Since quite some time tectum is utilizing the Ribcage Kit from Backbone to mount prime lenses on GoPros. Our initial goal was to modify the GoPro for highend productions and finally make them work for 3D. With the Ribcage Kit you can use a variety of prime lenses which enables you to pull focus and adjust the iris. In combination with the GoProHero3+ you have the possibility to use the Protune-Settings where you can lock your ASA as well as work with 2 different Gammas. Once you modified your GoPro, there is no turning back and the underwater housing can't be used anymore. As we modified our first GoPro it took us nearly 2h using the video tutorial from Back-Bone. Modifing the GoPro with the Ribcage Kit from Backbone is not a big deal, nevertheless we recommend to have it modified by your dealer.
The Ribcage Kit is a new front housing for the GoPro and has a C-Mount lens adapter as well as an anti-alising filter. The filter can be installed constantly or in a flexible way. We decided to install the filter in the flexible way. This has the advantage that you could still clean the sensor if there should be dust on it and be able to use the GoPro Camera as a infrared camera. If you take out the anti-alasing filter the GoPro is sensitive to IR-Lights. This easily enables you to shoot in complete darkness using IR-Lamps. Another challange was the right choice of lenses and the correct back focus. First we used our Kowa 10P C-Mount lenses which have a 10 mega pixel resolution and are computed for 2/3" sensors. Those lenses are technically ideal for the setup but the GoPro has a 1/4" sensor so that there is crop factor of 2.9 and even 6.3 using a S35mm-lens.
Therefore we researched on 1/3" CMount-Lenses and found the Computar lenses. Although the Kowa lenses are still superior in quality the 4mm and 8mm Computar lenses are an excelent choice for wide angle shots. The Computar lenses are quite inexpensive and are available at price starting at €100,-. The Kowa lenses in comparison cost around 800€. However, the C-Mount can be adapted to EF-, Nikon- or even PL.
To adjust the back focus correctly a monitor is essential. There would be the possibility to use the GoPro-App on the smart phone but the picture is not good enough and the delay to hight to work properly.
The adjustment of the back focus with the RibCage Kit is not a big problem. The lens or adapter needs to be screwed into the mount until the correct back focus is achieved. The lens or the adapter will then be fixated with small screws. Even though the procedure is quite easy we recommend to prep the cameras a day before the shoot to eliminate oversight.
In order to work safely with the RibCaged GoPro we had to modify the GoPro-Frame as well. Using the modified GoPro Frame enables you to still use all commonly used GoPro accessories. The mounting kit that is delivered with the Ribcage didn't turn out 100% stable. On one occasion the screw of the mounting kit broke during production but was easily reapaired afterwards. In the meantime our RibCaged GoPros were used in several commercials.
Our last test for the RibCage GoPros is the suitability for stereoscopic productions. As of now we have found two different ways to syncronize the GoPros. The GoProHeroDual System and the „Advanced SuperHero 3D System Cable“ von 3D Guy.tv. Although we are still in the testing phase it shows that both systems give you the possibility to start both cameras at the same time. It seems that for short takes the synchronisation was satisfying. Takes longer than 2 minutes will have a slight off sync. We are working on a final test where we will mount our RibCage GoPros in a small mirror rig. We will test the sync extensively as well as the stereo alignment and the Protune-Settings.