Sunday, February 20, 2011

Tiny C-mount Lenses on MFT Camera

One of the cool things about c-mount lenses is their size. Some are very small. If you're a rangefinder camera user (as I am), or if you're into the MFT system because it offers RF-like functionality (again, as I am) then this is probably something you'll find appealing (small fast lenses on compact bodies is a highly valued combination for RF users, because it facilitates quick discreet shooting in a wide variety of lighting situations, which is great for shooting in a photo-journalistic fashion (e.g., street photgraphy) or taking candid photos). However, all is not rosy and perfect. These little lenses can have their drawbacks, some of which can affect how you end up shooting with them.

My recent c-mount lens hunting and gathering produced these two beauties, which are ridiculously small.

Wollensak Cine Velostigmat 1-Inch f3.5

Wollensak-Keystone Cine Raptar 1-Inch f2.5

I pulled The slower lens, the f3.5, off a Keystone A3 16mm camera. The Keystone A3 is one of those wind-up pill-shaped movie cameras. The other lens (f2.5) also has a 'Keystone' designation (on the lens), and although I haven't yet verified it, I'm guessing that this lens was attached to a similar type camera.

Both of these lenses are considered normal focal lengths for 16mm film (they are 1-inch or about 25 mm). So, one would think that these normal lenses coupled to a MFT camera using a c-mount adapter would work without a hitch. But they don't, and this brings up a point (or two) about dealing in the wild world of c-mount lenses.

About the only thing standard with the c-mount "standard" seems to be the mount width and the thread (pitch, etc.). Also, while it's convenient to think of c-mount as the mount for 16mm film cameras, it is also the mount used for video and CCTV cameras, and these formats can have their own normal focal-length lenses. Sensor sizes vary considerably, so be careful when bidding on auctions or when hunting and gathering video and CCTV c-mount lenses.

These two lenses present a couple of issues of which one should be wary when purchasing or gathering c-mount lenses, particularly older 16mm lenses. First, just because the mount is a standard don't assume that the lens will work with a MFT camera and adapter. Case in point, note the thread depth of the lens on the left in the photo below compared to the thread depth of the lens on the right.

The thread for the lens on the left (the f3.5) is too long for a MFT camera. From the base of the lens, the thread extends about 4-5 mm. When attaching this lens, it threads past the adapter and contacts the beveled interior of the camera. In the photo below, you can see where the lens rubbed up against the camera interior.

Forcing the lens to mount flush with the adapter certainly would have damaged something, so when mounted, the lens sits away from the camera.

On the other hand, the thread for the f2.5 lens extends around 2.5-3 mm. This lens screws in firmly and sits flush with the adapter.

None of this really matters, because if you look closely at these two little beauties, you'll notice something is missing. Yup, there isn't a focus ring. These are both fixed focus lenses. The f2.5 lens doesn't provide an in-focus image at any distance, and the f3.5 lens—by virtue of its inability to mount through to the adapter—provides a focus distance of about 10-12 inches. So, what does one do? How does one use these lenses on an MFT camera?

Well, to focus the f3.5 lens, I unscrew the front element.

The f2.5 lens doesn't have a detachable front element, so I focus the lens by unscrewing it from the adapter.

This actually works quite well—in both instances—and probably more so with the f3.5 lens, because the threads on the lens are finer than the mounting thread. With the f2.5 lens I can focus the lens very precisely, but the amount of slop in the threads means that I have to either "focus" the lens and allow it to 'hang' from the mount, or I can hold the lens firmly against the mount as I focus. The difference between the two techniques produces significantly different focus points. I prefer the latter method.

With both lenses the processes can get a little fiddly as the aperture ring and the mounting (in the case of the f3.5 lens) have a tendency to move as well. The amount of dexterity required can slow down shooting considerable, and one loses that RF-like advantage of shooting quickly with small fast lenses.

As an alternative to loosening the front element or the mount, another possible way to focus these lenses is by using shims or extenders and setting an acceptable fixed-focus distance. One can then refer to a DoF scale to determine an in-focus range and use the camera as a fixed-focus point-and-shoot—that is, use the lens as it was probably intended.

This method is more applicable with the f2.5 lens than the f3.5 lens, because the latter lens is already "shimmed" by virtue of its inability to mount completely—hence the 10-12 inch focus point.

I picked up a couple of shims, a 1 mm and a .5 mm and three 5mm extenders.

With the shims, I can get focus points of about 6'8" with the .5 mm shim and 3'4" with the 1 mm shim.
The .5 mm shim at a higher f-stop could be very usable for shooting as a fixed-focus point-and-shoot. (I'd really like to pick up a shim or create one that will allow me to set the focus at infinity.)

With the three 5 mm shims I can get six inches with one, three inches with two, and 1.5 inches with three.

Obviously, the macro possibilities when using extenders is interesting.

So, these diminutive little lenses have an appeal, but they have their drawbacks—the least of which is the odd looks you might get from the DSLR crowd (we RF users are used to that mild annoyance). Compatibility and focusing could be an issue, and these are things one should consider carefully. In the end, what good are these little lenses if one can't use them for the intended purpose. Of course, the real appeal of c-mount lenses is the signature they are capable of producing, and it's really one of the biggest reasons to delve into c-mount-to-MFT.

The two lenses discussed here compared to a focus-capable Wollensak 25/1.5.