Big words are scary. Flange Focal Distance is right up there with the worst of them. It’s a career ending word – if you don’t get it, or use it on someone who doesn’t want to get it, it’s over.
Just kidding. If you can master aperture and shutter, the flange focal distance one is nothing. Ultimately, if you’re going to learn a fancy word you might as well get some benefit out of it. Let’s see how this one helps.
The lens mount
The lens mount is a construction that allows a lens to be attached to the camera body, for
- A tight fit so it won’t slip off
- In some cases, weather sealing
- Correct alignment to the sensor
- Ensuring the right optical distance to the sensor
The lens mount allows the lens to become an extension of the camera body so they behave like ‘one thing’. It can also have other useful features:
- Electrical contacts (the gold contacts in the above image) that pass on lens information to the camera circuitry, or
- Filter holders (the square groove in the middle)
Who would have thought that something that comes in between two expensive personalities could offer so much?
Why it’s important to keep the distance between the lens mount and sensor fixed
One of the most critical decisions any manufacturer makes in designing a camera system is pinning down the distance between the mount and the lens.
Why does it have to be so? Keeping the distance from mount to sensor flexible means designing a camera body that changes shape as well. There is nothing that stops manufacturers from doing this, except for one nagging problem: When you change the distance, the focus changes as well!
So, what are the advantages to keeping the distance constant? Here they are:
- Image circle remains constant.
- Autofocus can be calibrated for a fixed position.
- Lens designers can manufacturer lenses that takes maximum advantage of a particular kind of sensor, keeping in mind its internal filters.
- You can use the same lenses on multiple camera models with the same lens mount.
- You can invest in lenses and exchange bodies without worrying about whether it will work or not (not entirely true. Unfortunately, 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.).
The Flange Focal Distance
The distance from the lens mount to the sensor is called the Flange Focal Distance:
Here’s a table that shows a few popular cameras and the lens mounts on them (click to enlarge):
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|
There are two scenarios where the flange focal distance will change:
- The manufacturer abandons or modifies an existing lens mount for something else, or
- You want to use a lens made for another camera system or lens mount.
This is the part where knowing about the flange focal distance can help you get more lenses than advertised.
The birth of Lens Adapters
So what happens when the flange focal distance changes? Here are two scenarios:
In the first scenario, you want to adapt a lens of a mount with a larger flange focal distance to a camera system with a smaller flange focal distance. You can do that, as long as you find some way to make up for the extra distance (blue thingy). The device that makes up the distance is called a Lens Adapter.
In the second scenario, you can’t do the reverse because the lens will then have to be inside the camera body. Most times, you can’t saw open a camera body to stick a lens in. Yet, people have found ingenious ways to get it done, with some compromises:
You create another lens that manipulates the optical qualities of the first lens to make it work somehow. Such an adapter has one serious drawback – it changes the aesthetics of the original lens. The primary reason for using a lens is for its aesthetic benefits (one hopes). Why would you compromise that? Two good reasons:
- The lens in question has features you want but your manufacturer doesn’t provide a lens with those features. An example is using an ENG lens on a cinema body for its servo zoom and low f-number.
- The adapter actually enhances and gives you something extra, usually cheaper than what it would cost you to get the ‘original’. Examples include the Metabones Speed Booster, wide angle converters, 35mm adapters, etc.
All of these are lens adapters. The ones that are built solely to make up the distance will include:
- Lens mount of the lens
- Hollow space
- Lens mount of the camera at the other end
- Electronic contacts to get some or all of the same functionality (not always necessary or possible)
The second group will have a lens instead of the hollow space.
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 with a smaller flange focal distance might cause the protrusion to scratch the sensor or hit the mirror assembly in a DSLR.
Secondly, not all adapters give you infinity focus. Then there’s the problem of the image circle. Lenses made for smaller sensors or film sizes might seriously vignette on larger sensors, even if you could find an adapter. A good example is C-mount lenses on the Blackmagic Pocket Camera.
Next are the electronic contacts, which might or might work ‘as advertised’. A camera circuitry is usually 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 one must assume incompatibility until proven otherwise.
Finally, the biggest drawback of using lens adapters is distance error. No matter how well an adapter is made, there are always minute variances that change the distance. Poorly made adapters might not be perfectly level, and will cause optical defects. The better manufacturers provide thin slivers of metal that you can add (like a rubber gasket) to fine tune the distance. This process is called shimming.
Using an adapter is a matter of weighing the pros and cons, and then living with the compromises.
Examples of Lens Adapters
Look at the chart listing the flange focal distances. 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.
Having a smaller flange focal distance has three major benefits:
- It allows you to use many lenses,
- It makes camera bodies smaller, and
- It makes lenses smaller (generalizing here).
Having a larger flange focal distance also has benefits. The sensor is safely tucked away in the camera body, protecting it from dust and the elements while changing lenses, etc.
What to look for in a lens adapter:
- Precise machining, with a shimming kit if possible
- Metal – Strength to handle heavy unbalanced systems
- Infinity focus
- Electronic compatibility using CPU contacts
- Aperture control
- TTL metering
- Minimal optical degradation
Here are a few ‘established’ manufacturers who sell a range of adapters:
|Sony E-mount||Sony, Voigtlander, Cinevate, Metabones, MTF,|
|FZ Sony mount||Sony|
|Micro Four Thirds mount||Fotodiox, Novoflex, Zeiss, Voigtlander, Metabones|
|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?
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. Finally, it adds to the weight as well. Only when ‘direct’ options are unacceptable should you consider adapting lenses.
Remember how the lens mount is designed to integrate camera and lens into one seamless system? The lens adapter is now responsible for the same function. If it fails, it’s a third wheel in the relationship.