Measuring for focus - perpendicular distance or trigonometry?

[Stuart Brereton]

Collimating a lens should be done by lens technicians at the rental house. It’s not something to do in the field.

[Dom Jaeger]

5 hours ago, Max Field said:

Which AC's job is it to calibrate the flange distance on each lens? I tried doing it on one of mine and could get it close but never exact. Or is that the job of the rental house when preparing a camera/lens package?

I'd love to go purely off of measurements because I'm sick of having to squint-eye the monitor all the time.

The lenses in a rental house inventory should all be properly calibrated, but of course they can sometimes go out due to wear or damage, or they may have been adjusted to suit another ACs preferences or accidentally maladjusted. One of the tasks the camera team should do during prep is to check all lenses for focus mark accuracy. If there are any lenses that don’t seem to line up, then the service techs can double check them and adjust the back-focus (or whatever else may be the issue). 

Sometimes ACs prefer to have focus marks set for best contrast wide open, sometimes it’s more consistent to have them set at a deeper stop like T2.8, sometimes different cameras with different OLPF thicknesses may affect high speed lenses differently and require tiny adjustment to the lens back-focus, sometimes a lens will be set for best focus a little off-centre due to field curvature, etc. 

The actual adjustment is not something ACs would generally do, as it requires a knowledge of what you might need to adjust, how to dismantle a lens to adjust it (it’s not always just shims under the mount), equipment or experience to know how much adjustment is required, a supply of different shims if needed, and equipment to very accurately check the image produced after the adjustment. 

Operators of ENG cameras can often fine-tune the back-focus of their zooms with built-in adjustment rings, but cine lenses tend to be more rigidly set.