Back focus, focus, and the Blackmagic Cinema Camera with EF mount
We recently sent out the Blackmagic Cinema Camera (BMCC) with EF mount on a rental and the DP had some questions about back focus. We thought this was a good time to go over some basics, and explain why certain cameras and certain lenses behave differently when it comes to focusing.
In an ideal optical system, you could focus on any object you want at any distance. Lenses in the real world have to follow the laws of physics, though, so they have a limited range of focus. Most lenses are capable of focusing from some near distance to infinity. Some specialty lenses trade that infinite focus point for more close focus distance, like a microscope, for example.
The distance between the camera sensor and the back of the lens is called the flange distance. This distance is important for multiple reasons. If it’s set to the expected distance, the lens will behave as expected. The focus range will be what the manufacturer intends. If it’s too short, the lens will be able to focus “past” infinity, which is an inconvienience for video shooters, but not really a big deal for photo shooters. If it’s too long, the lens won’t be able to focus to infinity at all, and only able to focus on very close objects.
For photo lenses, it is common practice with Canon’s EF mount to mount the lens a little closer than “perfect.” This is because EF mount cameras and lenses have been made by dozens of manufacturers with widely varying tolerances. This way, all the lenses can focus to infinity on all the bodies, even if they are a little bit off. The least forgiving lenses are the wide angle lenses, and manufacturers like Tokina (who makes the 11-16mm f/2.8) fudged the distance a little bit short to allow for variances between camera bodies. Most higher end photo lenses even let you focus past infinity intentionally, allowing for an even wider tolerance in manufacturing. For photographers, this was no big deal. The numbers on the focus ring were a little bit off relative to real life, but nobody was using a ruler to focus, because autofocus was much more accurate anyway.
When Blackmagic designed the BMCC, they designed it to match with Canon’s cinema lenses. These lenses are pricey, and made to much more exact measurements than Canon’s photo lenses, even their L series. Most of the high-end lenses with EF mounts have very precise focus marks on the barrel. They stop at exactly infinity. Then, they released it to the public, and realized that certain lenses are really meant to mount much closer to the camera sensor. Oops! Plenty of wide-angle lenses that worked perfectly on cameras like the 5D mark III couldn’t focus to infinity on the BMCC. So they performed a recall, and every lens mount they made afterward was adjusted to allow for the wider tolerance of photo lenses. This is how our BMCC is set, to the manufacturer’s revised specifications.
So now, cinema lenses on the BMCC work, too, but their focus marks are off. No big deal, right? Well, kinda. If you’re using prime lenses, nothing else changes. If you’re using zoom lenses, you’re now up against a back focus problem.
Zoom lenses are incredibly complex optics, and the flange distance becomes even more important. The lenses need to hold a point in focus at multiple focal lengths, as you zoom in or out. If the flange distance is off, your image can be in focus when zoomed in and when you zoom out, you will lose focus. Your focal plane will shift significantly. For photographers, this was never a problem, since you just zoom and refocus. For most high-end video cameras, you can just set the back focus manually for each lens. For video on budget cameras like the BMCC, it can be a concern.
The upshot is that if you want to use zoom lenses, and you want to be able to keep focus during your zooms, you probably don’t want the BMCC. Unless you’ve tested all your lenses on the body and you’re okay with whatever level of focus drift you get. And chances are, if you’re happy with the way a budget wide lens like the Tokina 11-16mm f/2.8 performs, you’re not going to like the way the Canon CN-E cine zooms perform on the same body. And even if you like the way one copy of a lens performs on the BMCC, you might not like a different copy of the same lens. Or you may rent a BMCC from us and it may work very differently even if you’re using the same lenses you always use. The flange distance is a compromise, and some lenses feel it worse than others.
If you’re thinking about renting a BMCC and shooting with zoom lenses, please stop by and check it out, with the lenses you’d like to be using. We’re happy to help you figure out your rental before you make a reservation, but we can’t give you a refund if your lenses don’t work with our camera.