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Theta compensation - what is it?
Old 12-24-2017   #1
monopix
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Theta compensation - what is it?

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 (it's just an input to the CPU) and manually moving the switch through the positions appears to make no difference to the camera. Of the lenses I own, the 50mm f/1.7 and the 18mm f/4 have compensation, the others (25mm, 85mm and 135mm) don't.

So does anyone know what theta compensation is.
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File Type: jpg 50mm.jpg (28.9 KB, 31 views)
File Type: jpg 85mm.jpg (29.9 KB, 22 views)
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Old 12-26-2017   #2
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There is some info in the service manual for the Contax 167 ('digital information codes'). It looks like every MM lens faster than f2.0 uses theta compensation.
When theta compensation is active, the selected aperture is read back as 0.5 stops slower than without compensation. At the same same time, the maximum aperture is read back as 0,5 stops faster. As far as I can see, these effects compensate each other.

I have no idea, what this is supposed to do.
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Old 12-26-2017   #3
monopix
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Quote:
Originally Posted by dirbel View Post
There is some info in the service manual for the Contax 167 ('digital information codes'). It looks like every MM lens faster than f2.0 uses theta compensation.
When theta compensation is active, the selected aperture is read back as 0.5 stops slower than without compensation. At the same same time, the maximum aperture is read back as 0,5 stops faster. As far as I can see, these effects compensate each other.

I have no idea, what this is supposed to do.
Thanks for that. Makes no sense to me though. The 167 service manual is one I don't have. Could I at least get a scan of the relevant part? Thanks.

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Old 12-26-2017   #4
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Maybe some lenses don't give an accurate light reading because of their rather large/small fstop and that tiny cut in the Pin tells the Contax to compensate? The camera could give you the same info about shutter speed and aperture but expose differently?
The EOS 1n rs with its pellicile mirror delivered a slightly darker image to the film than a slr with a normal mirror... so the camera told you for instace 1/60 and exposed at 1/55.

Very interesting thread btw.
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Old 12-26-2017   #5
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Thanks Santino.

I was thinking along the same lines but surely that would apply to all cameras and lenses so why do only some lenses have it. Dirbel is saying the manual suggests it applies to all lenses with apertures larger than f/2 but my 18mm f/4 also has it.

The fact it's called 'theta' compensation suggests it's something to do with angles. I was thinking wide angles but then my 50mm f/1.7 has it which isn't a wide angle.

What is surprising to me is the apparent complete lack of information about it anywhere. I've searched all the Zeiss literature I can find and, other than a few references in service manuals, I can find nothing. I've even searched the Japanese and USA patent archives with no luck.
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Old 12-27-2017   #6
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Ok, that was a bit misleading. I meant there are no lenses faster than f2.0 without theta compensation. Slower lenses may have this as well.
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Old 12-27-2017   #7
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Monopix, since I couldn't figure out how to attach a scan of this size to a message, I have sent a scan with the relevant tables to the e-mail address from your web site.
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Old 12-27-2017   #8
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Quote:
Originally Posted by dirbel View Post
Monopix, since I couldn't figure out how to attach a scan of this size to a message, I have sent a scan with the relevant tables to the e-mail address from your web site.
Thanks Dirk. Still doesn't make much sense to me but at least I know more than I did.
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Old 12-27-2017   #9
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Quote:
Originally Posted by santino View Post
...The EOS 1n rs with its pellicile mirror delivered a slightly darker image to the film than a slr with a normal mirror... so the camera told you for instace 1/60 and exposed at 1/55.....
That's because the mirror is a half-silver mirror that allows light to flow through as well as reflect. Some light goes up into the viewfinder while most goes to the film.

I agree it's a very interesting thread.

I have no idea of what Theta compensation is. It rings a bell, but a dull one, something to do with development. Sadly, nothing specific.

B2 (;->
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Old 12-27-2017   #10
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This thread reminds me of Nikon's AI vs. AIS.
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Old 12-27-2017   #11
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Quote:
Originally Posted by css9450 View Post
This thread reminds me of Nikon's AI vs. AIS.
Well sort of, because MM lenses are the rough equivalent to AIs. Translated to Nikon, this discussion would revolve around two different versions of AIs - and no explanation what the difference is.

Back on topic, could this whole thing be related to the light transmission of some lenses (i.e. f-stop vs. t.stop)?
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Old 12-27-2017   #12
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Quote:
Originally Posted by dirbel View Post
Back on topic, could this whole thing be related to the light transmission of some lenses (i.e. f-stop vs. t.stop)?
Someone else suggested that on another forum Dirk. Could be I guess. But where would that leave AE lenses. Would it mean ... ah, hum, I don't want to start rumours. I'll leave that thought where it is for now.

One thought I had, as it only applies to MM lenses, could it be something only applicable when used in P or Tv modes. But I still can't guess what.

I've asked the question over in the official Zeiss group on Flickr. No response yet but maybe they're still on vacation. Maybe I should just email Zeiss and ask them.
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Old 01-29-2018   #13
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We who have Lenses in various focal length don't play with magical Addon Lenses capebel of widning your Lens viewe or making your 50 a short Tele-Lens. These addon Lenses often change focal point in whole Lenses. Therefore another distance setting is needed? Or am I now out on thin Ice?

My Photograpic Liberary gave no answeres, althoug I serched in almost a Feet and a half of Encyclopedias, but Internet is full information on such items we don't use.

As a youngster I remember a pals father had a Voigtländer reflex and an add on Tele Lens that demanded the Theta setting on the Camera. But that over 58 years since.
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Old 01-29-2018   #14
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Quote:
Originally Posted by johank View Post
We who have Lenses in various focal length don't play with magical Addon Lenses capebel of widning your Lens viewe or making your 50 a short Tele-Lens. These addon Lenses often change focal point in whole Lenses. Therefore another distance setting is needed? Or am I now out on thin Ice?
I'm not referring to any sort of add-on lens so not sure what you mean.

Quote:
My Photograpic Liberary gave no answeres, althoug I serched in almost a Feet and a half of Encyclopedias, but Internet is full information on such items we don't use.
Thanks for looking anyway.
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Old 02-20-2018   #15
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Theta Lenses are used in Photocopying and Scanner Machines, can it be that on close Distances more light reflects into the Lens and therefore we see the compensation?
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Old 02-26-2018   #16
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Maybe it has something to do with Scientology.
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Old 02-27-2018   #17
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Maybe it has something to do with Scientology.
Bad pun!
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Old 02-27-2018   #18
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I once tried to use a Schneider theta corrected lensblock, an industrial flat-field laser-engraving lens. Unfortunately it was designed for a specific wavelength and even with a combination of the green and blue filters I had available there was no hope of eliminating fringing.

Here is a brief general description of such a device: https://www.thorlabs.com/newgrouppag...group_id=10766

p.
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Old 02-27-2018   #19
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Quote:
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I once tried to use a Schneider theta corrected lensblock, an industrial flat-field laser-engraving lens. Unfortunately it was designed for a specific wavelength and even with a combination of the green and blue filters I had available there was no hope of eliminating fringing.

Here is a brief general description of such a device: https://www.thorlabs.com/newgrouppag...group_id=10766

p.
This might solve it. In to the first set of diagrams in that website, theta is the angle between the optical axis and an off-axis point one might want in focus. So this is a compensation for the focus-and-recompose-technique with flat-field lenses?
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