An adapter is a metal tube or ring with a mount that matches your camera on one side and a mount that matches your lens on the other. By the means of the adapter you can use a lens from different system on your camera. First, you attach the adapter to the lens, and then you attach them to your camera - just like any lens specially designed for your camera. However, this requires that the flange focal distance of the lens mount is equal to or greater than the flange focal distance of the camera mount. The flange focal distance is the distance from the mounting flange to the film or imaging sensor plane. The greater the difference in flange focal distances, the thicker the adapter.
Flange focal distances of modern digital SLR cameras, and popular SLR lens mounts that can be adapted to them:
|Camera series||FFD, mm||Adaptable lens mounts
|Nikon||46.5||Icarex, Leica R
|Pentax||45.5||Contarex, Contax/Yashica, Icarex, Leica R, M37, M42, Mamiya CS, Mamiya E, Nikon F, Olympus OM
|Sony DSLR/SLT||44.5||Contarex, Contax/Yashica, Exacta, Icarex, Leica R, M37, M42, Mamiya CS, Mamiya E, Mamiya ES, Nikon F, Olympus OM, Pentax K, Topcon
|Canon EOS||44||Contarex, Contax/Yashica, Exacta, Icarex, Leica R, M37, M42, Mamiya CS, Mamiya E, Mamiya ES, Nikon F, Olympus OM, Pentax K, Praktica B, Rollei QBM, Topcon
From the above table, it immediately becomes clear that Canon EOS cameras are the best if you like experimenting with vintage manual focus lenses, and Nikon cameras are poorly suited for this, since popular mounts such as Contax/Yashica, M42 or Olympus OM cannot be adapted.
One more important note. Canon EOS cameras have the shortest flange distance of any modern digital SLR camera, which is 44mm. Therefore, if the lens has a mount with a flange focal distance of less than 44mm, that lens cannot be adapted to any digital SLR camera.
Vintage SLR lens mounts that cannot be adapted to modern digital SLR cameras:
|Lens mount||FFD, mm
||Lens mount||FFD, mm
In the case of Canon FD, Konica AR and Minolta SR mounts, this is especially annoying, since these systems had many lenses with beautiful bokeh.
Some manufacturers may offer adapters for these mounts in variants with or without magnifying glass. With glass adapter, the image quality will suffer in terms of image sharpness, especially with fast lenses at large apertures (the most common adaptation and use case). With glassless adapter, you will loose the ability to focus at infinity. The choice is yours, however the only right way to adapt such lenses is to use a mirrorless camera.
Lenses designed for rangefinder cameras can only be adapted to rangefinder or mirrorless cameras:
|Lens mount||FFD, mm
||Lens mount||FFD, mm
Lens adaptation should be distinguished from its modification, which is usually performed when the use of a lens with a camera via an adapter is impossible due to shorter flange focal distance of the lens. During this modification process, the technician removes the original lens mount, grinds off a portion of the rear of the lens barrel and installs a new mount. For example, in order to modify a lens with the Canon FD mount to use it on a camera with the Canon EF mount, you need to grind off 2mm, which is the difference between the flange focal distances of these mounts.
Thus, modification is a custom-made procedure for permanently changing the mechanical construction of a lens (including its mount). Unlike modification, adaptation always implies the use of an adapter while keeping the original lens mount.
Exposure metering and focusing
Before taking a picture in fully manual exposure mode (M), the shutter speed and ISO must be selected by the photographer, as well as the aperture, which is controlled using the diaphragm ring on the lens barrel. Focusing is also done manually. Shooting in shutter-priority (S/Tv) or programmed auto exposure (P) modes is not possible because the camera cannot automatically control the aperture. But in addition to the fully manual exposure mode (M), you can shoot in aperture-priority auto exposure mode (A/Av).
If the adapter does not have a focus confirmation chip, your camera may refuse to take pictures because it does not "see" the lens. This is not surprising: after all, the adapter is just a piece of metal, and the camera cannot exchange any information with it. Therefore, when using an adapter without a chip, do not forget to enable the so-called "shoot without a lens" setting in the camera menu.
A couple of good advices at the end
1. Do not adapt any zoom lenses or slow prime lenses (except for such special tools as fisheye, macro and shift lenses). There are many inexpensive autofocus zoom lenses available for your digital SLR camera that can replace them and can be used without an adapter.
2. If you own a Nikon digital SLR camera, do not waste your time buying lenses like old Soviet Helios-44 with M42 screw mount, or similar, as well as adapters with magnifying glass. A variety of high quality manual focus Nikkor lenses have been produced for your system since 1959, many of them have beautiful bokeh and excellent sharpness, not to mention the high build quality. You should not adapt third party lenses to cameras that are not suitable at all for this.