Wednesday, 1 October 2014

New Bolex prime lenses

Recently, a set of three prime lenses were announced by Bolex, with C-mounts:


The lenses are quite small, with a filter thread of 43mm. Pancake lenses like the Lumix 20mm f/1.7 and Lumix 14mm f/2.5 have a filter thread of 46mm. These Bolex lenses cost around US$350 per lens.

So how is this relevant for Micro Four Thirds?



Even if the C-mount is one of the few mounts which has a shorter register distance than Micro Four Thirds, there still exists adapters. Here is just one out of many examples:


Using an adapter like this, you can use c-mount lenses on Micro Four Thirds cameras. However, most c-mount lenses have a smaller image circle than the Four Thirds sensor, so you will frequently get vignetting problems.

Bolex claims, though, that these three lenses have an image circle which includes also the Four Thirds sensor, so it should be safe to use them on Micro Four Thirds cameras.

This illustration shows the difference between the standard Super 16mm image format, which most C-mount lenses conform to, and the Four Thirds sensor size. As you see, Four Thirds is much larger, which is why many C-mount lenses vignette on Micro Four Thirds cameras.


There are more oddities to these lenses. Of course, they are all manual focus only. Further, they have the special "cine gears" on the focus rings.

These are used to mount a remote controlled servo motor to the focus ring. The purpose is to be able to control the focus in a very smooth manner. Servo motors for cine use tend to be very expensive, and are not seen used by amateur enthusiasts very often.

Finally, the lenses do not have any aperture diaphragm mechanism at all. You only have one aperture available, f/4.

For indoor use, I would say that is no problem. You can set the camera to a suitable shutter speed, and then select the ISO which gives you the proper exposure. The Lumix GH4 camera (my review) now even has auto ISO in manual exposure mode, which makes this easy.

For outdoor use, this is going to be a problem. Using the "sunny 16" rule, you need a shutter speed of 1/2000s at ISO 200 on a bright, sunny day. This is much faster than desirable for video use.

For movie use, it is often preferred to have a shutter speed which is twice that of the frame rate. So if you have a frame rate of 30FPS, set the shutter speed to 1/60s. This is called a "180 degree shutter", and the purpose is to get some motion blur on moving objects. Read more about this here, and see some examples illustrating how using a 180 degree shutter is different from a faster shutter.

To achieve a slower shutter speed outdoors, you need a neutral density filter (ND filter). Preferably a variable ND filter, like this.

An ND filter stops a large portion of the light coming into the lens, and lets you use a slower shutter speed.

Another typical use of ND filters, is to combine fill flash with a large aperture during daytime. Let's say you are taking a portrait photo during daytime, with the sun behind the subject. To avoid shadows in the face, you use a flash. At the same time, you want to use a big aperture, to blur the background.

The camera will need a very fast shutter speed, to avoid over exposing the background. The ND filter comes in here, allowing you to use a slower shutter speed, slow enough to fit inside the flash sync range of the camera. With the Lumix GH4, the flash sync speed is 1/250s, meaning that you must keep the shutter speed below 1/250s while using the flash. On the Lumix GM1, the flash sync speed is much slower, 1/50s, so you need an even stronger ND filter.

Conclusion


Given all this, are these lenses still interesting for use on Micro Four Thirds cameras?

I think the shortest lens is. It has a focal length of 10mm, which is a quite wide lens. There are no other very wide lenses like this, which are also very compact. And I think the f/4 aperture is quite adequate for most uses.

The other lenses, I don't see much use for. I would rather go for the Sigma 19mm f/2.8 lens, rather than the Bolex 18mm f/4. The Sigma lens is cheaper, and faster.

And rather than the Bolex 38mm f/4, why not get a lens like the Sigma 30mm f/2.8 or the Olympus 45mm f/1.8? I think those would be better choices for most uses.

Beyond these three lenses, there are heaps of cheap c-mount lenses available. However, most of them are poor quality, and will cause vignetting on the Four Thirds sensor.

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