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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 :)

Back Focus adjustment

T

Thomas Hencz

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Hi there all Red Owners. After having had countless attempts to set the back focus in the last year on my Red cameras I finally come across the perfect solution. It's Denz Flange Depth Controller. I just purchased one and it cost me a furtune. This is the most accurate instrument for setting back focus. I now calling on all Red camera owners to bring their camera in and I will set the back focus for your camera for $70. Please email me if you are interested info@t-stopinc.com

Kind regards,

Thomas Hencz FSF
DOP
 
Er...

Wrong section, dude, I'm moving it to the jobs/hire section. And this smells like spam...

"Once it is set you never have to play with it again." ...not true.
 
Ironically I am awaiting my first batch of these.
Still working out the pricing but probably just a hair shy of $5,000.
Works on all Digital cameras with standard 54mm PL Mount such as Red One, of course, as well as Sony F35, Arri D20/21, Weisscam HS-2, & Phantom HD.
Should be here within the next 2-3 weeks.

Also bringing in their Bogie Head as an alternative to the Tango Head.
 
Back Focus Greetings

Back Focus Greetings

Thomas Hencz said:
Hi there all Red Owners. After having had countless attempts to set the back focus in the last year on my Red cameras I finally come across the perfect solution. It's Denz Flange Depth Controller. I just purchased one and it cost me a furtune. This is the most accurate instrument for setting back focus. I now calling on all Red camera owners to bring their camera in and I will set the back focus for your camera for $70. Please email me if you are interested info@t-stopinc.com

Kind regards,

Thomas Hencz FSF
DOP
Click to expand...


At last!! Somebody who is actually getting involved on this problem. Over the last Year or so I have been maligned many times for raising the problems of back focus and HOW IMPORTANT it is to be correct.
I have seen guys buy a set of Prime Lenses, paying tens of thousands of dollars and then arguing that they are not very good or that the Camera is brand new and cannot possibly have a problem.
To those who still disbelieve heres a point worth thinking about. A set of Primes, set for 35mm will be probably have a back focus setting of -Zero and + 2/10ths. of a thousand part of an inch. The camera will be + or - zero. Most Red cameras can be anyplace + 0r - 0.002 thousand of an inch. [0.004 difference] Thats a huge difference, and can mean soft pictures from a sharp lens. Also its no good setting the lenses as you would a Sony Video camera's lens. Its not the same at all.
RED gives terrific pictures when its back focus is set and then not tampered with.
This is NOT a criticism of RED, the adjustment is built in, please make use of it.
Meanwhile I applaud the new tools for the adjustment of RED back focus.
Cheers,
Tony Covell.
Old school camera maintenance tech.:thumbsup:
 
I got my RED back from the "upgrade" and the backfocus was off by 8 microns even though they said they had adjusted it as part of the service. I informed them of this and was told that 8 microns is very minimal, but I don't agree. If wide open on T1.3 it means a lot!
 
mixup

mixup

Tony Lorentzen said:
I got my RED back from the "upgrade" and the backfocus was off by 8 microns even though they said they had adjusted it as part of the service. I informed them of this and was told that 8 microns is very minimal, but I don't agree. If wide open on T1.3 it means a lot!
Click to expand...
Hi Tony,
indeed 8 microns (8µm) is very little- 25µm goes for .001"
I think there is a mistake in the calibration on the Denz unit- they left out a "0" so 8 on their scale is 80µm
 
You're right, Jacek. I meant 80 microns. It was indeed measured with an FDC.
 
Depth value?

Depth value?

How do you know what depth value to set the mount to?

Because of the thickness of the cover glass on the sensor and the thickness of the OLPF there is no single path length for rays at the center and edge of the light cone for a f/1.2 lens.

Therefore there is no relation to the standard flange to focal distance used for a film camera on a PL mount where the light goes only through air to the film surface.

So what exact value for the flange to focal distance are you using on the RED ONE, and how did you determine the compensation for the thickness of the cover glass and OLPF since their refractive index and dispersion values are unpublished and "mysterious"?

What f/ value are you using for your calibrations or instrument?
 
another mixup

another mixup

Dan,
I think that you are a bit mixed up on this. The actual depth is irrelevant as long as it's equivalent of "in air" 52mm ( in case of PL lenses, 46.5mm in case of Nikon lenses, etc.) because that's where properly set lenses form image. If the tool provides adjustment that will make properly adjusted lenses perform well on camera set with that tool that's all you need...
 
Ray trace

Ray trace

OptiTek said:
Dan,
I think that you are a bit mixed up on this. The actual depth is irrelevant as long as it's equivalent of "in air" 52mm ( in case of PL lenses, 46.5mm in case of Nikon lenses, etc.) because that's where properly set lenses form image. If the tool provides adjustment that will make properly adjusted lenses perform well on camera set with that tool that's all you need...
Click to expand...

I think you need to use a ray trace to find that the path length is not the same for all rays through glass as it is through air. You can debate how much the loss of resolution is but you cannot debate the error in ray length?

Because of the added error the "best focus" will vary by f/ stop, there is no longer a single right setting of the flange to focal distance.

If you set it for wide stops then there is a shift of the DOF off center for smaller stops.

Rays at the outside of the light cone from a f/1.2 lens will take a longer path through the glass than rays at the center of the light cone. Rays at an angle like from a wide angle lens will take a longer path than rays that are in the center of the frame. There is no longer a single "depth" setting that produces the same result through glass as through air, so you can set one point in the field, but not all points, and that one point for a given cone angle but not for all cone angles.

Since the exact optical properties of the cover glass and OLPF are not published I was asking how the exact values were calculated since it would seem that there was no way that they could have been calculated without the right knowledge.
 
Dan Hudgins said:
Rays at the outside of the light cone from a f/1.2 lens will take a longer path through the glass than rays at the center of the light cone. Rays at an angle like from a wide angle lens will take a longer path than rays that are in the center of the frame. There is no longer a single "depth" setting that produces the same result through glass as through air, so you can set one point in the field, but not all points, and that one point for a given cone angle but not for all cone angles.
.
Click to expand...

Hi Dan,

Thats why Panavision lenses have 2 focus marks on their lens barrels, 1 for the very wide stops & one for the rest.

Stephen
 
Dan Hudgins said:
I think you need to use a ray trace to find that the path length is not the same for all rays through glass as it is through air. You can debate how much the loss of resolution is but you cannot debate the error in ray length?
Click to expand...

This is why modern lenses are somewhat more telecentric on the sensor side, than the traditional designs. Makes the OLPF, cover glass, microlenses, etc. play into this somewhat less. This is a non-issue with longer lenses, and can be a significant issue with fast wide lenses.
 
Dan Hudgins said:
Since the exact optical properties of the cover glass and OLPF are not published I was asking how the exact values were calculated since it would seem that there was no way that they could have been calculated without the right knowledge.
Click to expand...

I believe the SI-2k has a OLPF and sensor cover of 2mm. This 2mm 'seems' to produce huge BF issues with old ziess distagons T1.3 of 16mm and bellow (wide open). So much so each lens needs to have a PL mount shimmed per lens.

I think you see this problem much less with Red due to most people using 35mm glass. I'd like to hear from anyone regularly using S16 SS distagons with Red?

regards

Michael L
 
Primos

Primos

Stephen Williams said:
Hi Dan,
.

Thats why Panavision lenses have 2 focus marks on their lens barrels, 1 for the very wide stops & one for the rest.

Stephen
Click to expand...
Primos had the two marks in the first run only. It was a disaster for the ACs. I worked for Panavision in the late 80's and they quickly removed them from the later lenses too because of the constant complaints. It was clearly a design flow that they addressed. It has nothing to do with digital-there were only film cameras back then.
Dan- all I'm sayin that it's irrelevant for backfocus adjustment what filter glass you use and such because you look in the center of the sensor for focus.
I do actually see some falloff in corners of the Optimator image and since none of the others cover full frame you can't really tell.....
Still what you are looking for in corners is how even they are rather than the actual resolution- You are testing the sensor and the sensor has the same resolving power everywhere by default. If the resolution differs from corner to corner it means sensor tilt and this is what you want to test.
There is a manual with the Optimator that talks about it as well as some classes offered by BandPro for the Local Union members and independent DITs.
Details coming...
 
2k/ 4k back focus

2k/ 4k back focus

Hello people, I wonder if I change the resolution of my project 4k to 2k, will have a change in back focus? UP was running at 4K and switched to the MKII 2k and lost my focus back. someone can tell me what happens

Tanks

PS: the UP and MKII was colimated!
 
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