Understanding back-focus

Approximate Reading Time: 4 minutes

It’s 8:55 AM and your church service is about to start. Before your livestream begins your AVL team’s camera operator zooms in and frames a the tightest shot he can framing your worship leader’s microphone grill and adjusts the focus barrel on his lens. The camera operator then zooms out very wide setting up his first shot of your broadcast … an establishing wide shot of your room.

The worship team takes the stage and the lights come up. You are now live to the web! Your camera operator starts a slow zoom into the worship leader … but the camera (that he just focused a moment ago) is now out of focus! What happened?

Your Director cuts away from the camera and the cameraman again zooms all the way in and focuses on the worship leader’s eyes. He then pulls out to a medium shot.

Again, the shot looks slightly out of focus!

Is it time to get a new camera operator?  Or is the camera broken? Maybe the lens is bad?

Or maybe your lens’ back-focus just needs to be adjusted?

All camera lenses have two dimensions which the lens must compensate for. When you rotate the focus barrel on the front of the lens you are actually compensating for the distance between the subject and the camera. However the distance behind the lens, between the lens and the camera’s imager is equally important and needs to be adjusted for as well. This adjustment is called back-focus.

All broadcast lenses will have a small back-focus knob (screw) near the back of the lens. When loosened and rotated, the sharp-focus point between the lens and imager can be compensated for.

Cameras that do not allow interchangeable lenses usually do not have a back-focus adjustment on the lens because the back-focus distance (should) be set during the manufacturing process at the factory. If using a professional (film) cinematography lens and it doesn’t have a back-focus ring, the manufacturer expected back-focus would be adjusted during a shop check-out using film-camera lens shims. Still photography, or hybrid still/video DSLR lenses typically do not have a back-focus adjustment because the lens was never intended for video applications with zooms (despite what the DSLR camera manufacturer may say). When shooting stills, the need for back-focus adjustments rarely comes up because you can refocus in between shots if the camera, or subject moves. One never has the opportunity to zoom on-air when shooting stills.

So then, what is the process for adjusting back-focus?  Here are ten simple steps to insure your lens focuses correctly.

Step #1: Firmly secure your camera on its tripod (or a stable mount where the camera is stable and safe).

Step #2: Aim the camera at a Siemen’s Star chart. If your zoom lens is wide angle in focal length, locate the chart about five feet from the camera. If your zoom lens is standard in focal length, about ten feet from camera will do. If you are using a long stadium telephoto lens, put the chart twenty or more feet away.

If you don’t have a Siemen’s chart try downloading and printing one, or when all else fails use a practical object that is relatively flat and perpendicular to your camera (avoid many foreground and background objects in the frame).  A practical object should be high in contrast detail, such as a wrought iron fence.

Use house lights, stage lights, or a portable light to illuminate the chart.  You do not need the chart to be excessively bright (due to the f-stop setting instruction in the next step).

Step #3: If the iris is on “automatic,” change it to “manual.”  Now adjust the lens’ f-stop aperture as wide open as possible. Depending on your lens, this will likely be f/1.4, f/2 or f/2.8.  It is very important that the aperture is as wide open as possible.  Performing a back-focus adjustment with a small aperture (a large f-stop number) with then moderate or deep depth of field will prevent you from accurately adjusting back-focus.  So set the aperture wide (the smallest number).

Since you are manually setting the lens’ aperture to its widest position (the smallest number f-stop) it may be necessary to reduce the amount of light on the chart, or use an ND filter, or increase the camera’s shutter, or reduce the camera’s gain; so the chart is not over-exposed due to the wide aperture.

If your lens has “an extender” feature (a lever which when engaged magnifies the lens’ focal length making the lens more telephoto), turn it off.

Step #4: If your viewfinder or monitor has a “peaking” control, you can (temporarily) set it or your viewfinder’s contrast higher to making seeing the back-focus adjustment easier.  If you do not have peaking or contrast controls on your monitor or viewfinder, skip this step.

Step #5: Zoom in fully onto the center of your Siemens star chart (or an alternative object).

Step #6: Adjust the lens’ front (normal) focus control until you achieve optimal focus onto the center of the Siemen’s star.

Step #7: Now zoom the lens fully out to its widest field of view.  You may now have other objects in the foreground or background.  Ignore them, pay attention only to the center of the chart (or the exact area of a practical object if not using a chart).  Do not touch the front-focus barrel of on lens.  Gently loosen the back-focus screw on the lens and slowly rotate the back-focus ring until you achieve the best overall optimal focus of the chart. It may help to slowly rotate the back-focus ring past the optimal focus setting and then back again to optimal back-focus.

Step #8: Gently finger-tighten the back-focus ring screw so it doesn’t accidentally move unintentionally. Repeat steps #5, #6 and #7 until you no longer need to adjust the (front) focus or back focus in either the wide or telephoto field of view focal lengths. It normally will take two or three repeats of steps #5, #6 and #7.

Step #9: You are almost done.  Tighten the back-focus ring screw a bit more so it doesn’t accidentally come loose while operating the camera. Do not over tighten it.

Step #10: Return any settings you may have changed (viewfinder peaking and contrast, iris manual/automatic switch, aperture, ND, shutter, gain, extender, etc.) to your camera’s normal operating settings.

The back-focus distance adjustment is just as important as front focus which is why this test should be done periodically even if your cameras never leave their tripods. If you have handheld cameras or cameras where the camera operators hands are physically touching the lens often, these cameras should be back-focus checked as part of your normal morning set-up routine on each rehearsal or production day.

If the lens is not focusing accurately, all other optical and picture evaluation tests are meaningless, and worse, you could end up with unusable footage. If you follow these steps you can rest assured that your lens will track focus throughout the zoom range and you can then move forward with the task at hand, making great looking video.

About Tom D'Angelo

Tom D'Angelo is a TV production tech guy who loves the Arts and the Church

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