You seat the lens in the mount after a perfect exchange, you hit your marks while pulling focus, and when you see the footage, you’re devastated to see it’s all out of focus anyway.
What happened?
Well, many things could’ve happened, but there’s a chance it was the backfocus of the camera you were shooting on. And with one simple check, you could’ve caught the problem early.
Flange Focal Distance, Backfocus, and Depth of Focus
Understanding how things work is one step towards being able to diagnose the problem. Before we dive into the tests for backfocus, it’s important that you understand the underlying concepts of flange focal distance, backfocus, and depth of focus as well as why backfocus matters.
Flange focal distance is the distance between the lens mount flange and the film or sensor plane while backfocus is the distance between the rear element of the lens and the film or sensor plane.
Both of these distances are crucial to have correct. When they are aligned properly, the distance markings on the lenses are exact and on point.
When they are misaligned, even by a few microns, the distance markings on the lens could be wildly off and completely unreliable.
To understand how this works, you must grasp the concept of depth of focus (different from depth of field). Since I am no lens expert, I will let the authors of The Filmmaker’s Handbook
Depth of focus is the tolerance in the accuracy of the mounting or seating of the lens on the camera. Never confuse depth of focus with depth of field. Depth of focus is an area behind the lens (inside the camera).
It refers to the very small distance on either side of the focal plane where the film or video camera’s sensor can be situated and still record an acceptably sharp image.
Depth of focus increases as the iris is stopped down. However, unlike depth of field, it is least in wide-angle lenses. This means that a fast wide-angle lens needs to be very accurately mounted. On some lenses, even tiny pieces of dirt on the mount can throw off focus.
If a prime lens is not properly seated, its focus scale will not be accurate and a tape-measured focus setting will be inaccurate … [and] might not be able to focus to infinity.
Having the backfocus exactly correct is essential to keeping your footage in focus — especially if you are relying on the distance markers on the barrel of the lens. Even if you rely on a monitor or viewfinder, being able to focus to a true infinity is critical.
The Simple Way to Check Backfocus
Always take the time during camera prep to do this test. It should be towards the top of your to-do list and any camera assistant worth their salt knows it — when it comes to focus, camera assistants don’t mess around!
Because the distances behind the lens are so incredibly fickle, you have to be absolutely sure you can reliably trust how the camera is setup.
Unless you’re armed with an autocollimator, an easy way to check back focus is to perform this test:
- Place a Siemen’s Star Chart on a Wall. If you are on location, or don’t have access to a chart, use a dollar bill or some other complex pattern. You can also use a newspaper with a black line drawn down the middle and shot at a 45 degree angle.
- Position the camera at level height about 10 feet away. Be very precise on this measurement and make sure it is a distance that has a corresponding marking on the lens.
- Mount a zoom lens or mid-range prime lens. 50mm lens works well in these cases. With the zoom lens, you should zoom in until the chart fills the frame.
- Open up the iris of the lens all the way. You want to minimize depth-of-field so that you don’t mistake something in acceptable depth-of-field as something in critical focus.
- Focus by eye using a viewfinder or monitor. Preferably you want to use the sharpest, biggest monitor you can find.
- Check to see if your eye focus matches the lens marking. If the backfocus is set correctly, then your eye focus mark should be exactly on the “10 ft.” mark, or whichever distance you chose, on the lens.
By running this test, you can diagnose a backfocus problem and have it fixed by the rental house or do it yourself if the camera permits (see below).
If you are doing this test on a film camera, you have the option of focusing to the mark (i.e. 10 feet in our example) and running film to have developed. This is a more precise test.
When testing with a zoom lens, you should add the additional step of zooming to different focal lengths on the lens after you have focused by eye.
Again: do not skip this test during camera prep.
It will help you rest easy knowing the flange focal distance is properly aligned and that the backfocus has been correctly collimated.
Fixing Backfocus on Location
Problems don’t always manifest themselves in the comfort of a rental house. Most of the time, you’re stuck on location with the tools in your ditty bag and the know-how in your head.
If the problem happens to be the backfocus on your camera, you have a few options, but not too many. For the last time, I reiterate that you should check this problem thoroughly during prep to limit its chance of arising during production.
Still, no amount of preparation develops into a guarantee.
Film Cameras
Film cameras are wildly different beasts than their video brethren. They require different types of servicing, different types of tests done during prep, and are more mechanical than they are electronic.
When it comes to film cameras, the flange focal distance is serviced usually by a rental house or other experienced technician in a process called collimation.
You can learn more by reading this article, “How Backfocus is Serviced on Film Cameras.”
The takeaway here is that you won’t be servicing a film camera’s flange focal distance by yourself on location. It requires more precise tools and a practiced hand. This makes checking it during prep even more important, since you can’t correct any mistakes on set.
Digital Cinema and High Definition Video Cameras
Cameras that use digital technology, like the RED One, are usually able to be serviced on the spot, which is a good thing because the backfocus on these types of cameras is constantly thrown out of whack.
With digital cameras, you can’t check the flange focal distance during camera prep and think you’re safe. It’s a check that, if possible, should be performed daily or weekly at the very least.
If your check reveals that the backfocus of the camera is off, then it’s time for you to adjust the backfocus yourself.
I wish I could list out simple 1-2-3 steps for doing this, but every camera is different when it comes to adjusting the backfocus. Your best bet is to have a copy of the manual printed out or downloaded on a smartphone for reference. If you are curious about a particular camera, leave a comment and myself or someone with experience will let you know the steps.
Like I said, each camera is different, so take the time during prep to become familiar with how to do these adjustments so when you attempt them on set you aren’t under the tremendous pressure of time and inexperience.
Backfocus Off = Focus All Soft
In my last article about backfocus, I used this phrase and told the story about my friend who had lost a week’s worth of footage due to some bad checks at the rental house.
Don’t become one of those people with a horror story to tell! Always check, double check, and triple check the essential parts of the camera during your prep time.
With a film camera, if your lens markings aren’t focusing where they’re supposed to, well, it’s going to be hard to pull focus that way. With a digital cinema camera, you don’t want to be left relying on external monitoring to get focus sharp in case something happens to those devices.
Measuring by distance is the most fool-proof method to pull focus, but it will only be successful if the backfocus is precise. Check it during prep and rest easy at night knowing that the dailies production is screening will be sharp as a tack.
Are there any major problems you have had with backfocus? Also, I’ve heard the term “Harp Test,” but have never fully understood what it is — an Internet high-five goes to the person who can explain it in the comments!