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C-Mount Lenses

C-Mount lenses have become popular with the BMPCC because the adapter is very simple and cheap. But the situation with C-Mount lenses is kind of complicated, since this specific mount has been used for a long time and a variety of film & sensor sizes.

History and Origin

The C-Mount has a long history of usage. It is the oldest mount still in general use and is reviving fast, because of the quality of the glass made for it. Early cinema was all on 19mm film and the mount used was the British Imperial measure of one inch, 25.4mm. So a massive gamut of cameras used it from early 16mm Film Cameras (i.e. Bolex) to modern CCTV and Broadcast Television Cameras.

Image Circle

The C-Mount itself unlike other mounts, does not specify the image circle it is going to be used for. Unless you specifically know that a lens is designed for a 1“ (CCTV) sensor size or Super16, S16, Film size, you have to be careful and research if the image circle covers the BMPCC's sensor. Some standard 16mm Lenses do work, some don't. Generally, the wider the lens, the less likely that it will cover the sensor. Some lenses designed to cover a 1/2 inch sensor will cover the BPMCC sensor, but these are usually 25mm or more and will render as a FF 70mm lens, applying the 2.88 ratio. Ultra wide lenses for the BPMCC are highly sought after, with many accepting that purchase of a new cctv or machine vision lens is the only answer if they really need a wide angle.

Sadly, there are other difficulties in selecting a C mount lens: the ranges of lens that cover different sensors but have the same description eg 25mm 1.4 is a recurring description but can mean 4/3, 1 inch, 2/3 in., or 1/2” lenses! Either we have to have the catalogue number or else a very accurate picture of it. Best to buy from sellers who demonstrate the lens on S16, Nikon CX, or MFT sensor cameras. Note that on MFT cameras at least, the video mode is 16:9 and this increases the crop factor over the normal 4:3 still photograph. This means that a still shot vignettes more often that a video sample.

Difference between CS and C-Mount

The mount was specifically adapted for use on more modern cctv cameras by shortening the distance between the sensor and the lens by 5mm. This is the CS mount, presumably for C (Short) Mount. These cameras do not use large sensors, but 1/2 is the largest and 1/4 the smallest. Lenses designed for these sensors will disappoint users if used as C Mount lenses: the normal focus range is severely restricted to close focus or even macro focus. The image formed by these lenses is well in front of the sensor unless inserted far into the camera, which is normally impossible.

They do make interesting macro lenses.

There are specific adapters, 25mm dia, 5mm deep, that enable normal C mount lenses to function as CS lenses. These adapters also come in smaller eg 1mm and larger sizes eg 40mm and have no glass, merely adjusting the distance between lens and sensor. You cannot make a CS lens into a C lens by using one!

Sensor Coverage

As the mount has been in use for a century or so, the lenses simply cannot be said to be always suitable for use on the BMPCC, which uses a Super16 sensor, larger than a standard 16mm. Seek a demonstration of the lens with this sensor combination on the net or from the seller.

The actual coverage of the lens is always a circle, unless it is tilted by special adapters. The minimum standard coverage is of a diameter of 16mm, so the sensor should have a diagonal measurement of this amount or less. A lens hood or filter ring may also restrict coverage. Some users find that grinding away the front of the lens, which may contain information, will enable coverage of the S16 sensor or even a m4/3 sensor which requires a coverage diameter of 22mm.

The earlier lenses have interesting aberrations that make them very interesting from the point of view of isolating a central figure. Generally speaking, the longer the focal length of a 16mm lens, C mount lens, the larger the sensor that may be covered. Some C mount lenses, 230mm, have been used on 135 format, FF, cameras with no vignette.

Some early 8mm cine lenses, often on a C mount, but also on a D mount(?), are actually 16mm lenses, sold as 8mm suitable. D mount lenses, a 16mm dia mount may cover a Super16 sensor if 'long' enough. At least 35mm focal length may be suitable(?)

Lens Corrections

During the long period of existence of C-Mount lenses, various optical challenges had to be adressed by the manufacturers. For example, the Bolex 16mm cameras at some point used beam splitting technology by introducing a prism into the optical system. This allowed the photographer to see the actual image he/she was going to film. Since every optical element changes the characteristics of the light which actually reaches the film gate (or sensor nowadays), some of the lenses made for Bolex cameras compensate for that optical element, especially to avoid spherical aberration. So, when used without the prism - for example on the BMPCC, they are “overcompensated”. Check out this article from 1976(!) that explains the “bolex problem” in depth.

But theres more: The sensor usually has an extra optical plane to filter out Infrared from the sensor, unless using a video camera specifically made for infrared recording. Basically a little piece of glass sitting in front of the sensor. This also is a an optical element that changes the characteristics of the light and thus is compensated for by modern lenses. When using a lens made for film cameras, this is not the case.

Don't be disappointed by this, it does not mean old C-Mount lenses don't work on video cameras, just don't expect the same quality that a “modern” lens, specifically made for a video sensor would deliver. A lot of people actually prefer this to the sharp and crisp look of modern lenses as well.

Lenses reported to be working with the BMPCC

C-Mount to M4/3 Adapters

Since the flange focal distance of the C-Mount (17.52 mm) is slightly smaller than that of the Micro Four Third Mount (19.25 mm), the lenses need to stick a little bit into the camera mount of the BMPCC. If this is possible with a particular lens depends a lot on the diameter of the lens barrel, but also on the way the C-mount Adapter is made. There is a variety of Mount Adapters, expecially with the more “budget” ones, often of unknown origin. In that case, it is common practice to order from a specific EBay seller, once you know which one you need. There are adapters in the price range from roughly 5$ to 60$.

Check our list of adapters and research before making a choice for your specific lens.

Lens Modification

Some lenses can be modified to fit an adapter, popular example is the Kowa 8mm (LM8HC). The lens needs to be modified in a lathe, here is a video showing the process

There are some people that can do this modification, lens repair, collimation and other lens magic at reasonable prices:

lenses_adapters/c-mount.txt · Last modified: 2014/09/20 10:35 by