Back Focus and Primes

[Tom Gleeson]

I am familiar with setting the back focus of a camera with a zoom lens but I am wondering what the procedure is and is it possible using Prime lenses? I rarely use zoom lenses and generally I use my EF Mount Canon CNEs or PL Primes on larger budgets. Without easy access to a zoom could I set my 24mm to infinity and point at an infinity distanced object and then adjust back focus? Would placing an object at 10 foot and setting the lens to 10ft and then adjusting be a better choice? Thankyou for any input.

 

[Dan Kanes]

The best tool for the truest backfocus is something like a Denz Flange Depth Controller.

It can demonstrate perfect optical backfocus on an infinitely adjustable camera like red DSMC2 or call out the number of shims on a camera that is shimmable.

I would basically not trust any lens to give you accurate backfocus as your main method of setting a true 52mm optical depth, except maybe a verified null lens with an autocollimator in front of it.

I say optical 52mm because the thickness of various OLPF and Sensor cover glass across various cameras needs to considered so a lens believes it is on a true 52mm system (compatibility from film to digital lenses) using a null lens or something like a Denz, optitek Optimator, or Lumacheck is more reliable than trying to use a mechanical flange checker - which is another nice tool for certain things - but not for digital cinematography cameras.

The denz can also be used to demonstrate parallelism (sensor flatness) under the right conditions because of the set of parallel lines it generates.

In a hot pinch you could set your infinity using a prime as you said, but it’s possible you might be inaccurate somewhere in your chain.

So - possible - yes , optimal - not always.

I should add - for the Zoom as you mentioned - that can work quite well if you are maintaining infinity throughout the zoom range - meaning - lens is Parfocal, not changing focus while zooming.

The lens could not be shimmed accurately but you could still deliver par-focality in the field using the method you describe as a form of field-triage.

Rental house or lens / camera techs should / would benefit from calibrating all of their gear so it matches, but we all know the real world is rarely the ideal world.