LensRentals has found themselves in a unique position recently. Their warehouse is full of gear, and they have the time to test it.
Roger Cicala, the founder of LensRentals, often goes above and beyond to test his equipment. This time, he tasked Ben Berggren, a camera op and A/V Technician with LensRentals, with a $10,000 machine to test the flange distance in a slew of high-end cameras. What they discovered was that the distance is not always the same, and this has some knock-on effects.
What’s Flange Distance?
Here are an image and description from Wikipedia. “The distance from the mounting flange... to the film or image sensor plane.” I’m sure the image above will get the point across.
What’s interesting is that right after that quote, it says “usually measured to a precision of hundredths of millimeters.” This is striking, because Berggren did this and found that camera manufacturers are sometimes off by much more.
Why It’s Important
If your flange distance is off, your focus is generally off (it depends). A wider lens will see bigger problems here. Cicala noted that “a 15mm lens changes [focus] 2" for each 0.01mm change in flange-to-sensor distance. Focusing further away makes the change greater.” That’s a huge deal, and it seems like some cameras are many times off the mark.
You could just adjust your focus to fix the issue, sure. However, there are two issues that stand in the way for cinematographers. The first is that many sets will measure their focusing distance with a literal tape measure (that’s why there’s a hook on some higher-end cameras). The second is that “if your flange-to-sensor distance is longer than it should be by a few hundredths of an inch, then wide-angle lenses won't reach infinity focus.”
That’s a huge deal and a major pain. Fortunately, it’s something that’s fixable with shims. Unfortunately, a lens will need to be re-calibrated every time it’s put on a new camera, and a shim won't work if the sensor is a few millimeters farther into the camera then it should be. Note that this really only applies to cine glass, since often, photography lenses will focus past infinity.
If you want to take a deeper dive into the issue, LensRentals has put up the juicy details on their blog. They’ll have the second half up soon, with a look into how this affects smaller DSLR and mirrorless cameras.