Author Topic: Theta compensation - what is it?  (Read 475 times)

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PeterR

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Theta compensation - what is it?
« on: December 23, 2017, 02:30:07 PM »
This is something that I've tried to find an answer to for a long time with no results so maybe the combined intelligence of the FW community can come up with an answer.

What is theta compensation in respect of camera lenses?

Here's what I know...

Some Contax camera service manuals mention lens theta compensation. Apparently, some Zeiss/Contax lenses have it and some don't. To indicate to the camera whether the lens has it or not, the switch that tells the camera if it has a AE or MM lens attached has an extra position that differentiates between lenses with or without theta compensation. There's nothing in the manuals that indicate what difference it makes to the camera and manually moving the switch through the positions appears to make no difference to the camera. Of the lenses I recently bought, the 50mm f/1.7 and the 18mm f/4 has compensation, the others (25mm, 85mm and 135mm) don't.

So anyone know, or would like to guess, what theta compensation is.
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charles binns

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Re: Theta compensation - what is it?
« Reply #1 on: December 23, 2017, 03:07:51 PM »
theta compensation is used to account for non linear optical displacement as a result of the Steinhold-Margolitz effect which is basically reticular distortion of light spectra in un aligned polychromatic lens couplings.

Google it if you don't believe me.

PeterR

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Re: Theta compensation - what is it?
« Reply #2 on: December 23, 2017, 03:19:10 PM »
Well I had Googled theta compensation (obviously) but got mainly info about a Ricoh 360 degree camera.

Googling Steinhold-Margolitz effect got me info about caesaean sections and Googling reticular distortion of light spectra has got me a lot of stuff that makes no sense to me.

Are you being serious? Got a link?
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John Robison

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Re: Theta compensation - what is it?
« Reply #3 on: December 23, 2017, 03:44:52 PM »
Well whatever it is I'm sure is simply isn't done in polite company.

02Pilot

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Re: Theta compensation - what is it?
« Reply #4 on: December 23, 2017, 03:49:08 PM »
I think it was mentioned in one of the original episodes of Star Trek. Something about reconfiguring the warp drives by inverting the theta compensation so they could close a rift in the space-time continuum. But maybe that's not what you were looking for.
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Indofunk

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Re: Theta compensation - what is it?
« Reply #5 on: December 23, 2017, 04:06:43 PM »
theta compensation is used to account for non linear optical displacement as a result of the Steinhold-Margolitz effect which is basically reticular distortion of light spectra in un aligned polychromatic lens couplings.

Google it if you don't believe me.


There are so many fower fouls in this post I don't know where to begin ;D So I shall just leave you with this:


Francois

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Re: Theta compensation - what is it?
« Reply #6 on: December 23, 2017, 04:20:35 PM »
And this
Francois

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Francois

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Re: Theta compensation - what is it?
« Reply #7 on: December 23, 2017, 04:22:10 PM »
But on a more down to earth side, could the switch simply be to go from calculated f/stops to t/stops?
That would explain why you don't see much of a difference.
Francois

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EarlJam

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Re: Theta compensation - what is it?
« Reply #8 on: December 23, 2017, 07:18:22 PM »
The Contax AX had the ability to autofocus manual lenses by moving the film plane to adjust back focus. I wonder if that's what they refer to as theta compensation?

https://cameralegend.com/tag/contax-ax-review/

PeterR

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Re: Theta compensation - what is it?
« Reply #9 on: December 23, 2017, 10:01:32 PM »
The Contax AX had the ability to autofocus manual lenses by moving the film plane to adjust back focus. I wonder if that's what they refer to as theta compensation?

https://cameralegend.com/tag/contax-ax-review/

Don't think so as the switch that detects if the lens is compensated is on all the MM version cameras.
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Francois

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Re: Theta compensation - what is it?
« Reply #10 on: December 23, 2017, 10:25:55 PM »
I'm starting to think that this has nothing to do with optics but more to do with some trademarked peculiarity...
For some reason I believe it's entirely related to the exposure system, a bit like the Matrix metering on Nikon lenses that require a chip...

Maybe it's in the camera's instruction book.
Francois

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charles binns

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Re: Theta compensation - what is it?
« Reply #11 on: December 24, 2017, 12:13:19 AM »
I can't believe that none of you have heard of it before!

One of the first things I was told about when I first picked up a camera.

PeterR

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Re: Theta compensation - what is it?
« Reply #12 on: December 24, 2017, 08:33:06 AM »
I'm starting to think that this has nothing to do with optics but more to do with some trademarked peculiarity...
For some reason I believe it's entirely related to the exposure system, a bit like the Matrix metering on Nikon lenses that require a chip...

Maybe it's in the camera's instruction book.

I've read most of them and can't find anything.

Given that theta is normally used to denote an angle, I originally thought it was something to do with wide angle lenses but that doesn't seem to be correct. In the camera, all that happens is the CPU gets a signal to say what type of lens is fitted (AE, MM with θ or MM without θ) but what it does with it is anyone's guess.
« Last Edit: December 24, 2017, 08:42:56 AM by PeterR »
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PeterR

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Re: Theta compensation - what is it?
« Reply #13 on: December 24, 2017, 08:46:02 AM »
I can't believe that none of you have heard of it before!

One of the first things I was told about when I first picked up a camera.


I think you're looking for timewasters.com Charles. This is filmwasters.com.
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Francois

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Re: Theta compensation - what is it?
« Reply #14 on: December 24, 2017, 02:29:42 PM »
I think I'm starting to get an idea. It's a long shot but it might just be that.
A while ago I was reading about using old lenses on digital cameras. In the magazine they were explaining that it isn't recommended because the sensor requires the light to hit it at a perpendicular angle while old lenses are not corrected for that. That makes the exposure uneven across the field.
Now if the sensor for the meter uses some sort of scrim to prevent stray light and make the exposure more precise, the same problem would arise. So the camera could have either two sensors or a mechanical interlink that would move the scrim out of the way to compensate for the problem.

That's the only way I can think of that would mix exposure and angle.
Francois

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charles binns

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Re: Theta compensation - what is it?
« Reply #15 on: December 24, 2017, 07:11:23 PM »
I think I'm starting to get an idea. It's a long shot but it might just be that.
A while ago I was reading about using old lenses on digital cameras. In the magazine they were explaining that it isn't recommended because the sensor requires the light to hit it at a perpendicular angle while old lenses are not corrected for that. That makes the exposure uneven across the field.
Now if the sensor for the meter uses some sort of scrim to prevent stray light and make the exposure more precise, the same problem would arise. So the camera could have either two sensors or a mechanical interlink that would move the scrim out of the way to compensate for the problem.

That's the only way I can think of that would mix exposure and angle.

Francois, you're just making this up as you go along.

Francois

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Re: Theta compensation - what is it?
« Reply #16 on: December 24, 2017, 08:52:01 PM »
Do I ever make things up?

The theory was right for the early CCD sensors in a sense. They are composed of light tunnels with the silicon sensor at the back. Light had to go through the sensor tunnels to reach the sensitive part in order to prevent a light overload of the device and increase contrast and sensitivity. Since scattered light is a problem inside those cameras (the sensor is shiny like a mirror), the tube solution was a simple but highly efficient way to fix the problem. This also fixed the size of the photosites. The photons that hit the sensor proportionally increase it's electrical charge that is read by a set of transistors.

But I don't see why it couldn't be the case for the Contax too. Remember, they were top of the line and at the forefront of every technological advance there was. I don't believe any other manufacturers went to the trouble of making ceramic pressure plates covered in thousands of tiny holes and use a magnetic vacuum pump to suck the film against it at every exposure simply to offer the best film flatness possible.
There were cameras that used CCD sensors for the exposure meter (Nikon comes to mind).
 
Francois

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PeterR

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Re: Theta compensation - what is it?
« Reply #17 on: December 24, 2017, 09:48:10 PM »
I was thinking it was something to do with exposure and particularly with wide angle lenses and it makes sense but there's no evidence of that. The 159, which was the first camera to include this, has a very basic sensor in the viewfinder that meters off the focus screen so the angle of the light will have no effect on the sensor.
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jharr

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Re: Theta compensation - what is it?
« Reply #18 on: December 24, 2017, 11:26:53 PM »
And this one is for Francois' "scrim" x 2 and "mechanical interlink" in the same reply.


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Francois

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Re: Theta compensation - what is it?
« Reply #19 on: December 25, 2017, 02:36:17 PM »
Thanks!
These are lovely and more durable than my poinsettia  ;D
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