Red 18-85 t2.9 focus issues

Ken Diewert · November 18, 2011

Hey everyone,

I'm new here but am searching for at least a partial solution to a Red focus issue that came up yesterday. We're shooting a low budget feature on an Epic with the only lens available being the Red 18-85 t2.9. Our canon mount is supposed to have been here weeks ago. I'm pulling focus on an Epic for the first time. We're using a Redrock follow.

I don't have the ability to collimate the lens but I'm sure we have picked up a back focus issue. The markings on the barrel of the lens are too coarse to measure with a tape and set focus. so I've been using a combination of zoom and magnify. Peaking has been relatively useless as we are shooting fairly low lighting. Yesterday I noticed on a scene we were shooting at 6' distance at 32mm, that the focus was not the same at 32mm as it was at 85mm. Since then, my confidence has been lacking. I'm feeling good at 70-85mm and. But near and wide has got me a little rattled.

Has anybody got any experience or advice with this issue?

Thanks in Advance

Stephen Williams · November 18, 2011

The lens & camera needs to be checked, however you could just try to adjust the Back focus to the lens, it's rather easy with epic.

Dom Jaeger · November 18, 2011

The trick with setting back focus on a zoom is to start at the long end, and get the focus sharp. Then zoom to the wide end and adjust back-focus (or in your case the camera flange depth) until it's sharp.

Then go back to the long end and fine tune the focus. Depending on how much the flange depth was changed, it may only need a tiny readjustment of the lens focusing barrel.

Then go back to the wide end and fine tune the camera flange depth if necessary.

The zoom should now hold focus through the zoom range.

Do the adjustment with the lens at T2.9 focused at 6 or 8 feet, double check it at infinity.

Ken Diewert · November 19, 2011

Thanks,

We ran some tests on a chart and there were definitely some back focus issues. We got through shooting Friday and the owner is taking both in for calibration.