PL Mount has some Play

[SF Geek]

I noticed that with especially arri lenses that I'm getting some play in the mount. The locking ring seems to be the culprit. The mount itself seems firmly locked to the body but the ring, even though I can tighten it to kingdom come, still allows a little play. I noticed that on my Red lens that the flange is not beveled, but on the Arri lenses it is. The Red lens seats snugly but I can get some jiggle out of the arris. Anyone else have this problem?

 

[Finner]

Hi SF

Here's a test for you to try. Set your PL mount back focus at a typical room temp, then turn your camera on for four or more hours in a reasonably warm location. I am hearing multiple cases of the flange focal depth changing throughout the day with cameras throwing lenses off. Aluminum is appearing not to be the best mount choice. Not sure if it is just the mount or if the aluminum body is also attributing to the problem?

 

[SF Geek]

I don't mean play like in the focus being off. I mean I can jiggle the lens very slightly when it's mounted. I tried other arri lenses and got the same result, but to differing degrees. I noticed it when i was prepping. Even racking focus on a 16mm standard speed the frame would adjust position and i don't mean from lens breathing. The center point would change.

 

[Finner]

I know exactly what you mean and what you are saying is your immediate problem.

What I am letting you know is that there will likely be further mount problems even after your jiggly mount is fixed. Problems that will not be fixed nearly as easily.

 

[Jeff Kilgroe]

The heat expansion issue is probably not the problem here... Sounds like a sloppy PL mount that should be reported to RED. It may be in need of service.

As for the heat expansion issue, I've heard one first-hand account of that and read of one more. Someone else (I'll let him reveal who he is if he chooses) pointed out to me that Clairmont made their own sub-mount plates for the F900 out of Invar (a type of alloy that doesn't expand/contract with temp changes) because of this issue on that camera.

Most all metals change as temperature shifts... In some ways aluminum isn't as bad as others like brass. For now, I think this is an issue that deserves further investigation. I would love to test myself... alas, I am still without a camera, trying to wait patiently.

 

[SF Geek]

Thanks Darren. I hope I don't have multiple issues here. About the heat throwing off the back focus, does it go back after it cools down? Because it did seem that the lenses were a little of at the end of a shoot and when I checked them the next day, everything lined up.

 

[Mitch Gross]

Clairmont's rebuild of the F900 is hardly a secret; they used to run ads in American Cinematographer with exploded views, touting the product.

 

[Finner]

Thanks Darren. I hope I don't have multiple issues here. About the heat throwing off the back focus, does it go back after it cools down? Because it did seem that the lenses were a little of at the end of a shoot and when I checked them the next day, everything lined up.

Mike I do not have my camera right now so I can't do the test I mention. It would be cool if you could report back on your results if you have the chance to do the test. The complaints I have heard on the issue come from very credible colligues. Throw up your widest lens as it will be the easiest one to see FFD problems with.

 

[SF Geek]

I'll try it out. I only have the 18-50mm to test it out. I'll try it at 18mm at 1' and just run the camera on AC for a few hours.

 

[Finner]

Make sure you hard mark the focus for 18mm before the test.

 

[Stephen Williams]

Clairmont made their own sub-mount plates for the F900 out of Invar (a type of alloy that doesn't expand/contract with temp changes) because of this issue on that camera.

Hi,

Jim originally talked about making the mount out of Invar for that very reason.

Stephen

 

[Andrew M.]

And what lenses bottom part is made from?

This back part has significant role in the back focus distance as well.
If the pipe holding the last two-three lenses expands or shrinks then add this one to the equation of the termal expansion.

BTW I found out that 16 mm MP is not that sensitive for the focus set, however longer ones like 65, you move the focus ring just by a bit and chart goes out of focus completly.

Finer, why then you advice to do the test at 18mm on 18-50 zoom?
Did you find out that lower the focal length more sensitive lenses are to the back focus?

This one I didn’t test thought. What I am saying is just that longer lenses are more sensitive to the focus ring adjustment.

 

[Pawel Achtel]

And what lenses bottom part is made from?

This back part has significant role in the back focus distance as well.
If the pipe holding the last two-three lenses expands or shrinks then add this one to the equation of the termal expansion.

BTW I found out that 16 mm MP is not that sensitive for the focus set, however longer ones like 65, you move the focus ring just by a bit and chart goes out of focus completly.

Finer, why then you advice to do the test at 18mm on 18-50 zoom?
Did you find out that lower the focal length more sensitive lenses are to the back focus?

This one I didn’t test thought. What I am saying is just that longer lenses are more sensitive to the focus ring adjustment.

Andrew, this is how optics work :nerd: . Long lens has shallow dof in front, deep dof at back plane. Wide lens has deep dof in front, shallow at the back plane. Therefore you always adjust the back focal distance on a widest lens.

Have you ever done BF adjustment on a 2/3" zoom lens? You are supposed to do this every time you change a lens. Anyway, Open iris all the way up, focus when zoomed in then you change back focus lever when zoomed out. You repeat this 2-3 times and you are set.

 

[chuck colburn]

Depth of FIELD in front of the lens and Depth of FOCUS behind the lens.

 

[Brook Willard]

Hi,

Jim originally talked about making the mount out of Invar for that very reason.

Stephen

I hope they return to Invar for the new mount in Epic.