lens of less than about 40 mm. Special bellows can be built to allow much shorter distances, but they retain all other problems of bellows. Many types of flexible materials used in bellows are translucent in the near-IR, so extension tubes may need to be used with IR-sensitive cameras.
In certain situations, an extension ring that houses a focusing helicoid can be used instead of bellows or fixed extension tubes. This is the case, for instance, of close-up photography with special lenses that do not have a built-in helicoid. A focusing helicoid can also be stacked together with extension tubes.
Focusing is very critical in these types of photography. Since the depth-of-field can be a fraction of a mm, it is very important to use equipment that allows the distance between camera and subject to be adjusted precisely and without backlash, sagging or slipping. A focusing rack and/or focusing jack are typically used. For work at high magnification, a second-hand microscope stand (sometimes modified) can be used.
The viewfinder of DSLRs is neither very accurate nor sufficiently bright to allow easy focusing at high magnification and/or with narrow lens apertures. In this respect, high-end film SLRs were much better, and several of them had interchangeable viewfinders and screens useful for this purpose. Most film SLRs also had a screen with microprisms and a split-circle telemeter at the center. Both were very useful in close-up photography, although they became too dark for macro photography and photomacrography. Modern DSLRs, instead, are designed to rely on autofocus, and are not really meant for macro work. As noted also on other pages of this site, SLR makers used to produce a good variety of lenses and accessories for macrophotography and photomacrography between the 1950s and 1980s, but stopped almost completely since the 1990s, probably as a result of a new emphasis on mainstream consumer equipment that could be sold by the millions, at the expense of specialty items that, at best, could sell only by the thousands.
Third-party focusing screens are available for Nikon DSLRs, and some of these may be useful for the present purpose. However, these screens are delicate and somewhat difficult to change, and not really meant to be interchangeable. They may also introduce a slight focusing offset. Aside from these, viewfinder magnifiers may be useful in some circumstances. A bright illumination of the subject, however, remains the best help for focusing.