Focus does not work on my lens at a certain zoom distance & aperture

[John W. King]

Hello all,

 

I use a BMPCC with an Angenieux 12-120mm f/2.5 lens; to attach the lens, I use an Arri-pl mount.

 

Now, I have an issue with this lens, and I'm not sure if it can even be solved: I get an out of focus picture when the zoom distance is below 20mm while at an aperture below f/5.6. No matter how much I adjust the focus ring, the picture is entirely out of focus.

 

Is this natural for which the lens does not work with the Blackmagic's sensor, or am I just a special case and there's something wrong with my lens?

 

Thanks,

John

[Gregg MacPherson]

Sounds like a collimation problem. Flange focal distance is off. A lens technitian will say it better. Dom Jaeger may drop in and comment.

 

I have to say, the 12-120 was a great workhorse in the 16mm days, a wonderful hand held cine verite lens. Mine had a light, smooth zoom to the fingers and was cheap to service. They are not that sharp. The Zeiss 10-100 of that era were very sharp. But Angenieux, the name, perhaps is seductive to contemporary shooters using BMCC etc.

[Dom Jaeger]

If you can get a sharp image at the 120mm end but it gets softer as you zoom out to 12mm then the back focus of the lens is off. This is the distance from rear element to sensor. How you have adapted the original lens mount (Arri Standard to PL?) can affect this critical setting, that's the most likely culprit. But it could also be the camera lens mount adapter, or the lens itself could be off. A technician could sort this out, but first make sure whatever adapters you are using are properly fitted.

[John W. King]

Thanks Dom and Gregg.

 

I was messing around with the camera and lens this morning, found out that the lens has a slight amount of wiggle room while mounted. Turns out if I pull the lens a little bit away from the mount, the focus works fine while at 12mm.

 

I tried to take off and remount the lens, but I'm still getting that wiggle room. On further inspection, it looks like someone chipped away at the slit on the back end of the lens in order to make it wider (at least that's what I think, I could be wrong and it may just be a natural opening).