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Setting the Back Focus on B4 Lenses
One of the huge advantages of B4 lenses is that they are designed to be ‘parfocal’ which
means they hold focus all the way through the zoom range. This means you can zoom in,
getfocus on an object and then when you zoom out it will remain in focus.
The back focus ring is typically located at the rear of the lens, close to the mount
Back focus is critical in ensuring that your HD or 4K B4 lenses retain focus right through the
zoom range. When you first mount a B4 lens on your camera you should check that the back
focus is correct. Back focus should also be checked when you swap lenses, and it’s a good
idea to periodically check back focus at the start of major projects.
To check the back focus of a B4 lens:
1 Place a focus test chart, or a flat white item with good high contrast detail,
approximately seventy feet, or as far away as possible, from the camera.
2 Set the iris on the lens wide open and compensate for exposure by using the built in
NDs if you need to. You’ll want a shallow depth of field so you can exaggerate any
focus issues.
3 Zoom into the chart and set focus.
4 Now zoom out. If you reach maximum wide and the focus remains sharp, then your
back focus is correct and you don’t need to make any adjustments.
However, if you zoomed out and the chart drifted out of focus, the back focus is
incorrect. Zoom all the way out and adjust the back focus ring on the rear of the lens
close to the mount. The back focus ring typically has a twist knob used to loosen and
tighten. Simply loosen the twist knob and rotate the ring until your chart is in focus.
5 Now zoom in and out slowly and check if your chart remains in focus all the way
through the zoom.
6 Repeat steps 4 and 5, zooming in and out and making adjustments to the back focus
ring. If your lens holds focus all the way through the zoom range your back focus is now
set correctly.
15Using Servo Zoom Lenses
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