The PL mount is an excellent standard that Arri gave us several decades ago and has been the industry standard alternative to Panavision’s camera mount ever since. The PL (Positive Locking) mount is large enough to accommodate sizable rear elements and strong enough to support the largest of professional cinema lenses (with proper support of course). More and more cinema is moving over to Nikon F, Canon EF, and even the Micro 4/3 standard. So why is everyone trying to slam a PL mount on their grandfathers old set of Nikon AIS lenses? It’s simple. All three of the still photo mounts I mentioned have their limitations that can really disrupt a cinematographers flow. For example, Nikon, Canon, and M34 all have a locking pin that keeps the lens set in it’s place and you push the little button to release the pin. Most of those camera mounts have a very weak leaf spring that keeps a bit of pressure on the lens mount to stabilize the lens. Certainly not as much pressure as PL mount fully tightened. Still photo mounts usually have one position that the lens attaches to the camera in and that’s it. you can’t rotate the lens relative to the camera whereas PL mount, depending on the lens manufacturer, can have up to four mounting positions, each 90 degrees apart. Not a deal breaker but still just another reason PL is superior for cinema. I can go on all day about the benefits of PL mount over Canon or Nikon mounts but that wouldn’t help many people. I receive more and more emails and phone calls from people asking if we can convert their Leica R or Zeiss ZF lenses to PL mount. Here’s the important part of this post… Sure, it’s possible. But it’s also possible to put a Ferrari motor into a Hyundai. I guarantee it would be cheaper, quicker, more reliable, and all around easier to simply purchase a brand new Ferrari than it would be to custom install a Ferrari motor into a little Hyundai. The same goes for PL mounts on still lenses. I can see the appeal of putting a PL mount on your favorite “rare” Leica lens but it’s probably not what you had in mind. This sort of conversion would take a lot of design time and prototyping which any engineer can tell you is very expensive. In engineering and machining, its always cheaper to order large quantities than it is to order a single part. When prototyping, ordering thousands isn’t possible since you don’t know if it will work. That’s the whole point of prototyping. This means that a prototype design and fabrication can cost 5-10 times more than the actual end product. Panvision did this a while ago with a bunch of Leica R lenses branded under the Dalsa Digital Cinema name. The company went under shortly after so there are only a few sets of rehoused Leica R lenses floating around the cinema world. To get a bit more in-depth of why simply adding a PL mount requires so much design and engineering, here is a quick little sketch I made that illustrates the flange depth issue behind PL mounting still lenses. A bit more of the technical stuff. The problem with changing from a still photo mount to PL is all in the dimensions. Particularly flange depth and back focus. The two different specifications are critical when combined. Any error in either measurement can be devastating. Any given lens is designed and set to a specific back focus, the distance from the rear element to the sensor. This measurement is extremely critical to within .0005″ or 12.7 microns. Changing the distance from the rear element to the sensor will result in the inability to obtain infinity focus or accurately adjust focus according to the scale on the lens. So that’s that. You cannot change the distance from the rear element to the sensor. Period. In my illustration I’ve use a standard lens originally designed for a Canon EF mount which you can see at the top. The distance from the rear element to the sensor is represented by the blue dotted line which, as we already covered, cannot change. The red dotted line is the flange depth. Flange depth is a manufacture specific dimension. It’s the measurement from the surface of the mount on the camera to the sensor. Canon EF mount has a flange depth of 44mm. The flange depth of the camera is critical to the lens because the lens is seated on the camera mount. Which means that the distance from the rear surface of the lens mount to the sensor is also 44mm. PL mount is 52mm, represented by the lower illustration with a red line as well. The distance from the surface of the PL mount to the sensor is 52mm on all PL mount cameras. The flange depth is essentially just empty space. A blank distance that gives the lens area to focus the light onto the sensor. So when we have a still photo lens that is designed to have only 44mm of empty space and we apply the same dimensions to a 55mm empty space you have problems. Remember, the distance from the rear element to the sensor cannot change which means that you have to move the mount but not the glass. In the illustration above, take a look at the PL mount on the lower graphic. It’s place much farther forward on the lens barrel. This shows exactly what the problem is in most cases. the difference of the 44mm Canon EF mount and the 52mm PL mount is a mere 8mm but those 8mm make a huge difference. You essentially have to remove 8mm of material from the back of the lens to accommodate a PL mount. In this case that could mean cutting into critical components such as aperture linkages, focus mechanisms, and anything else that is located at the rear of the lens. This is where the expensive part comes in. All of these components can’t just be thrown away. They have to be re-engineered and repositioned to fit in a space that will be 8mm shorter than it was before. This isn’t always the case as some lenses are smaller or constructed differently. For example, telephoto lenses traditionally have a large empty space from the rear element to the mount which leaves a lot of empty space to work with. There are usually still a lot of mechanical components that have to be addressed, but they just aren’t crammed in like most other lenses. So some lenses would be easier to work with than others, but it’s always going to be a custom one off job until someone decides to take a nice new large supply of still lenses and rehouse them to accommodate a PL mount all while repositioning components to function properly. Oh wait… Zeiss already did that with the ZF.2 and CP.2 lenses.
Like I said before: Yes, putting a PL mount on just about any lens is possible. It’s just not going to be quick, cheap, or easy.
