Ever since I bought the Panasonic GH5, I was shocked by the fact that lenses for Micro Four Thirds (MFT) cameras are so expensive. Though, I am able to salvage my Canon lenses by using an adapter with and without speed boost glasses, I am still in search for some other options. Needless to say, I happened upon some C-mount lenses and I am amazed, to say the least, by the performance of some of these lenses.

There are thousands of C-mount lenses out there with various focal length, sensor coverage, and above all, quality, but not all of them can be used on MFT cameras, so how do we choose?

Sensor Coverage Size. The first thing to consider, when choosing a C-mount lens is the sensor coverage size. Whenever you encounter a C-mount lens, one of the specification of the lens will be what size sensor it can cover. If this is not specified, ask the vendor or seller on eBay. Why is this important? Because nowadays, most C-mount lenses are designed for security monitor cameras, these cameras have very small sensor size, usually designated as 1/3″, 1/2.5″, 1/2″, 2/3″ or larger one 1″. Lenses that are designed for sensor size less than 2/3″ will probably not suitable for MFT cameras without scarifying some quality issues, such as heavy vignetting. Some camera has a special mode to support such lenses, for example, Panasonic MFT camera line have something called ExTele mode or ETC mode. What these mode does is to use a smaller area of sensor to capture image. But this effectively increases equivalent focal length. For example, GH5 has ExTele mode which increase the crop factory by 2.7X.

Lenses designed for sensor size of 2/3″ or 1″ most likely will work on a MFT camera. For those MFT camera with even smaller sensor size, such as Blackmagic Pocket Cinematic Camera, C-mount lenses designed for 2/3″ sensor will work very well, probably without any vignetting. C-mount lenses with short focal length tend to vignette, even if they have 1″ coverage, however, vignetting might not be an issue for video since not all sensor area are used and if you crop it further to 2.35:1 ration, vignetting could be completely gone.  This is just my observations and there might be exceptions.

The famous Fujian 50mm f/1.4 is actually designed for 2/3″ sensor and it works fine on my GH5 without vignetting and without using ExTele mode. However, it actually is not very sharp based on my tests and since it is designed as 5MP lens.

For longer focal length C-mount lenses, they seem to work very well with MFT cameras, even a C-mount lens for 2/3″ sensor. The famous Fujian 50mm f/1.4 C-mount lens is actually for 2/3″ sensors (as I was told by a security camera vendor). Just the other day, I encountered a 75mm f/2.8 C-mount lens designed for 1″ sensor, it does not have any vignetting at all, even for stills.

Sometimes, vignetting issue can be relieved by stopping down the aperture, just like with regular lenses. Here is an example:

Vignetting might not be an issue when shooting video as not all sensor area are used. Stopping down aperture can further reduce it. This video shows little vignetting because its aperture was stopped down to about f/5.6

There seems to be a trend to make C-mount lenses for APS-C sized sensors. This is probably due to popularity of mirror-less cameras such as Sony A6xxx, Canon M series, and Nikon N1 series camera. Some of these C-mount lenses for APS-C are very good and some are not quite there.

These are two popular C-mount lenses designed for APS-C sized sensors. They usually comes in complete with C-to-M43 (or C-to-EOS, C-to-AI) adapters and some even comes with extension tubes. These lenses are gaining popularity lately, and there are even Make II versions (with green or red ring around it).

Resolution. Besides sensor size consideration, resolution is another very important specification. Ironically, security cameras are notorious for their image quality. There are many reasons for this, one being the sensor size and the other being the resolving power of lenses used. Since most these security cameras use C-mount lenses, and hence the vast number of selection for C-mount lenses, it is very important to pick the right lens with right resolution.

So when you are searching for a C-mount lens, more than likely you will see something like 3MP, 5MP, 8MP, or 10MP (highest I have seen), this is particularly true for modern C-mount lenses, might not be there for vintage ones. These are resolution indicator, for example the 3MP is for 3 mega pixels sensors, 5MP is for 5 meg pixels sensors, etc. Because security sensors are usually small, thus smaller pixel size on the sensor, even if the 10MP lens is for 1″ sensor, the lens must have very high resolving power for the smaller sensor pixel size, else the image will be very blurry. Therefore, it is better to get those high MP C-mount lenses.

Here is am example image using an 8MP 75mm f/2.8 C-mount lens on a GH5. The other building is at least one kilometer away and it still captured those numbers.

75mm f/2.8 8MP C-mount lens used in above example.

Shopping of C-mount lenses. I live in Shenzhen, China, I have access to many stuff unimaginable out side of this city, like a huge building full of vendors selling LED lights, or electronic components, or security cameras (6 floors of them). You can buy almost anything within the building for a specific type of business. So one day, I happened up one lens, a 16mm f/1.4 C-mount lens and bought one. From the conversation with the sales staff (mostly relatives of owner), I got the feeling that they just want me to pay and go away, though if I ask more, they do give me more information that I might not know.

I have also seen similar things being sold on Chinese online shops, Taobao, and sometimes I venture into AliExpress. One thing I found out is that there are a lot of mis-information about a product. For example, the 75mm f/2.8 C-mount lens is being listed as f/1.8, or with two different pictures. Actual minimum distance for the 75mm lens is about 2 meters, yet it is listed as 0.7m for many listings in online stores. I guess you really have to ask them to get correct information. So when you shop online, particularly Chinese online stores, be careful, have a grain of salt about their listings, ask them questions.

16mm f/1.4 C-mount lens that is truly amazing. It can focus as close as 10cm and have magnification of 0.21x. Here two bought from two different vendors, they seem identical, same casing, same layout, yet one is better than the other. 

Another thing is actual specification about “minor” things. For example, the 16mm f/1.4 C-mount lens has 0.3m as minimum focus distance in most of online listings, but I was told that it is designed for machine vision and can focus very close by the store sales staff (yeah, they volunteered to push the sale). And yes, it is true, I was able to stack (taking multiple images at different focal planes) an image out of it, I was truly amazed by it. Had not the sales staff at local store told me that it can be used as macro, I would not have done this.

Image stacked from 8 images using the 16mm f/1.4 C-mount lens on a GH5

Expectations. Most of these C-mount lenses are cheap as they are mass produced and they have simple structure. From what I have seen, those made for computer/machine vision are better than those for security cameras and tend to be more expensive. The build quality seem to be acceptable and give me most likely utilitarian feel, nothing fancy.

There are some quirks when using them — though turning either focus ring or the aperture ring is very smooth, sometimes, the focus ring and aperture ring are swapped, so when you think you are turning focus, you are actually turning aperture. Or sometimes the focus ring is just too small, you essentially have to use your thumb and index finger to turn it.

Most of these C-mount lenses are intended to be used in fixed configuration for a long time, so neither the focus nor the aperture rings are adjusted frequently and once they are adjusted, they will be locked in position. This is why you see locking screws on them. This might be inconvenient for regular photography or videography as both rings are changed constantly. Sometimes, when one ring is locked, say the aperture ring, it is hard to turn the other, say the focus ring.

A 37mm UV filter is put on the 75mm lens, a 37mm variable ND2-400 is put on the 16mm lens with 33-to-37 adapter and a 37mm ND2-400 is put on the 25mm APS-C capable lens.

One more thing, sort of important, is accessories for these C-mount lenses. For example, to shoot a video with aperture wide open, to soften the background, during the day is impossible without an ND filter. I usually set the aperture to the sweet spot of the lens (f/4 seems to be right for all C-mount lenses I tested so far) and use a variable ND filter to set the right exposure. However, most of these lenses are very small and have filter thread that is not common in photography world. I have seen filter thread size of 33mm, 35mm, and 37mm. It is relatively easy to get filters for 37mm, but hard to find a filter with 33mm or 35mm filter size, so a step up filter adapter ring is needed to make the common thread size to be 37mm. From here, we can add ND filter, UV filter, CPL filter, even lens hoods, etc.

It looks pretty cool with a hood on it and most of these C-mount lenses designed for APS-C sensors come with it.

Conclusion. Over all, I am very happy with my collection of C-mount lenses for my GH5, they are truly amazing lenses. To get one for yourself, here are things to watch out:

• Sensor coverage size, ask vendor or seller about it if not listed, get those for 1″ sensor if you have normal MFT sensor or 2/3″ for super 16 sensors.
• Resolution of the lens, is it 5MP, 10MP? The higher the better.
• Ask a lot of questions about specification, very often, specification can be wrong due to typo or simple ignorance (some listings are just sold by “blind” merchant with zero industry knowledge.
• Sometimes, look can be deceiving, two items might look the same, but actually different due to different components being used.
• Accessories and step up rings are just as important.
• Expect some weird stuff like small focus ring, etc.

This concludes the first part of blog about C-mount lens. Next blog will actually be doing some test such as resolution tests, etc.