Adjusting Back Focus of Video Lenses
Most video viewfinders have less
resolution than the camera recording chips and it is difficult to see focus
of wider lens angles. It is wise to often check if the lens is "holding
focus" through its zoom range.
Most professional video lenses have
a "Back Focus Adjustment" at the rear of the lens next to the lens-mounting
flange. Back in the tube days of video cameras this adjustment was important.
Today there is more accuracy in the mounting of video chips and lens mounts
thus adjustments are needed less often. Note "Back Focus" refers the
distance between the rear element of a lens and the camera focal plane.
Work with less light so the lens
opens up to maximum aperture. Place the camera on a tripod. Use the ND setting
on the filter wheel if necessary. Set the lens to manual zoom. Support the focus
chart at right angles to the camera lens.
First focus your eyepiece! To adjust
back focus you need a focus chart or an object with fine detail. Focus the lens
on a near by object or focus chart. Zoom the lens all the way in. Now zoom back
until detail starts to blend together and refocus. If the focus stays the same
throughout the zoom range, the back focus is OK. If you have to refocus between
telephoto and a wider-angle position, back focus needs adjusting. At a wide-angle
position, release the screw on the back focus ring and turn it. Notice that
focus goes out in both directions just like focusing the lens normally. You
will use this to reset back focus.
Zoom in and focus with the regular
focus ring, then zoom wide and focus the back focus ring. Continue doing this
until you no longer have to adjust the back focus ring. Carefully tighten the
back focus screw and recheck if the lens still "holds focus" for the
whole zoom range. Now the lens distance calibrations should also be correct.
Motion picture lens back focus can only be changed with shims or lens mount
adjustments and should be done by a motion picture lens mechanic.
The advantage of the Seimans Star
is it makes seeing focus easier. Go to Professional
Equipment Advice/Focus Pattern to print out the Seiman's Star.
© Copyright 1999-2004 Ron Dexter.
All Rights Reserved.