Once the back focus is set, tighten the set screw. Then you check the back focus again. Obviously, with the back-focus ring locked down, you can’t move the ring to see if focus is right when the lens is at a wide angle. Instead, just use the lens-focus ring. If you move the focus ring in either direction away from the optimal focus setting (which you set when the lens was at full zoom), the focus, as evaluated by the Siemens Star Chart, should get worse. If that’s the case, then the lens back focus is set properly.
Cinema zoom lenses don’t have a back-focus ring that can be adjusted. Instead, they’re checked and adjusted like cinema prime lenses. So, what about prime lenses and back focus? Since a prime lens doesn’t zoom, there really isn’t a focal length change that needs to be compensated for. But what does need to be accounted for is the distance from the back of the lens to the image sensor—that’s the back-focus distance.
When you use prime lenses, you may use several different lenses to achieve various focal lengths. It’s important that the prime lenses you use will all focus to the same focal point. This assumes that the primes you use are properly collimated so they focus properly.
When you use multiple lenses, you don’t want to have to think about back focus with each focal length change. The back focus really needs to be about the lens mount. In this case, the critical distance is from the image sensor to the lens mount. This is called the flange focal distance. Since we’re not zooming, why do we care if the flange focal distance is off? We care because we want our actors to be in focus. If the lenses are all up to grade, but the flange focal distance is off, all the distance marks on each of the lenses will also be off. Someone trying to pull focus might hit all the marks, yet everything will be out of focus. On some cameras, flange focal distance is adjusted by using mechanical shims—essentially, thin spacers to increase the distance. On other cameras, the flange focal distance is adjusted by moving the sensor. This is what the back-focus adjustment would control on the CION you mentioned.
Back focus does relate to zoom lenses. The term is also used when talking about prime lenses.
Some DSLRs have an electronic compensation. That’s what Nikon calls AF Fine Tune, Canon calls AF Micro Adjustment and Sony calls AF Micro Adjustment. This doesn’t change the back focus or flange focal distance, but what it does change is how the autofocus system works in order to compensate for any flange focal distance discrepancies.
So, what’s the technique for adjusting flange focal distance or back focus? Because it varies by camera (and most people don’t have access to shims in their kit), I won’t detail adjusting flange focal distance. Instead, you can verify it. You can use a similar back-focus chart (Siemens star) just as you would for zoom back focus. But this time, the distance from the camera is critical.
Place the chart some distance that’s marked on your lenses. It could be 10 feet or something else, but it needs to show up on the focus ring. You must measure the distance accurately with a tape measure. Measure from the chart to the focal plane of your camera. (The focal plane will be indicated somewhere on the front or side of the camera; check the camera manual if you aren’t sure.) Open up the aperture as wide as it will go. Again, you don’t want a deep depth of field to cover up errors. Then, focus the image using a monitor. Don’t look at the lens focus ring while you’re doing this. You want to do it by eye. Once you think you have the camera focused, take a look at what the lens is telling you about the distance. Does it match the actual distance? If so, then the back focus is set correctly. Once again, this assumes your lenses are in good shape. To be on the safe side, check all your lenses with the same procedure.
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