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  • Hey all, just changed over the backend after 15 years I figured time to give it a bit of an update, its probably gonna be a bit weird for most of you and i am sure there is a few bugs to work out but it should kinda work the same as before... hopefully :)

Wierd backfocus issues - something new - is something wrong with our camera?

J

jacknusa

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Hi,

I'm sure you have all read multiple "backfocus threads" but I think this one is different, so pleas stay with me...

Saw that the wide shots were kind of soft after our last shoot, so I decided to re-check backfocus.

I used a 16mm lens @ 3meters with a Siemens star as the target. and sure enough back focus was a little off, so I do the following:

a) Unscrew the two locking screws on the (PL) mount collar (which took some effort because they were TIGHT).

b) Set the lens @ 3m

c) Turn the collar to achive best focus.

and now it gets scary!

d) Tighten the two locking screws (not crazy tight)...

...and now the focus is off!!! - way off!!! - I reajust the focus ring on the lens, and best focus is now about a meter off from the 3 meter mark that was best focus before tightening the locking screws.

Thinking that I'm doing something wrong, I repeat this process more than 10 times - but the conclusion is clear, tightening the locking screws changes the backfocus adjustment - by quite a bit.

Slowly I realize that my only shot at getting correct backfocus with the screws tight, is to dial the collar to a point where focus is quite a bit off with the screws loose, and then thighten them - check how much focus is off - open the screws, and "guestimate" the needed correction - tighten the screws again, and after 10 or so iterations of this, I arrived at what I hope is a correct backfocus setting.

So, what's going on - is this the way it is, or is something wrong with our camera?

Can someone please shed some light on this?

Jack

P.S. And before you ask, I was very careful to make sure that the ring did NOT turn while I was tightening the screws.
 
This has been reported by quite a few people. You have to watch as you tighten and try work out where you should be before you start tightening. It sounds like you worked that out for yourself.
 
Uhm, ok, so I guess every thing is "good" then...?

Still seems a little funky - and, it further makes me feel that the process (ith this funkiness) should be properly documented by RED.

Or, maybe it is and I'm just looking in the wrong place - I'm looking in the RED one operations manual.

Jack
 
I have adjusted back focus on several cameras and never had this problem. How much are you loosening the screws. They should be loosened just enough so that there will be some friction when turning the back focus adjustment. Also, make sure that 16mm lens is dead on if you are using it as a back focus tool. If you are still having the problem, then I would talk to Red.
 
I've had the same problem as well. The only good thing is, since I got it set correctly I haven't had to change it, regardless of what lenses I'm using.

I also find that if I keep trying to set and lock the focus, sometimes it'll lock without throwing itself out. It's slightly frustrating during prep, although prep is much faster with the Red than 35 so I really can't complain. I'm sure it would be much more stressful if it was done on set.
 
Ive always done it as a two man job. Im actively watching the focus while someone else is tightening down the screws - slowly. Then you can see if the focus shifts and adjust before the screws are too tight.

One more tip: leave the tape measure extended from the camera while checking focus, with the measurements visible. Then you can watch the entire field of focus move. Quite handy.
 
So, s816mm35mm (Steve) is not having this problem, but most others are? is it an issue with only a few cameras...?

Here is a quote from Brent@red I found when I searched the forum:

"Loosen the two hex screws on the PL mount ring, set your backfocus, lock the two hex screws down. DONE!

BC"

No mention there of the more hit and miss procedure that we encountered, and that others in this thread are reporting.

So, which is it? Are there cameras out there that behave one way, and camera that behave another?

Hmm...?

Jack
 
I've also encountered this phenomenon, but only on later model cameras. I'm pretty sure this problem doesn't exist on our camera (#247). I wonder if anyone with the new mounts have this problem too?
 
Same problem here but you can do it without it moving. All the time watch a monitor closely in case it does shift. Turn the screws very slowly little by little, constantly changing back and forth between the two. Even when it starts to get a little tight keep on going only very small turns bit by bit and still changin back and forth between the two.

Works well for me and if i try and do it any other way it just slips.:pinch:
 
Not having the opportunity to adjust backfocus yet, I might be out of turn with this idea but it occurred to me that after you have adjusted your backfocus and before tightening the screws, couldn't you apply a piece of tape to hold things in position while you tighten it up? Another idea would be to also apply two pieces of tape with a line marked between them spanning the gap, like Doug did to give you a visual reference to see if it had moved at all. Just a thought.
 
Mark,

The problem is not that the ring is moving, it's not - it's just the tightening of the screws that causes it.

Jack
 
CrewPix.com said:
Yup. The back focus changes when you tighten the screws. It takes some practice to get used to how far off you have to make it so that it will be right after you tighten them. Not fun.
Click to expand...

Does this translate into bad engineering or bad machining? Will RED acknowledge the issue and fix it?
 
chuck colburn said:
Good morning Stephen,

I quite agree with you!
Although I think the concept of an adjustable mount standard could be made to work properly.
Click to expand...

Hi Chuck,

Don't you think it's a soloution looking for a problem? Unless the mount can be adjusted to be parallell to the sensor I don't think it's a real answer.

Stephen
 
Stephen Williams said:
Hi Chuck,

Don't you think it's a soloution looking for a problem? Unless the mount can be adjusted to be parallell to the sensor I don't think it's a real answer.

Stephen
Click to expand...

I do.

The way it's being done by using threaded pieces is a problem area no matter what the pitch is. There are however other ways of doing it, but they get complex and therefore pricy. Linear travel that is basically slop free and maintains parallelism requires complex mechanisims like the moving parts of a real zoom lens.
 
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