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Distance Scale

You need to DIY a clearly marked distance scale and affix it on the RC controller, so that the joystick can be used as a focus control lever. You also need to add a pointer to the joystick to indicate the focal distance.

THE TINKERING

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You need a total of nine marks:

          infinity, 20ft, 10ft, 7ft, 5ft, 3ft, 2ft, 1ft, CF (for Close Focus)

Go to Channel Monitor in the Frsky RC controller to see the joystick position.

Follow this table to line up each distance mark with the respective joystick position.

Set up SBUS Channels & RC Controller

Go to the Remote in your camera’s menu. Assign a SBUS channel to each camera function you want to be controlled by the RC controller.

Spend some quality time with the rather complex menu of your Frsky RC controller to map out the key for each camera functions. You need to assign a button/switch/dial/lever to each SBUS channel. Your preference in control interface will probably differ from mine, so I won’t go into details on the RC menu setup.

You need to assign focus to the left joystick. You also want to use one of the dials for aperture (iris).

I made a DIY distance scale with some scrap plastic sheet and some epoxy glue.

I printed individual distance marks with a label printer, then I cut each label into arrow-shaped mark. I made it extra-legible by using yellow sticker over a black background.

Custom Lens Curve

Here comes the secret sauce that makes distance-accurate focus pulling possible.

You need to create a custom curve for each lens. The curve translates lens motor positions into physical focal distances. it is very accurate because you will measure each of your particular camear + lens combo with a tape measure, eliminating any inaccuracy from flange depth imperfection or lens sample variation.

Another benefit of lens curve is smoother focus pulls, because you are saving the joystick’s resolution for the most useful range of focal distances and redistributing the focus throw more evenly. Otherwise, you will be wasting 1/3 of your joystick range on values beyond infinity focus and another 1/3 of the range on distances below 1 foot, resulting in very touchy focus pulls.


This diagram shows you the steps to make a nine-point lens curve (click to enlarge):


     

Here are my settings for each SBUS channel. You may have other preferences.

I created a profile for each lens in my kit, each has its own 9-point lens curve.

One caveat to note. Each lens curve is calibrated for the particular camera+lens combo used for the measurement. It may or may not work accurately if you use it on a different camera body or a another copy of the lens, despite being the same model. There is a certain amount of precision tolerance in the manufacturing process of cameras and lenses. Variance in the flange depth of the lens mount or differences in the lens motors may throw off the back focus. Wider lenses tend to be more sensitive to such back focus variance. You should always double check for distance accuracy and make any necessary changes to the lens curve when you switch to a new camera+lens combo.

Closing Thoughts

I hope this guide helps shorten the path for those who is interested in building their own BMMCC camera kit. It took me two months to figure this out, but now with this guide, it should be a rather straight-forward kit to set up.

It is not perfect. I wish the lens motor is even smoother. A follow focus style handwheel would be a much better control interface than a small joystick for focus pulling. But overall, this setup is reasonably reliable and has served me quite well in many mission-critical situations.

I also intend to write this guide as a proof of concept for the implementation of distance-based focus pulling with a small camera. I believe camera makers are not paying enough attention to the importance of a proper focus control system in DSLR / mirrorless cameras. More than a decade has passed since DSLR took over the world of low-end (and some high-end) film productions, but we haven’t seen much progress in the tools for focus control. Some innovations in this area is badly needed. If a model plane controller can do such a good job in focus control, I am sure the big camera manufacturers can design a much better system.

In a separate article, I wrote an open letter to Panasonic to ask for better focus control in their newly-announced Lumix-S system. Click here for the FULL ARTICLE.

And to Blackmagic Design. SBUS support is a GREAT feature. Please continue to make new cameras with SBUS support. I will buy the new Pocket Cinema Camera 4K in a heartbeat if it supports SBUS. Maybe I will buy two.

I’d love to hear from you about this article. Please share your thoughts and experience in the comment section below.

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CONTENT COPYRIGT

I am have no affiliation with any company whose products are mentioned in my articles unless otherwise stated, nor do I accept any financial reward from them.

DISCLAIMER

Jack Lam is a cinematographer based in Beijing and Hong Kong. His body of works includes TV commericals, seasonal TV drama series, and theatrical feature films. His commercial clients include Cathay Pacific, Lenovo, Airbnb, Alibaba, and Mandarin Oriental Hotel Group. His feature films in China are commercial successes.

Jack also works with DJI as a design consultant for their cinema products.

Jack is a graduate of Tisch School of the Arts at New York University, where he received two Excellence in Cinematography Awards. He then further pursued his craft at the prestigious FAMU conservatory in Czech Republic.

To see a showcase of Jack's works , click here >

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