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Harcharan Singh
09-14-2014, 09:33 AM
Hi
We are facing focus issues with our Ultra
Prime lenses specially 16mm.24mm,32mm
and 50mm on all our Dragon cameras.But 85mm,100mm
and 135mm are okay.
All the lenses are okay with our EPICs.
Is anyone else facing same kind of issue
or is there any solution to it.
Thanks in advance.
Harcharan

Phil Holland
09-14-2014, 09:37 AM
Have you adjusted your back focus?

That is rather important.

Harcharan Singh
09-14-2014, 10:37 AM
Hi
No adjustment done on focus.
There is a focus adjuster near the mount
of camera but never used it.
Can we get some help from you?
Thanks
Harcharan

Phil Holland
09-14-2014, 10:53 AM
Yes Harcharan. There is a small screw on the top of the camera that needs to be removed, near the front. Then there's another screw that adjusts the back focus underneath that. Turning that either way will change your back focus.

If your back focus off it could be the difference of your scales reading accurately or inaccurately. It also can have some negative impact of the "optimal" quality of the image.

A good way to check if your back focus is spot on or not is to grab a few lenses and measure out 15 feet from the optical center of the camera. If it reads something off (and it can be way off if it's out of whack) you need to adjust it.

This is of course assuming the lenses themselves are within spec.

Harcharan Singh
09-14-2014, 11:00 AM
Hi
Thanks .
But dont you think this adjustment
in anyway will harm the lenses
which are showing proper focus?
Harcharan

Phil Holland
09-14-2014, 11:04 AM
Wait. What's the focus issue you are having?

Post an .R3D maybe of the problem?

David Battistella
09-14-2014, 11:07 AM
You can download the red focus operation guide here, which explains where all the back focus controls are.

https://www.red.com/downloads?category=Documents&release=final

battistella

Gunleik Groven
09-14-2014, 11:18 AM
The issue of backfocus and unprecise lenses will have similar effects.

If some of your lenses are ok, and others not, it is hard to determine on a forum... :)

If you have a wide lens and set it to infinity and point it at infinity fully open, that should be sharp... :)
If the sharpness gets better "beyond or before" the infinity mark, that could suggest you are off.

then check with the next-widest in a similar fashion.
If the tendency is similar, you may have a back-focus issue.

If it's different, you most likely have lenses out of whack.

It's a good place to start to check if backfocus is off (given that you have a lens you trust)

Harcharan Singh
09-14-2014, 11:36 AM
Hi
Thanks for your replies.
We have 4 sets os Ultraprimes
which are perfect on EPIC with no
focus issues.
But same lenses with focal lengths
16mm,24mm ,32mm and 50mm are not
showing proper Focus shooting at 6K. But
with resolution at 5K the focus on Dragon is
also okay.
Harcharan

David Battistella
09-14-2014, 11:40 AM
Wait.

you are saying that a focus mark at say, six feet is in focus with the camera set to 5k and out of focus with the camera set to 6k?

that one is quite strange....

battistella

Gunleik Groven
09-14-2014, 11:52 AM
That does indeed sound intriguing...

How do you check focus?
And are the marks on the lenses correct on the Epic AND on the Dragon (at 5k)
16mm should get soft/vignette at 6k.
But I guess that is not what this is about.

Harcharan Singh
09-14-2014, 12:05 PM
Hi
We know the 16mm issue..
But we are having focus issue.
And it is in all our 4 ultraprime sets.
and all our Dragons..plus minus a little
Bit...will check tommorow with tips given
And revert back.
Thanks
Harcharan

Harcharan Singh
09-14-2014, 12:06 PM
Wait.

you are saying that a focus mark at say, six feet is in focus with the camera set to 5k and out of focus with the camera set to 6k?

that one is quite strange....

battistella
Yes

Gunleik Groven
09-14-2014, 12:10 PM
Hm...

Can you share 3 R3D's of a focuschart?
If you don't have a focuschart... well... something else, like a wall with tons of details.

Snaps.

1: Epic @ 16mm, wide open 6ft 5KFF
2. Dragon @ 16mm, wide open 6ft 5k
3. Dragon @ 16mm, wide open 6ft 6kFF

Same lens.

David Battistella
09-14-2014, 12:18 PM
Yes


Wow. Ok. I'd advise sharing r3ds and a log file with red support right away.
The weird thing also is that it's only the wider lenses. If you can share r3ds here as well that would be good.

the distance the lens is projecting onto the sensor (back of the lens to the sensor plane) does not change so the focus should not change.
Does the compression change? Is it on the r3d or just in the monitor?

Battistella

Phil Holland
09-14-2014, 12:23 PM
I suspect curvature of field and coverage. Are you confirming focus center frame? 6K FF is outside of the optimal coverage in field and edges on these primes. In terms of matching the general image performance from MX 5K. Dragon should be set to 5.5K for the same FOV.

Though the last time I shot with the 32mm UP I didn't experience any real issues.

Blair S. Paulsen
09-14-2014, 12:33 PM
Please post the results of the investigation Harcharan. I have some theories about how the larger imaging target might be adding distortion by scattering light from the outer edges of particular elements in the lens. IIRC the wider focal lengths you are having issues with are Distagons, while the 50mm and up are planar design.

Cheers - #19

Harcharan Singh
09-14-2014, 09:16 PM
Hi
Will do the tests today and post the snapshots?
Thanks
Harcharan

Nick Gardner
09-14-2014, 10:12 PM
Sounds like a back focus issue. Wider lenses have a shallower depth of focus which would make a back focus issue more pronounced than longer lenses. Depth of focus not to be confused with depth of field.

Nick

Gunleik Groven
09-14-2014, 11:35 PM
That is why I wanted the test done on 16... :)

David Battistella
09-15-2014, 12:46 AM
If it is a back focus issue the markings should not be spot on (assuming they are all shimmed and tested on a projector and then the back focus is set on each camera) I use my REDfocus now for all of this kind of thing. This way I know that the target to the focus plane is spot on. If anything is out then it's a lens thing.

But you guys are correct, get the widest lens on there, set a known distance and set the lens to that distance from the chart (I used 3 meters or six feet before I had the RED focus), get a large monitor, get in 1:1 zoom mode on the camera and set that focus as close as you can get it on the chart in the center of the frame. Then all the lenses should fall in line and be spot on at that distance.

This is for primes. with zooms you have to see how they hold the range.

Battistella

Blair S. Paulsen
09-15-2014, 01:09 AM
The one thing I'm having trouble attributing to back focus error alone is the differences in focus results between 5K and 6K frame sizes when the sensor (Dragon) and the lens (UP wides from multiple sets) are the same. I suppose that the depth of field decreases as the the image circle increases so a slight error might reveal at 6K that is below the circle of confusion at 5K but I'm dubious that alone would account for it. I still have to wonder if a lens designed for S35 might exhibit some unaccounted for issues with stray light, internal reflections, the edges of elements, etc when trying to properly cover the larger aperture.

Cheers - #19

Phil Holland
09-15-2014, 01:23 AM
Heh. This is one of those things where we need a bit more info.

Also, I imagine if one of us was on set it would be sniffed out in about 3 minutes.

Gunleik Groven
09-15-2014, 01:49 AM
we are too expensive....

Harcharan Singh
09-15-2014, 08:31 AM
Hi guys,
We had the Dragon but didnt have any
Ultra Primes to test today.All lenses were out
with Epics...
So give me day or two and I will come with
R3Ds
Thanks
Harcharan

Andre W.
09-15-2014, 10:23 AM
With back focus issues, longer lenses take much more drastic movements to see a shift. Thats why usually if doing a back focus calibration with a lens on a body, you do it with the widest you have. 14, 16, 18mm are usually good ones to check it with. A very minor back focus adjustment can move you focus with these WIDE lenses DRASTICLY, where as that same minor adjustment will not have effected your 85 or longer lens.

So if you feel that your 16 is spot on, I would use that lens to set your back focus on the camera. Chances are it's a very minor adjustment that needs to be done, and you will notice that the rest of your lenses should still fall in line, with your longer lenses perhaps popping a little better.

Michal M.
10-24-2014, 06:49 AM
Hi,
it is possible to know the distance between both positions of chip when you correct back focus issue?
Thanks