BMCC-EF Losing Focus During Zoom

Hi guys,

I recently had my camera replaced because of the rolling banding issue. Since I have gotten the new camera, I have noticed focus going soft when I zoom. I am using a Canon EF-S 17-55. I took the camera to a local shop and tried a 70-200 f2.8 is ii and got the same result.

With the 17-55, setting focus at 17, it stays pretty sharp until 35. From 35 to 55 things go wacky. I didn't check where things went bad on the 70-200.

BMD have been very helpful in both situations. They replaced the last camera and they have offered an RMA to send this camera in for testing. But, I hate to send it in if I am missing or overlooking something or if it is a lens problem that may have been present on both lenses tested.

Has anyone else experienced this? Could this be caused by the BMCC or is it a lens issue?

Thanks in advance for any insight!

Most DSLR lenses are not parfocal. This mean they don't hold focus as they zoom. They are not designed to because when shooting stills you don't zoom after focusing.

To make a zoom lens parfocal is very complex. This adds to the cost. This is one of the reasons cine zoom lenses are so much more expensive than DSLR zooms.

So there is nothing wrong with your camera or lens. This is normal behaviour for a DSLR lens.

Most DSLR lenses are not parfocal.

Hi Adam,
Thanks for the response.

I agree that the 17-55 is not parfocal, but it is suppose to be close to parfocal (whatever "close to" means). I didn't notice much deviation in focus on the 17-55 on my other camera. It didn't jump out at me like it does now. It is very obvious at this point.

Also, I just checked on the 70-200. I thought that it was parfocal, but it seems that I was mistaken. So that test is out.

Thanks again Adam.

Most DSLR lenses are not parfocal.

Hi Adam,
Thanks for the response.

I agree that the 17-55 is not parfocal, but it is suppose to be close to parfocal (whatever "close to" means). I didn't notice much deviation in focus on the 17-55 on my other camera. It didn't jump out at me like it does now. It is very obvious at this point.

Also, I just checked on the 70-200. I thought that it was parfocal, but it seems that I was mistaken. So that test is out.

Thanks again Adam.

The 70-200 is not par focal. The difference between different cameras comes down to different sized sensors.

Even on non-parfocal lenses, I think you will have better luck if you set focus at 55 then pull back to 17 rather than focusing wide and expecting it to hold as you zoom in close.

Even on non-parfocal lenses, I think you will have better luck if you set focus at 55 then pull back to 17 rather than focusing wide and expecting it to hold as you zoom in close.

Thanks Marc,

Yeah, that's how I usually do it and how I noticed that it went soft in the first place. It does it both ways. I just used the 17 going to 55 as an example.

Thanks for the input everyone.

The 70-200 is not par focal. The difference between different cameras comes down to different sized sensors.

Thanks Sean,

Both cameras were BMCC-EF's (same sensor). I just received a replacement for a camera with rolling banding issues.

The concern that I am having is that the lens didn't show as much focus variation during zoom on the first camera as it does on the replacement.

You may find the back focus tolerances on the mount are not identical. It only needs to be different by a few microns to change how a lens focuses.

That is the nature of the EF mount (or any DSLR mount) and you have no way of adjusting the back focus.

On a cinema camera with a PL mount you can adjust the back focus to that it is accurate. This is important as the lenses are all accurately calibrated for the focus puller. DSLR's don't need to be that accurate and so there is much more tolerance in the mount.

Dwight, you may have better luck if you focus first at the telephoto end, then zoom to wide, roll film, zoom in. No guarantee though but may be worth trying for static scenes.


Rick Lang
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I would just have an assistant pull focus and not have to worry about it


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Thanks for the information guys!

I feel a little better knowing that it is most likely not a defect. I do some zooming during live events which is where I noticed it. I don't zoom while shooting anything else. The picture is clear and sharp and I like the results I am getting, zoom/focus issue aside.

What triggered my concern was the difference between the two cameras with the same lens. But, If this is normal behavior with this lens, then I am happy .

Your help was and is always appreciated.