Welcome to our community

Be a part of something great, join today!

Register Log in
  • 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 :)

OLPF change affecting back focus?

"checked out" what does that mean?

IMHO there is no proper way to use a standard collimator on the RED. The only way to set it properly is to use a well calibrated wide prime. And a high magnification viewing system.
 
How is the back focus set up/adjusted at RED headquarters before being shipped to the customer?
 
Richard Bradbury said:
My contact at the lens service house suggested that the only possibility as far as he could think of was that a different optical low pass filter was introduced into the camera between numbers 500 and 1000, and it's affecting the back focus differently. He thought he could see a difference in the filters on the collimator, but wouldn't swear to it.
Click to expand...

Sorry, but I don't get what you mean by the last sentence? Can you explain exactly what he was doing?

I've also used the same serial number cameras you have and haven't seen any problems like you describe so that would lead me to suspect it's not a universal, build-related problem as you suggest. Weird...
 
Mike Prevette said:
"checked out" what does that mean?

IMHO there is no proper way to use a standard collimator on the RED. The only way to set it properly is to use a well calibrated wide prime. And a high magnification viewing system.
Click to expand...


Why wide prime not 65 or 80mm
Wide primes are not that sensitive to focus setting?

I had trouble to set focus on 65, just a touch and focus was off.
On 16mm it was very easy.
 
Focus shift with stop

Focus shift with stop

eastco said:
How is the back focus set up/adjusted at RED headquarters before being shipped to the customer?
Click to expand...

There may be a small focus shift with the lens stop used when there is an OLPF and sensor cover glass+micro lens array behind the lens, even if you adjust for the Paraxial Ray path length.

There may also be a small focus shift when the angle of the light varies from parallel, some lenses have their ray crossing point closer to the sensor than others, so the distance through the glass plates will be longer or shorter for some lenses or others. The focus shift is about 1/3 the path distance difference or maybe a few 1/10000" or just maybe enoough to be a little soft in the corners.

With f/1.2 lenses the sides of the rays cone take a longer path through the glass plate than the center rays, or in the corners one side is shorther than the center and the other side, so that makes the "best focus" distance shift not only from center to corner, but from full open to stopped down.

To avoid this shift only use f/8 and remark the lenses. Otherwise there will be a small shift, not much for a film camera, but at 4K small shifts might be a little soft.

In another post someone was having problems with the compression, thought it was heat, but may be that the image is sharper when he stopped down in bright sun and thereby causing an overload of the image compressor with too much detail. So in bright light when you stop down you may get a focus shift that has nothing to do with the lens mount, just that you set the backfocus indoors in dim light with the lens more open. In otherwords the mount is not shifting, the lens is not shifting, just the normal optical shift of "best focus" for that lens from full open to stopped down due to glass in the light path (all lenses show a very slight shift in their "Best Focus" from one stop to another, 35mm film is not sharp enough to see this much in a good lens but at 4K you might make it out). The only way to fix that is to compute lenses just for the OLPF and coverglass-micro lens and do not change or adjust where they are.

The mount should be set for your zoom since the zoom cannot be remarked, then remark your prime lenses.

Anamorphic lenses are VERY fussy about the backfocus and cannot be remarked and get right focus, so if you shoot scope, set the mount for the lens at the stop you will be shooting at.

The lens and f/stop used when the mount is setup may not be the same as what you are shooting at, so you should check the backfocus at the stop and with the lens you will shoot with. The error is small, and if you focus on the monitor at 1:1 pixel, the marks do not matter (except for INF), the focus will just be a little soft at large stops, which is what the OLPF is ment to do anyway, blur the image over four pixels so that two green and one blue+red pixel are always exposed to avoid colored spots. If the corners go softer than the center, it does not matter except on charts since those parts of the frame do not have in focus things most of the time anyway.

I should add, that if you use lenses that were aligined to their focus mark on a lens projector or Collimator, those do not have the optical plates (OLPF and coverglass+microlensarray) so the alignment could show a slight shift when the "Best focus" is checked on the camera. If you adjust the lens mount for one lens aligined on a lens projector or Collimator, then put another on that is opticaly at a different distance for the rays to cross, the "Best focus" can be a little different ON THE CAMERA even though the lenses were right on the lens projector or Collimator, due to the shift in optical path length with angle through the OLPF and coverglass+microlensarray. So if you rent lenses, rather than have your own, the rental house would probably not have aligined the lenses focus marks on a RED ONE (tm) of the same serial number as yours, and therefore you would need to remark the lenses for each stop if you are going to focus by tape, and hope that you can go to INF when you need to.

The reverse can also happen, if you mark the lenses for use on a RED ONE (tm) and then put them on a film camera, you might get some soft shots on the film camera, but with film being what it is you might not notice so much. Anyway, if you are going to a filmout the lens in the projector will probably be far enough out of focus that the camera lens issues will not be visible from the back of the theater(?)
 
Displacement varies with angle of rays as well as thickness

Displacement varies with angle of rays as well as thickness

Andrew said:
The Effect of Inserting a Flat Glass Plate into the
Optical Path Downstream from a Lens

http://doug.kerr.home.att.net/pumpkin/Glass_Plate.pdf
Click to expand...

What some people seem to get confused about is that they seem to think there is one setting of the backfocus that will fix this problem for all lenses that are made for a film camera, there is no single setting of the backfocus that will correct for this plate thickness since the length of the rays through the plate varies with the f/stop, the angle of the lens (ray crossing point), and from the center to the edge of the frame introducing curvature of field as well as negative spherical aberration.

The "plates" do three things:

1) there is no longer a single "best focus" for all stops (not that there was, its just worse now). The lens needs a new focus mark for each stop.

2) the "best focus" error is larger more for larger stops. (unless lens had positive sperical aberation before).

3) the "best focus" for the center and corner are not the same, i.e. the lens is no longer "flat field" (not that it ever was, just maybe worse now, although it might be better for some odd lens.)

The thicker the "plates" behind the lens the more this shows up. The faster the lens stop, the more this shows up. Bolex made "RX" lenses to "fix" the issue for their Reflex camera but only up to 50mm (?) the slower long lenses had enough travel on their mounts to sort of focus by eye.

If the "plates" are less than 0.01" total its not so bad, maybe just a little shift of the "best focus" on the different lens stops, but thicker you might see something in the end result. Does anyone know the total thickness and index of the OLPF and coverglass+microlens array?

How much shift in backfocus are people seeing between lenses in 1/10000" (I guess they check wide open)? Most lenses are retro-focus or tele-photo so the point where the rays cross is not as different as when "short" focus wide angle and "long" focus lenses were used.
 
www.DANCAD3D.com said:
The "plates" do three things:

1) there is no longer a single "best focus" for all stops (not that there was, its just worse now). The lens needs a new focus mark for each stop.
Click to expand...

Could we set "best focus" for a little F stop (1,4) and when closing F stop to 4, 8 or 16 the increased depth of field would get us in the tolerance of the "best focus"?.

I never heard about this plate "issue"... I never worked with olfp before...

Any RED could comment on that?

Thanks

Pat
 
DOF at 4K

DOF at 4K

Tresch said:
Could we set "best focus" for a little F stop (1,4) and when closing F stop to 4, 8 or 16 the increased depth of field would get us in the tolerance or the "best focus"?.

I never heard about this plate "issue"... I never worked with olfp before...

Any RED could comment on that?

Thanks

Pat
Click to expand...

At 4K there is almost no DOF, being just a little off focus means that you are not getting a 4K image of what you want in focus, something may be in focus like the actors ear, but you might be focusing in front of his noise as well.

To get a 4K image, you need the subject in focus. The point of the OLPF is not not get 4K with a high MTF, but to fuzz the image up so that the high MTF comes below 2K. None the less, some detail comes through in the LUMA from the De-Bayer near 4K, and that is what makes the image look in-focus or out-of-focus.

When shooting at f/1.2 using a 75mm at 4K DOF is very short, the charts made for film cameras relate to something closer to a 1K image, check to see what size spot the DOF table was made using.

When you look at the circle of confusion at high magnification you can see the shift in "best focus" for the stops, for film cameras a lens stop shift of 0.001" might just look like the follow focus guy was a little off, but at 4K if you see the image with all its glory 0.001" may be enough shift to notice.

Why purchase a 4K camera, just to shoot everything out-of-focus and say that it is within some DOF table made for old film cameras (or the old out of date DOF marks on the lens)?

BTW: at f/16-f/22 you will be getting out of the 4K range due to diffraction, maybe someone can test this on their chart?

The simple fix is to set the backfocus short so that all lenses will go to INF, then focus by the 1:1 pixel display, and only use prime lenses that focus by moving the whole lens in and out. You cannot set the backfocus too short or you will run out of travel on wide angle lenses. If you have a fixed focus wide angle lens you would need to fiddle with the mount, so try to only use lenses that move and focus on the image not the marks, or tape a new mark on for the stop/lens on the camera...
 
Andrew said:
Why wide prime not 65 or 80mm
Wide primes are not that sensitive to focus setting?

I had trouble to set focus on 65, just a touch and focus was off.
On 16mm it was very easy.
Click to expand...


Andrew wide primes have a larger depth of field in front of the mount, but a much shallower depth of field on the imaging plane. The same but opposite is true for telephoto lenses. They have a shallow depth of field to the subject, but a lot of depth of field on the chip.

So you use Wide lenses to set flange distance as it's much easier to see if your exactly on or not.
 
Dancad has made a very important point here, that explains a lot of what I've been seeing recently. Since the exit pupil of lenses differ so greatly, the OLPF effects them all differently. Since the lightrays would be passing through it at dramaticly differing angles. It would explain why I could set the camera bang on with a 14mm 35 prime, but have it be subtly off with a series of S16 zooms I put up. Or even change between wides and telephoto's within a prime set. We are talking .0001" differences here, but as he said a 4k chip is not very forgiving.

How very scary.
 
After thinking about it, it might be time to invest in a Zeiss or Century HD Collimator. That way you set each lens to Infinity every time you swap ughh. But at least the marks would jive.
 
We are still looking into it, but anytime we have seen this symptom it has been resolved with tweaks to the PL MOUNT itself (either backfocus, tightening the set screws on the front plate of the MOUNT, etc).

Know that we have a pretty extensive backfocus process with top end equipment. Believe me, Jim and Jarred would not tolerate anything less.

BC
 
Mike Prevette said:
Andrew wide primes have a larger depth of field in front of the mount, but a much shallower depth of field on the imaging plane. The same but opposite is true for telephoto lenses. They have a shallow depth of field to the subject, but a lot of depth of field on the chip.

So you use Wide lenses to set flange distance as it's much easier to see if your exactly on or not.
Click to expand...

Thanks Mike, now I got it.
Just by playing with the back focus I see that most back focus misalignment could be corrected just by remembering the offset or by programming focus motor with the bit different offset.
Except infinity, here the best is to overshoot infinity, and press the infinity mark reset on the motor.
This way if the other lenses are bit further from the sensor you will have room for correction directly on the motor setting. Is this correct thinking?

If I set infinity bit shorter, say at 30 feet, so I will get perfect infinity focus when focus ring shows actually 30 feet, will I screw up something else or all will work OK except the marking will be slightly off?

Sharp MAX collimator:

http://www.digiprimes.com/downloads/brochure/DigiBrochure.pdf

Would be nice to have back focus ring on the lenses like this one, see second to last page.
 
Angle of attack on "plates" changes with focus in primes

Angle of attack on "plates" changes with focus in primes

Mike Prevette said:
After thinking about it, it might be time to invest in a Zeiss or Century HD Collimator. That way you set each lens to Infinity every time you swap ughh. But at least the marks would jive.
Click to expand...

That's a good idea if the near marks go with the INF, on some lenses it seems the INF stop does not fully relate to marks like 1m or 2m sometimes, so in that case only INF would be right...

Another issue is that in prime lenses that move to focus the angle of "attack" on the "plates" changes when you focus the lens in and out! Making the backfocus comp only right on for one stop and focus distance of a particular lens.

With lenses that move in and out a half inch or more the angle can change quite a bit, maybe as much as changing lenses. Zoom lenses may be better here if they are true zooms where the rear group stays still and only the front moves to focus.

I would adjust the focus by taping out 2m (6 feet) and putting a bear flashlight bulb there, then set the lens to its 2m (or 6 foot) mark, and do the back-focus with the monitor on 1:1 pixel mode. That way you are setting the focus at the distance it will be used, and can check the stop that will be used to see that the mark is in the center of the DOF to give you the maximum DOF, if you are at one end of the DOF it is easer to get soft footage, you should be able to rock the focus ring a few mm each way and see that the "best focus" at the stop you are using in dead center on the two marks. If the lens has backlash, you always need to focus going from INF, when you need to go to INF you need to shift the fixed mark to compensate for the backlash in the lens. You can adjust the flashlight lamp power so that it does not over expose.

Very important, if people are not always moving the focus from INF to 2m they will think the backfocus is off. You cannot move the focus a little back and forth in lenses with backlash to "hit" the focus and expect the marks to line up, focus 10 times from both directions and see if the right "best focus" spot shows the fixed and moving marks stop with gaps that are not the same.

When lenses that have INF stops get backlash wear, they do not quite make it to INF, so when you put the Collimator on you will be mis-adjusting the backfocus.

You can check for backlash sometimes by pushing in on the front of the lens, if the focus shifts then you will never get to the INF stop just by turning the focus ring. Better to have lenses that focus PAST INF, and go from past INF back to INF to hit the mark with a Collimator.
 
What about focusing by eye with the aid of a bare maglite bulb? Set the focus marks on the follow-focus wheel that way and just pull focus. Nevermind the marks.
 
You must log in or register to reply here.
Back
Top