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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 :)

Any 32mm lens off on the RED

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niterider

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Has anyone else had this issue? Basically any 32mm lens we have put up on our RED ends up being 3-6in off. We have switched out RED bodies and the issue still happens. We tested 4 different Cooke S4's, a Master Prime, and an Ultra Prime. Clearly something is going on with the RED and 32mm length lenses, and even RED themselves cannot come up with an answer for us. Anyone else have this issue?
 
Thus far, this issue has only been happening on the 32mm length lens. We have tried at least 4 different bodies and there is always an issue with it being 3-6in off. Having a Master Prime off alone seems strange, let alone so many other 32mm lenses.
 
If I were going to call this strange phenomenon something, it would be... "the RED One OLPF sweet spot effect".

But, hey, what do I know about these things?
 
niterider said:
Thus far, this issue has only been happening on the 32mm length lens. We have tried at least 4 different bodies and there is always an issue with it being 3-6in off. Having a Master Prime off alone seems strange, let alone so many other 32mm lenses.
Click to expand...

Why in the world would the focal length have anything to do with a back focus issue. I would think every lens would have the same issue. It doesn't seem like a very logical result.
 
This length has everything to do with something going on internally with the RED. This focal length works fine with other PL mount cameras, but to go through 4 Cooke S4 lenses in one day, all with the same issue proves great results especially with us switching out the RED bodies too. There has to be someone out there with similar results, or people arn't checking back focus when they throw the 32mm up on the RED. I would love to see if someone out there who has a 32mm could do a test and confirm this result.
 
Thanks Tico, even after reading that thread it still doesn't really mention any thing about the issue happening only with 32mm lenses. We are aware of back focus issues with different # REDs, however this issue is specific to the 32mm lens, whether it be on an early generation model or a brand new one. Any other ideas out there...?
 
So it happens with 32mm but not 35? How about zooms at 32mm?
Sounds fishy to me. I'd ask to see the lenses on a projector if I were you!

BTW, when back focus is out, wide lenses are further off than long ones. If your 32 is off (and you know it's properly collimated) then your 18 or 14 should be even worse.

Edit: Congratulations on your 1000th post, Brent. Please get a life. Love, Brent
 
I have checked with a 35mm S4 lens and no problem. I do not have a 32mm S4 but I can check with a 32mm Zeiss Mk2 Standard Speed tomorrow if you like. I can also recheck the 35mm cooke s4 again to be sure. Did you record the image and analyze afterward or are you just going by the monitor image.
 
This makes no sense. I would definitely run some tests with those lenses on other RED One bodies. I'm not seeing anything unusual here at 32mm (or any other focal length) on either the Cooke 18-100 or the RED 18-50. I don't have one here now, but I've used 32mm zeiss primes on more than one occasion and never saw anything like what is being described.
 
Alright we just checked our Cooke 18-100, again it was off at 32mm. Now I must mention that this issue is at close focus on our lenses (less than 5ft.) But when we measured the 18-100 at 3ft it was off by 3in. But I must reiterate that every 32mm we have checked at focal lengths 5ft or less is off by 3-6in. s816mm35mm if you could check your 32mm standard speed tomorrow at less than 5ft. I would love to hear your results.
 
Plate effect...

Plate effect...

AngusChandler said:
As was almost suggested, throw up an 8 or an 11 (or even a 16) and check the focus. It should be wacked right out... The issue would be hidden on longer lenses, it might just be that 32 is the first lens short enough to show the problem.
Click to expand...

The off backfocus adjustment keeps coming up.

This is caused in part by several problems:

1) You cannot put a plate behind a lens like the thickness of the sensor cover galss and OLPF without causing negative sperical aberration. Please stop questioning the existence of this issue, you can see it with any optical design software, the thicker the plate the more problem, the closer the lens the more the rays get longer off center so the more curvature of field you get, you can also see that with any good optical design program. With a plate behind the lens it will no longer track focus on its scale, if you set backfocus for the scale at INF then all the other numbers will be off a little, more at closer distances. If you set the backfocus for 6 feet, then INF will be off and closer points will be off, but less than if you set for INF, my Brother thinks it is best to set the backfocus and lenses focus mark for 6 feet since that is where you film all the time. Since the lens cannot focus to a point with the cover glass and OLPF you no longer have the ability to change lenses and keep one backfocus setting, you can only seek "Best Focus" (not sharp focus) on the monitor. Zoom and Anamorphic lenses cannot be adjusted to track with the OLPF and coverglass since they need a fixed flange to focal distance, you will have to adjust the backfocus a little for the focal length and focus distance used.

2) The microlens array works best with parallel light, with shorter focus lenses like a Cooke Series I 24mm or 32mm the rays are at an angle off center so you get some "best focus" issues off center vs. on center.

3) Many lenses have play in their mount, try pushing back on the front of the lens and see if the focus shifts, then put out and see if it goes the other way, focus goint to INF, the away from INF, if you do not stop in the same place the the mount has too much play to focus by scale.

4) Most lenses have scales that are not right on, but on a movie camera you would assume the cameraman was just a little off focus when he shot.

5) Retrofocus wide angle lenses have a rear group that is more like a 50mm lens, if you use a wide angle lens that has a shorter focus rear group the problems with the OLPF, coverglass, and microlens array will be worse.

6) The effect on different lenses of the plate effect will vary depending on the lens iris stop used, and the amount of positive spherical aberration the lens has without the plate behind it, since the plate may reduce the postitve spherical aberration in some lenses, and make it go negative in others.

7) When a lens shows negative or postive spherical aberration the point of "best focus" becomes subjective since at one point the image will be lower resolution and higher contrast on low frequency detail, but at another adjustment of the focus ring the resolution for fine detail will be higher, but the contrast will be lower near high frequencies. The size of the image used for the viewfinder alters the subjective point of "best focus", at low magnification you will pick one point, at high another.

8) You cannot use a lens projector to set the scale of a lens used on a camera with a cover glass and OLPF in the camera. You would need to have the microlens array, coverglass, and OLPF etc. in the lens projector in their right places from the focal plane in order to see the rays take the same path, about (they are going the other way, so the order the rays spread is reversed, you start with a sharp image and end up with blur in the "subject" plane). If you do that, and test many lenses with that setup vs. a projector with just air between the focus pattern and the lens, you should find small differences in the point of "best focus" (on the focus scale) with different lenses and different f/stops and different subject distances.

9) Heat and humidity may make small issues with the sensor board and lens mount since the movement that can put the focus off is less then 0.0002".

Most of the focus errors will be small. For motion picture work you would not pay much attention to them since the projector lens would be more out-of-focus than your camera negative. For broadcast on SD the small focus errors you can see in the RED ONE (tm) 4K images will be too small to matter much or even be visible. If all the errors go in the same direction then you might see them at the HD level, but there are more important things, like color and lighting that will be clear to the viewer, the HD monitors have built in sharpen, and the compression will also sharpen and fuzz parts of your images without your control.

A Bayer sensor camera needs the OLPF to reduce the chroma moire and aliasing, its role is to fuzz up the focus so light hits four pixels rather than just one, you cannot use the camera without the OLPF and get good results with the lens stopped down.

Accept the things you cannot change?
 
Sorry, but in the real world this is just not particularly true. Really, a lens needs to be set to an absolute standard based on a collimator. And a camera needs to be set to an absolute standard. Any issue with an aberration on an OLPF or a misalignment of it or the sensor head would create problems with other focal lengths as well.

Really it sounds to me that your camera's mount is just slightly off.
 
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