Even though wide lenses have a great depth of field, they have very critical
depth of focus and the mounts have to be very accurate. It is very difficult
to judge focus at infinity. Use a Siemens star focus chart at a very close focus
where you can see focus at the widest aperture. If the focus marks agree with
the object distance, you are probably OK. If not use this formula to see how
much error there is in the lens mount.
d = lens displacement from infinity position
f = focal length of lens in inches
a = distance focused on in inches
d= f squared / a-f
To correct the flange focus of a zoom that is adjustable, as most video lenses
are, zoom in and focus on a close object, zoom back focus again and repeat this
until the zoom holds focus at all focal lengths.
Some wide-angle attachments for professional lenses require the back focus to
be adjusted. The focus scale now doesn't work. After the attachment is removed
you have to reset the back focus. Use a Siemens Start (radiating lines from
a point) to check focus. The focus scale now doesn't work. On consumer camcorders
some wide-angle attachments up to about .6 work with the whole zoom range. Wider
attachments rely on the auto focus system to adjust and find focus. It is almost
impossible to see focus on the new LCD viewfinders.
To check the focus at the film plane you can place a ground glass to fit in
the gate or against the aperture plate of cameras that have removable movements.
Sand all the sharp edges off the glass to prevent scratching any parts. A right
angle prism is easier to use than a mirror to look at the image. Make a holder
for the prism and ground glass to make viewing easier because you need one hand
to hold a magnifier (like a 50-mm lens) and one to focus the lenses. Photo stores
can supply 4 x 5 ground glass. Some are plastic and are OK. You can also grind
glass or one faced of a prism against another piece of glass with fine glass
© Copyright 1999-2004 Ron Dexter. All Rights Reserved.