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Infinity Focus Problems With 18mm Cooke Speed Panchro S2 (Not Rehoused) On RED DRAGON 6K

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As title states, I'm having problems with infinity focus on my 18mm Cooke Speed Panchro f1.7 (Arri-S mount, no rehoused) on my RED DRAGON 6K (WEAPON), and there could be a number of factors in play I was hoping someone could shed a light on the most likely issue.


The "symptom" (or issue) I'm facing is when turned to infinity, the image is currently out of focus (it's not soft like focusing past infinity, it's actually out of focus - not totally so, but it seems to be focused on a plane before infinity).


There are a few things in play here which I will state below (which I think any of which could be causing the issue).


(1) I have a RafCamera Arri-S to Arri PL adapter fit on.

I'm not new to this adapter, and I'm using 2 of these for my Cooke Speed Panchro 32mm and Cooke Kinetal 50mm respectively on RED GEMINI and RED KOMODO and they work okay, no issue at all. They focus fine to infinity, and focus ring is smooth. However, with this newest lens and adapter I got, without the adapter on, the focus turns quite fine as well; once the adapter is on, the focus ring is noticeably a little bit more resistant to turning (but still turn-able with a follow focus unit).


(2) The 18mm CSP seems mechanically different to my existing 32mm CSP and 50mm Cooke Kinetal.

The 18mm I noticed has 2 rings, one for focus, one for aperture. The 32mm CSP and 50mm Kinetal does not have a seperate ring for Aperture, and aperture is adjusted by turning the front of the lens. (I'm not sure if this is even a factor, I'm simply stating an observation).


(3) My RED DRAGON 6K has a Low Light Optimized OLPF (filter) fit on.

I just took over this unit, and it comes with Low Light Optimized OLPF - I'm in the midst of getting the Skin Tone Highlight OLPF instead (because I don't shoot Low Light), and I've kinda read that Low Light generally can affect infinity focus, though again, I'm not sure if it actually causes inifnity focus to go off this much.


(4) I'm not sure if my RED DRAGON 6K has had its backfocus adjusted.

The backfocus may have been adjusted for the previous lenses it housed, but I'm not sure and I can't really check because the seller was a production house that has clocked over 3.3K hours on the unit, and they've used it with other cameras for multiple productions and shoots - they don't really remember if they did adjust the backfocus on the unit. Although, does backfocus affect infinity focus? That one, I'm not sure.


(5) My RED DRAGON 6K uses the older Titanium PL Mount.

I'm not sure if this is even a consideration, but it simply is an observation against my RED GEMINI which uses the (newer) Magnesium PL Mount - as far as I know, the difference between the two PL mounts is only in terms of make and weight, but I'm not sure.


I'm not looking for the one solution at this stage, but I was hoping to save some time hopefully if I could get help and eyes from more experienced shooters on what might be the most likely culprit as to what's happening here.


Thanks in advance!

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Back-focus absolutely affects infinity focus.

The way almost all old lenses focus is the same - at infinity the lens is positioned closest to the sensor/film plane and by shifting the whole lens further away you focus closer. If either the camera flange depth (distance from mount to sensor) or the lens back-focus (distance from lens mount to focal plane when set to infinity) are out of tolerance then the focus scale will be shifted and the lens will either go through infinity or it won’t reach it. If you can’t reach infinity then the lens is sitting too far away from the sensor.

The shorter the focal length, the more an error will show up. This is because a long focal length needs to move a lot further away from the sensor to focus than a wider lens. You can notice this when you focus an old telephoto compared to a wide angle - the long lens will shift quite a lot as you focus from infinity to minimum, while the wide lens will barely move. So your 18mm will be more affected by a small back-focus error than your 50mm. 

It could be that the adapter is a little out, or your camera flange depth, or the lens itself needs adjusting (if you just bought a lens from eBay there is no guarantee it is correctly collimated.) It could be a combination of these factors. Or (and this is common in old lenses) there could be some wear causing play in the lens focus threads - try pushing the lens closer to the camera to see if infinity gets sharper. The OLPF can sometimes affect the perceived focus of wider lenses slightly, but any error should go away if you stop the lens down a couple of stops. 

A lens tech or rental house service department can check all these things pretty quickly on a collimator or lens projector. 


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Oh wow thanks for the in depth response! Sadly, I don't know of any lens technicians in Singapore where I am, though I suppose I can ask around.

Theoretically, assuming the lens is correctly collimated (it came from a Japan community seller who claimed on the listing that the lens has been pulled from a working environment, serviced and kept in almost mint condition; but I understand that's not to say it's collimated), then adjusting the backfocus on my DRAGON might make a difference to the issue I'm facing, yes?

I could check if the adapter is fitted on properly, I think it's snapped in, but I could give it another nudge probably.

I did try stopping down a couple stops and it did get sharper, but I thought it wasn't a long term solution because I always shoot wide open, and I thought that sharpening to be something that happens anyway whenever lenses go down a couple stops.

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You could adjust your camera’s flange depth but if it’s the lens that’s out then you will be offsetting your camera for one lens and all your other lenses will be out. Perhaps not by much, but it’s not really the way to do it. Ideally you set the camera depth to the Arri PL standard and then any correctly set lens should work. If all your other lenses seem ok I would probably leave the camera alone and just try to tweak the lens, but you probably need a tech to do that.

Maybe Camwerkz, who are a Singapore rental house with a service department?


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You make a really good point on not adjusting the backfocus for the 18mm alone and potentially making other focal lengths and other lenses not fit! I'll have to try figure out the "default" backfocus for Arri PL standard on RED DSMC2, if it comes to that - but I'll probably run another test of the 32mm and 50mm on the DRAGON before I tweak the backfocus.

Thanks for the suggestion! I worked with them as cam operator under their production house before, but I never knew they also do lens servicing. Good to know!

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