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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 :)

Red back focus

Keep an eye on this thread, Matt. I see it's drifting into the Twilight Zone(shilling zone?)
Jacek Zakowicz, OptiTek-dot-org
 
Matt,
I'm talking about setting the chip up to the right backfocus. I am also taking for granted that the lenses you use for this are already set-up (infinity is at the mark, footage checks out ect.).
Jacek was right about this is just physics, depth of focus uses magnification as part of the equation, and at infinity, it is close to zero for these lenses. When you are using a chart, say at 6 ft., that's when using a wide lens is important, as the magnification factor changes, and teles are less accurate.
And Jacek, I do not get a cut of Centurys (now Schneiders) action. Wish I did.
Circle of confusion is also part of the equation, what is that number?
 
Have you try that approach?

Have you try that approach?

Ken Robings said:
Matt,
I'm talking about setting the chip up to the right backfocus. I am also taking for granted that the lenses you use for this are already set-up (infinity is at the mark, footage checks out ect.).
Jacek was right about this is just physics, depth of focus uses magnification as part of the equation, and at infinity, it is close to zero for these lenses. When you are using a chart, say at 6 ft., that's when using a wide lens is important, as the magnification factor changes, and teles are less accurate.
And Jacek, I do not get a cut of Centurys (now Schneiders) action. Wish I did.
Circle of confusion is also part of the equation, what is that number?
Click to expand...
Hi Ken,
I know you don't work for Century, but didn't know you wish you did:thumbsup:
I just tried to keep this on topic away from the urban legends that keep cluttering up valid info...
Anyway, your approach would work if you came up with a tele lens with 10+ turns from CF to inf. In reality there is such a long and quick throw in focusing in order to keep full travel below 1 turn that the error margin is far greater than even the lowest precision. Sometimes you need to think about practical application of theories... If you tried it you'd know it does not work.
As far as cof ideally it should match the pixel size- .005mm which happens to be 1/5th of the traditional .001" In reality I don't know...
And while we at the factor topic: how about the MTF- 10%, 20% minimum?-it certainly makes the difference.
Jacek Zakowicz, OptiTek-dot-org
 
Jacek,
I didn't say I wanted to work AT Century, I just want a piece of the sales. I am having a great time engineering (with Stewart) the Ruby zoom.
Has Red ever stated the CoF on its spec sheet? I have not looked.
To figure out the DoF, you need the actual CoC, not just close.
Jacek, will you be at CineGear this Friday, I will look you up.
The more we can correct uban (cinema) legends, the better.
 
I completely agree that urban legends have no place here, especially in a thread such as this. I also agree with what Jacek says: there's a difference between theory and practice. In practice, there is a lot of slop in a standard lens and not enought precision in setting to an absolute distance. At least not for this purpose. That's why the RED Null is a fairly wide lens with an extremely fast aperture that is absolutely locked at infinity. And that is why it is accurate to within fie microns, which will simply not be the case in the real world when one uses a regular lens with adjustable focus.
 
So are you guys saying that Back Focus shouldn't be attempted in the field and the camera and lenses should be sent in to be adjusted? And I've lost track of what equipment is needed. Can it be done by eye with a good HD monitor or do you need special calibration equipment?
 
The Red Null allows an in the field camera PL mount collimation. PL lenses would not be field adjustable afaik.
 
Back focus rules

Back focus rules

Tyler Black said:
So are you guys saying that Back Focus shouldn't be attempted in the field and the camera and lenses should be sent in to be adjusted? And I've lost track of what equipment is needed. Can it be done by eye with a good HD monitor or do you need special calibration equipment?
Click to expand...
1. Have your lenses setup correctly(and all the same obviously)-that is not going to change no matter what.
2. Pick the widest/fastest lens to setup your camera- this one also has to have the 2-3 closest focusing marks checked accurate. If they are not have qualified tech mark them after setting up infinity.
3. Set up a resolution chart at one of the close distances you had verified on your test lens. It has to be precisely measured, within few mm or 1/16" or so, not skewed- square to the camera. Light it so you can see well. Putora chart is very popular, I prefer trumpets because I can precisely nail best resolution.
4. set the test lens focus scale at the distance from the chart -wide open and adjust the back focus on the cam until best resolution.

OR YOU CAN PICK UP THE OPTIMATOR AND, AS ONE OF THE GUYS FROM SALT TEST PUT IT"CHECK THE BACK FOCUS IN LESS TIME THAN IT TOOK TO MOVE OPTIMATOR FROM ONE CAMERA TO ANOTHER"
Jacek Zakowicz, OptiTek-dot-org:beer:
 
I'd like to try and answer my own question about what size the Circle of Confusion is, but first some history;
In 1960, the first ASC Manual staes the CoC for 35mm film is 50 microns (.002")
26 years later (ASC Manual 6th ed.), this has changed to 25 microns (.001") CoC
For 16mm film, that year the spec was half that, 12.5 microns (.0005")
This where I should quote the current Manual, but I don't have it (to cheap).
Fujinon's spec for 2/3" HD is 5 microns, which is the size of one pixel (in a Sony f900).
If I needed a CoC for a 35mm size sensor today, I would go with .00635mm, 6.35 microns (.00025"), almost one tenth of where we started 50 years ago.
Jacek, Mitch, good luck with your backfocus tools at CineGear this weekend
 
Coc

Coc

Hi Ken,
Thank you for the Coc rundown. I would agree with your numbers- now who would calculate DOF for the new size Coc. Congrats on the conversion- you beat me to it:) I'll see you at Cinegear.
Jacek Zakowicz, optitek-dot-org
 
Jacek & Ken have it down. I would add that while I have not yet seen the Optimator in person, the Null is just as quick & easy to use and it is dead on accurate to within five microns. With Ken putting the CoC at 6.35 microns, that means that the Null is accurate "within Nyquist" or simply put is beyond the accuracy the RED One can resolve.

To answer Tyler's question, these devices are for adjusting the backfocus or flange depth of the camera. To adjust the lens itself one must use a lens collimator. For this I would suggest you contact a rental house or lens service facility such as Optitek, Focus Optics or Duclos. A lens collimator is a large and expensive bench device and not something a camera owner might generally own. A device for checking the camera such as the Optimator or the Null is something useful for a camera owner to have in the kit.
 
Mitch Gross said:
To answer Tyler's question, these devices are for adjusting the backfocus r flange depth of the camera. To adjust the lens itself one must use a lens collimator. For this I would suggest you contact a rental house or lens service facility such as Optitek, Focus Optics or Duclos. A lens collimator is a large and expensive bench device and not something a camera owner might generally own. A device for checking the camera such as the Optimator or the Null is something useful for a camera owner to have in the kit.
Click to expand...

I was waiting for someone to separate camera and lens back focus.
Very different, yet equally important.
 
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