Flange distance accuracy for zoom lenses

[rob spence]

If the flange distance is out on a camera a prime lens will over focus or under focus at either end of the scales ( close up or infinity )...is this the same with a zoom lens, or are there also problems at different zoom positions?

[Simon Wyss]

To my knowledge it is about the same. Most varifocal or zoom lenses have the strong positive power image forming part in the rear, followed by the afocal part that allows to change focal length, and the focusing part at the front. Things look somewhat different with tele zooms or when only longer focal lengths are considered. A true zoom lens offers correction of geometrical issues but I think the closer you have the iris to the film or sensor plane, the more critical is that technical setting. Only the manufacturer can tell you more.

[Satsuki Murashige]

The zoom lens will also not hold focus when zooming. If the lens should be parfocal (as all cine zooms are), this is a quick way to check and see if the FFD of the camera is correct.

[Dom Jaeger]

Yes, if the camera flange depth (or indeed the lens back-focus setting) is out, a zoom will not hold focus through the zoom range. It will get progressively softer as you zoom out to the wide end.

 

It is similar to how a set of primes would behave if the camera flange depth is out - a. long prime will only be a little off its focus marks, while a wide will be way off.

[Peter Welander]

Is this sort of effect with zooms common with Bolexes? I was shooting with the ubiquitous 18-86 Switar and found it to be OK at the long end to middle, but I could not get it to focus at all at the wide end. Soft as anything when at 18 mm. Primes on the same camera worked pretty well.

[Dom Jaeger]

It's not necessarily common to Bolexes, it's just a lens (or camera) out of factory tolerance. It's easily checked by a technician and adjusted back if necessary (with cine lenses anyway). Sometimes it's a case of a lens not seating properly in the camera mount.

 

Usually the camera flange depth and lens collimation are pretty stable. They can go out through damage or wear but more often the cause of maladjustment is someone having fiddled with them.

[rob spence]

Interesting Dom, the reason I'm asking this is because I am using a stills zoom on a Sony F3 including using lens adapters...the whole thing could be a recipe for disaster! Having said that I haven't really come across any softness as yet , so perhaps the collimation just so happens to be correct . Although considering it's a stills zoom ... vivitar series 1with pentax mount, connected to a pentax/EOS converter mount ...then to a third party EOS/sony F3 mount I'm amazed.

[Robin R Probyn]

I doubt its par focal (wont hold focus from tight to wide) being a stills lens.. because they don't have to be and its alot cheaper to manufacture).. so you most likely have to check focus anytime you change the frame size anyway.. but if you cant get infinity you have a problem..

[rob spence]

It's a push pull zoom too so even if it were parfocal they are more difficult to zoom in/out without altering the focus accidentally with your hand. Seems to focus all the way through the ranges ok though.

[Dom Jaeger]

A smaller aperture gives you more tolerance at the film plane (depth of focus), so if it's a slow zoom (like many stills zooms) and it doesn't go down to a short focal length, you can have some error in flange depth without it being obviously apparent in the image.