Don’t worry, it’s not as hard as it looks! When you have objects of different sizes and weights, the first thing you should do is find the center of gravity of each object.
Luckily, the devices used in videography are usually symmetrical, which means you wouldn’t go too wrong in assuming that the center of gravity is in the exact middle.
The second step is to fix a spot from which you want to start measuring the lengths. It can be anywhere, but a good place to plant your flag would be at one end of the train, so to speak. This point is called the Datum, as you can see from the above drawing.
Each object has a weight, W1, W2, W3, etc. Correspondingly, the distance from the datum to the center of gravity of each object is d1, d2, d3, etc. Simple, right?
Now all you need to do is multiply each of them and add the results: (W1 x d1) + (W2 x d2) + (W3 x d3) and so on. If you’ve read the section on Moments, you will have guessed that W x d is the moment of each individual object.
To find the Center of Gravity, use this formula:
[(W1 x d1) + (W2 x d2) + (W3 x d3)] / [W1 + W2 + W3]
That’s it! The result you get is the center of gravity of the entire system, taken as a whole. If you try to place the system on a needle at the center of gravity, it shouldn’t fall.
On a shoulder rig, the center of gravity should fall on the shoulder/spine. When you take your arms off the rig, it shouldn’t fall forwards or backwards.
Many people like to use chest pads to help support a front-heavy rig. If it works for them, that’s fine. But what if the operator wants to tilt down or up? What if you want to smooth pan by twisting your torso while keeping your legs firm? How do you get your rig off your shoulder without killing yourself?
For these reasons and more, I always try to keep things simple and elegant, unless it is impossible. Rule of thumb: When something impossible comes up, it’s always because of an unreasonable human expectation.
Start with what HAS to go together
The brain, SSD module, Side handle, Redmote, Redvolt, PL mount and Redmag always go together. I’ve bunched them together as the red box, weighing in at 3,515 grams. I’ve selected the back side of the Epic as my datum, and the distance to the center of the box is about 111 mm.
Note: The values in the schematic or in this guide might be incorrect or assumed. Please consult the manufacturer’s documents for actual figures, and make your own calculations.
The big numbers are the weights in grams. The small numbers are the distances in millimeters. The colored dots represent an approximate position of the center of gravity of each object.
Add everything that has a FIXED place
The lens obviously goes in front of the PL mount, but we have a bunch of lenses for our production. Either the length of the lens changes or the weight changes, and one does not correspond exactly to the other in our case. E.g., the Red Pro wide angle primes are heavier than the Zooms.
In any case, by varying the length or the weight, the difference is minimal – only in our special case. It is safe to assume an average distance of 303mm from the datum and a maximum weight of 2,932 grams. This is the blue box.
The matte box goes in front (green box), and I’ve taken an average length, because the actual length will vary depending on how many flaps there are and how much they are opened. Again, you don’t have to be precise here, you’re not designing a rig, just putting together one.
The follow focus goes to the right or the left, as shown as the yellow box. Either way its effect on the rig is negligible.
What isn’t negligible is the Bomb EVF, which weighs in at 600g (grey box). It’s exact distance varies based on the operator’s comfort and physical features, so I’ve given it a general location in relation to the shoulder pad.
Underneath all this, are two 15″ 240mm steel rods, 60mm apart, centered along the optical axis. It’s weight isn’t negligible, and is shown as a white rod with a black border.
As you might have noticed, everything below the datum line is in front.
Where do you place the shoulder pad?
If you try to calculate the center of gravity of the above ‘front-heavy’ system, you’ll see that it’ll take a lot of weight at an approximately equal distance at the back to balance this rig. Sometimes, there’s no other way than to do it this way, but we should first consider the easier alternative.
The ideal way is the bring the center of gravity (the middle of the shoulder pad, approximately) as forward as possible, and for this reason, I recommend getting it underneath the camera body directly. This is what I’m aiming for, as indicated by the grey line that runs right through the middle of the camera (red) and shoulder pad (orange).
If you look at camera systems designed for shoulder use, like the Alexa and the F65, you’ll see they have deliberately designed it this way. In the case of the Red Epic, if and when further modules are added to the brain to extend the length of the camera system, it might be okay to push the shoulder pad back, but not here. Why?
The further you push the shoulder pad backwards, the tougher it will get to balance this system. If you have studied the formula for moment carefully, you might have noticed that the larger the length, the more the impact of the weight. 1 pound over 4 feet is the same as 4 pounds over 1 feet. The further you push your front-heavy system forwards and away from the shoulder, the longer you’ll have to stretch back to counter its effect. If you don’t, the amount of weight you’ll need to add will be heavier than your camera system!
You can’t solve every problem by adding weights at the end. Remember, adding weights will also increase the total weight of the rig. It’s okay if your operator is a green monster; mine isn’t.
Keep the images of the earth and bubble in memory. Try to keep distances as short as possible.
In the next part we’ll add the rest of the gear, and finish rigging our Red Epic.