Blackmagic Forum

I would like to use the very light weight Canon EF-S 10-18mm f/4.5-5.6 IS STM on my Pocket 6K.

The problem with the lens is, the f-stop changes while zooming even with manual exposure.

For example, when the lens is set to 18mm and the f-stop to 11.0, zooming out to 10mm changes gradually to f-stop to 9.1 with the zoom.
It happens both with shutter speed and shutter angle.
Auto Exposure is OFF.

Any idea how to get a stable, fixed f-stop with this lens on the BMPCC6K?

Sadly, I don’t know, but you might save up for a cine zoom close to those focal lengths. For example: the Tokina CinemaATX 11-20mm T2.9 is parfocal and constant aperture.

The Canon 10-18 is also parfocal and is performing good enough for many of my projects, which makes the aperture problem even more frustrating.

The Tokina 11-20 weights over 4 times as much, which can be a problem in some situations.

Will keep trying to make the Canon 10-18 work.
It would be a great light weight option.

vivoices wrote:The Canon 10-18 is also parfocal and is performing good enough for many of my projects, which makes the aperture problem even more frustrating...

Ah, that’s good. I agree my suggestion wasn’t the best when you want to be light and inconspicuous.

So many stills zoom lenses ramp the iris with changes in focal lengths. Very frustrating. If a lens goes from f/4.5 to f/5.6, you should be able to set it at f/5.6 and have a constant aperture. I think it works on some lenses. One of the irritants that people don’t seem to care about until they want to use their stills lenses for video.

The open aperture ramping from 4.5 to 5.6 is not the problem.

Since the f-stop is the ratio of the lens' focal length to the diameter of the entrance pupil (aperture), the diameter (D) has to changed with the zoom's focal length (f), in order to keep the f-stop (N) constant.
N = f / D

The Canon EF 70-200 and the EF 17-40 for example seem to handle the diameter change internally and mechanically. I can zoom their full range at e.g. f11 and the f-stop stays all the way at f11 on the Pocket 6K body.
The consumer level EF-S 10-18 on the other hand seems to rely on a camera body to control the diameter electronically to keep the f-stop constant while zooming. On Canon bodies, the f-stop stays constant while zooming the EF-S 10-18.

The Pocket 6K body does communicate with electronic lenses.
So it should be possible for Blackmagic to update their firmware to make the EF-S 10-18 work properly on BM cameras, especially because the EF-S 10-18 is parfocal and zooming smoothly, and therefore very well suited for video.

Excellent suggestion! You seem to have proven that Canon cameras are managing the change in iris for zooming on the EF-S 10-18mm which puts this problem in a whole new light.