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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 :)

Parfocal f'ed up?

Arthur Woo

Arthur Woo

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When a lens doesn't hold focus through the zoom for a parfocal cine lens, is there any obvious thing that could cause it (e.g. needs collimation)? I tested another copy of the same lens on the same camera and the other one held focus fine throughout the zoom range. Currently not able to send to a lens technician for another month so I'm guessing at this point.
 
No grating, grinding or other evidence of damage? Suspect back focus, no?
 
Scott Crawley said:
No grating, grinding or other evidence of damage? Suspect back focus, no?
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No outer physical evidence of damage. Focus/iris/zoom are all smooth.

Correct me if I'm wrong, but I thought backfocus was set on the camera? I tested a second copy of the lens on the same camera, and the other one came out fine, which I *thought* ruled out back focus.
 
On Red cameras you can adjust the back focus. Each lens, even if it's the same lens from the same manufacturer and especially with zooms, may need it's own back focus adjustment.

There's also the chance the lens itself needs work.
 
Phil Holland said:
On Red cameras you can adjust the back focus. Each lens, even if it's the same lens from the same manufacturer and especially with zooms, may need it's own back focus adjustment.

There's also the chance the lens itself needs work.
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Thanks Phil, the camera I tested on had its back focus set correctly, but I didn't know that it might vary from lens to lens, I'll check again.

There is a chance the lens needs work, but I bought it sight unseen with promises of it working perfectly and 100% ebay rating. Hopefully it is the back focus.
 
If it's that 25-80 you posted in the 3rd Party PL thread, then it's most likely the lens. My russian is rusty, but I think Lomo's slogan roughly translated to "No lens is made the same way twice!"
 
Mike P. said:
If it's that 25-80 you posted in the 3rd Party PL thread, then it's most likely the lens. My russian is rusty, but I think Lomo's slogan roughly translated to "No lens is made the same way twice!"
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lol yeah...

i contacted the guy i bought it from and he said to check the backfocus. i don't have a red focus, so i tried using the siemens star method (difficult because even though the markings are in meters, they don't quite line up with the beginning/end point). It seems to have helped a bit. gotta tweak it more.
 
You're talking about that lens's back focus, right? If when using your EF mount everything is peachy on multiple/all of your lenses, I'd argue that the lens is the thing that's off... Dumby mounts are pretty straightforward and red's mount implementation is equally straight forward (back focus shouldn't drift between mounts).

I have used a recently serviced 25-80, and it could NOT hold focus through its zoom.
 
I don't know that lens, but some zoom lenses have a built in lens back focus adjustment too. Worth checking for.

If not, as you found another copy of that lens worked parfocally, then there would seem to be hope that yours could be adjusted to do so too with the correct shims.

But without a known good reference you could end up chasing your tail. If you don't rent lenses then you could just set your camera adjustment to match the majority of your lenses, and then get the odd one out lenses adjusted to match those.

Overall, it's far better to have both the camera and all your lenses correct to the PL specification to allow for renting lenses as well though.

Bear in mind I've found that even the large rental houses, while internally consistent, can be very slightly different to each other. This is inevitable with lineup variations and manufacturing tolerances.
 
The Russian 25-80's are easy to set backfocus on. There is a threaded part on the rear, you can twist and lock in place with a set screw.

I have 2 INOP 25-80's that are waiting to be turned into one that works.

Matt Uhry
www.mattuhry.com
 
Matt Uhry said:
The Russian 25-80's are easy to set backfocus on. There is a threaded part on the rear, you can twist and lock in place with a set screw.

I have 2 INOP 25-80's that are waiting to be turned into one that works.

Matt Uhry
www.mattuhry.com
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Thanks, I will check it out!!

Mike P. said:
You're talking about that lens's back focus, right? If when using your EF mount everything is peachy on multiple/all of your lenses, I'd argue that the lens is the thing that's off... Dumby mounts are pretty straightforward and red's mount implementation is equally straight forward (back focus shouldn't drift between mounts).

I have used a recently serviced 25-80, and it could NOT hold focus through its zoom.
Click to expand...

The seller mentioned checking the camera's back focus. My back focus was off anyway from trying to figure out the infiniti focus issue w/ a Tokina 11-16. After adjusting it helped quite a bit, it's not as bad now - but focus still drifts. I'm going to look at Matt's suggestion about adjusting it on the lens, I didn't know that was an option and see if the Ekran has an adjustment point.
 
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