Back Focus Specifics

[Brian Stevenson]

Having an incorrect flange focal depth with prime and zoom lenses will obviously make all of your witness marks useless. A zoom will not hold focus over its entire length. But it seems that if you're focusing by eye that you can still get sharp focus when the cameras FFD is off. What I want to know is if this is CRITICAL focus, can you obtain a sharper image, or is there more resolving power to the lens when the FFD is spot on? (Assuming the lenses are all calibrated and have been checked.)

 

 

[Satsuki Murashige]

I think in general you would still have the ability to achieve critical focus unless you were on a wide lens and near infinity focus. Since there's very little tolerance for back focus on wide lenses, all it takes a few microns to make hitting inf. focus impossible. I suspect that there may be additional issues focusing with lenses that have a curved field of focus like anamorphics, Cookes, and other vintage optics. Would have to test to make sure though.

[Brian Stevenson]

Thanks, I did not know that different focal lengths had varied tolerances for FFD.

[Satsuki Murashige]

No problem. The area of focus projected by the lens is called the 'depth of focus' should you want to read up on it:

http://en.m.wikipedia.org/wiki/Depth_of_focus

[David Hessel]

With anamorphic lenses the FFD can be very critical, many anamorphics will not resolve to a sharp image if the FFD is off. As with spherical the wider the lens the more sensitive it is.