The lens mount
The lens mount is a construction that allows a lens to be attached to the camera body, so that it is correctly aligned to the sensor, at the right distance, to take full advantage of the combination.
A lens mount can also have other useful features, like electrical contacts (the gold contacts in the image) that pass on lens information to the camera circuitry, or a holder for filters (the square groove in the middle), etc.
Flange Focal Distance
The distance from the lens mount to the sensor is called the Flange Focal Distance. Since any given lens is designed to have a fixed image circle, any change in the flange focal distance will change the size of this circle.
Therefore, the flange focal distance is a fixed property as far as a lens mount is concerned. Sensors and lenses come and go, but lens mounts are meant to last forever.
Yet, there have been cases of major camera manufacturers changing or abandoning their long entrenched lens mounts, causing untold grief and suffering on countless souls who invested in them over the years.
Here’s information on each mount, in increasing order of the focal flange distance:
|Mount||Focal Flange Distance in mm|
|E Sony E-mount||18|
|FZ Sony PMW-F3 mount||18|
|m43 Micro Four Thirds mount||19.25|
|M Leica M mount||27.8|
|FT Four Thirds mount||38.67|
|FD Canon Manual FD mount||42|
|EF Canon EOS EF mount||44|
|EF-S Canon EOS EF-S mount||44|
|A Minolta/Sony A-mount||44.5|
|K Pentax K-mount||45.46|
|F Nikon F-mount||46.5|
|R Leica R-mount||47|
|PL Arri PL mount||52|
What do you do with the flange focal distance? Simple, the rule of thumb is: the lower this distance, the more universal your mount. What does that mean?
E.g., You can’t use Canon EF mount lenses on a Nikon DSLR like the D800, because the F-mount is designed so that the mount sticks 46.5mm away from the sensor; while the EF mount ‘needs’ 44mm to work the way it is designed to. One will have to break the F mount to the get the lens 2.5mm closer to the sensor.
On the other hand, you can use Nikon glass on a Canon EF mount with an adapter whose width will make up the difference.
You might be able to use a lens not designed for your camera, if the focal flange distance of the lens is greater than the focal flange distance of your camera mount. However, this is not universally true.
There are other factors which determine which lenses can be adapted to which mounts. One of these is the fact that some lenses have protruding elements that move when focusing or zooming, etc. This protrusion is acceptable because the flange focal distance the lens is designed for will take this into account. However, using such a lens on a camera like the Sony FS100, with its 18mm flange focal distance, might cause the protrusion to scratch the sensor.
Just because a smaller flange focal distance is more universal doesn’t mean it is always ideal. One of the advantages of having a large flange focal distance is that the sensor is safely tucked away in the camera body, protecting it from dust and the elements while changing lenses, etc.
The device that allows a lens designed for one mount to fit another mount is a Lens Adapter.
Other than the physical nature of the connection, adapters can also provide electrical contacts to pass on information from the lens to the camera.
However, this does not always work well in practice, because a camera circuitry is usually only designed to read and ‘understand’ the data coming from lenses by the same manufacturer. There are also patent and licensing issues on cutting edge technologies like auto focus, image stabilization, sound dampening, etc. The trend in the digital age has always been of protecting proprietary information rather than sharing it, so assume incompatibility until proven otherwise.
Using a manual lens (with no electronic circuitry or wizardry inside) is easy if the physics allow for it. In some cases, an older manual lens encased in metal is a much better alternative for video work when compared to the plastic digital equivalent that the manufacturer provides.
When buying an adapter, always check to see if the lens focuses on infinity; if your intention is to make the lens work exactly as it would on the mount it was designed for.
Study the adapter to see what optical compromises it is making to make the physical connection possible. Sometimes adapters only work well over a limited aperture range, or don’t pass on the full contrast range, etc. Make sure what you’re getting into, especially if the adapter costs more than the lens!
I’ve had limited success with adapters, especially cheap versions, so if you just want a manual ‘dumb’ adapter, they might be good enough.
- Precise machining
- Strength to handle heavy unbalanced systems
- Infinity focus
- Electronic compatibility using CPU contacts
- Aperture control
- TTL metering
Here are a few ‘established’ manufacturers who sell a range of adapters:
Sony, Voigtlander, Cinevate, Metabones, MTF,
|FZ Sony PMW-F3 mount||Sony|
|Micro Four Thirds mount||Fotodiox, Novoflex, Zeiss, Voigtlander|
|Canon EOS EF mount||Fotodiox, Novoflex, Leitax, Schneider|
|Canon EOS EF-S mount||Fotodiox, Novoflex, Leitax|
|Sony A-mount||Leitax, Fotodiox|
|Nikon F-mount||Leitax, Fotodiox, Schneider|
|PL mount||MTF, Engineers who specialize in custom-made solutions for adapting Medium Format lenses|
The bottom line is, unless you are experienced enough to have strong preferences for certain third-party lenses, I suggest you stick to the lenses directly made for the mounts of your camera body. It is not uncommon for Hollywood DPs to have custom mounts made to adapt specialized lenses for a certain effect. They have the budgets, so why not? The ultimate example of this is Stanley Kubrick’s use of a Zeiss lens made for NASA, with an f-stop of f/0.7!
Only when ‘direct’ options are unacceptable should you consider adapting lenses. Why?
You’ll probably need one adapter for each lens you intend to use. Constantly switching the adapter might loosen the adapter, or chip the adapter or the mount. Plus, you’ll be carrying two rear lens caps. Not fun.
I will have to stress that you do your own research, read reviews written by others and perform your own measurements and tests before purchasing any adapter for a professional shoot. The selection of an adapter is as critical as the selection of a lens.