Focus This On That

At AbelCine, we get a lot of questions on camera and lens combinations. With so many new cameras hitting the market, often with new lens mount types, it’s no surprise that people are trying to determine the best lenses for their new camera system, while others are trying to utilize lenses they already own with a new camera. We all know that lenses often outlive our cameras, so it only makes sense to get the most out of them.

With so many questions coming in, I wanted to give you a brief breakdown of how to determine which lenses can be adapted to which cameras. I’ll also give you an overview of the different adapters available today, including mechanical, electronic and optical adapters that will help you get just about any lens on any camera.

FOCUS ON THIS

To determine if a given lens will work on a specific camera, we have to look at a variety of factors. The first and most important factor is flange focal depth. This is the physical length from the lens mount (the flange) to the sensor or film plane. Lenses are designed to focus light at specific distances. For example, a PL lens focuses light at 52mm from its mount, and an EF lens focuses light at 44mm from its mount, while an E-mount lens focuses at only 18mm.

Sony a7S with a PL lens attached via an adapter.

If you were to put a PL lens at a different distance from the sensor, other than its specific flange focal distance (52mm), then the image it produced wouldn’t be in focus. In fact, if the lens was off even by more than 0.01mm (10 microns), then the focus scales on the lens would no longer be accurate. We call this a flange focal or back focus issue, and I’m sure many of you have come across this before. Every lens mount has a specific flange focal distance, and getting it right is critical.

MECHANICAL ADAPTERS

One of the easiest ways to adapt between lens mounts is just to put something between the lens and camera mount to physically correct the flange focal distance. A great example of this is Zeiss CP.2 lenses. These lenses have interchangeable mounts, meaning you can remove one lens mount and attach a different one. When you do this, you’re essentially adjusting the flange focal depth of the whole lens.

In the image below, you see the same Zeiss CP.2 lens with various mounts on different cameras. Notice how the distance between the rear of the CP.2 and the marked sensor plane is always the same; the lens mounts simply position the lens at the correct distance for focus. The great thing about the Zeiss solution is that it enables the lenses to function without adapters for a given camera system. However, we could also use the lenses with a simple mechanical adapter.

Zeiss CP.2 lenses have interchangeable mounts that can mount to different camera systems.

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