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A tech lens curiosity: the bulb inside
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PostPosted: Sat Apr 09, 2022 8:10 am    Post subject: A tech lens curiosity: the bulb inside Reply with quote
Someone is selling a curious tech lens which has a shielded back glass, with a cross hole and a small bulb behind, while the whole construction is equipped with a battery compartment. I wonder what could be the purpose and the use of such a lens? And what kind of performance (IQ) to expect?

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The images are taken from the sale page.


PostPosted: Sat Apr 09, 2022 8:42 am    Post subject: Reply with quote
Very curious.

Given the built-in red filter, maybe some kind of dark-room accessory?

Or maybe some antique slit-lamp device for early optometry / microscopy?


PostPosted: Sat Apr 09, 2022 8:59 am    Post subject: Reply with quote
There are three scales on the lens' front, one of which is clearly aperture, while another looks like a distance scale in meters starting from 1.2 and going up to 10. If it is the case, micrometry seems to be a less probable option.

I was also curious about the small red thing in the cross slot. But it that really be an IR filter? It looks more like a simple light stopper.


PostPosted: Sat Apr 09, 2022 9:20 am    Post subject: Reply with quote
It kinda looks like a crystal/prism in the rear cap, I wonder if it's for spectrum, and the bulb is a test mechanism.
Astronomy? Chemistry?
Definitely an interesting piece.


PostPosted: Sat Apr 09, 2022 10:16 am    Post subject: Reply with quote
You are right, that is certainly a small prism!


PostPosted: Sat Apr 09, 2022 10:59 am    Post subject: Reply with quote
Or perhaps an old form of light-meter? Comparing light seen through the lens against that of the calibrated lamp light?


PostPosted: Sun Apr 10, 2022 3:35 pm    Post subject: Reply with quote
Very interesting and puzzling.


PostPosted: Sun Apr 10, 2022 4:44 pm    Post subject: Reply with quote
alex ph wrote:
There are three scales on the lens' front, one of which is clearly aperture, while another looks like a distance scale in meters starting from 1.2 and going up to 10. If it is the case, micrometry seems to be a less probable option.

I was also curious about the small red thing in the cross slot. But it that really be an IR filter? It looks more like a simple light stopper.


I'm not so sure those smaller numbers are distances, and the big ones apertures.

There are three sets of smaller numbers, but they all are part of the series 1.2 / 1.8 / 2.5 / 3.5 / 5 / 7 /10 / 14

All of those are suspiciously close to a factor square root of 2 (~ 1.42) apart, which makes me suspect that they are aperture values, just not the standard series we are used to.

I suspect those large-font values (22 . 32), (26 . 32) and I think the third one may be (22 . 30) are not related to aperture, but more likely some speed / sensitivity or other measure of whatever this was used for.

I still suspect some type of exposure meter, comparing (maybe diffused) light dimmed by the aperture to the calibrated brightness of the lamp filament. Possibly the little prism allows superposition of a bright lamp-generated line when looking through the device, not entirely unlike the disappearing filament systems used in optical pyrometers?

Definitely an intriguing mystery Wink


PostPosted: Mon Apr 11, 2022 2:13 pm    Post subject: Reply with quote
My bad, one numbered scale looked the most like the distance one, but it's indeed a kind of partial repetition of the other scales.

The idea of a calibrated lightmeter is attractive by its complexity. I still dificultly catch the working principle of the whole. Did you pay attention to two small perpedicular stripes attached around the exit pupul? They could have something to do with the measurement. Meanwhile the lateral hole is puzzling. As it is off the optical axis, which purpose it could serve for?

And I might be wrong, but isn't the rear glass of the lens mat instead of being transparent?


PostPosted: Mon Apr 11, 2022 4:25 pm    Post subject: Reply with quote
alex ph wrote:
My bad, one numbered scale looked the most like the distance one, but it's indeed a kind of partial repetition of the other scales.

The idea of a calibrated lightmeter is attractive by its complexity. I still dificultly catch the working principle of the whole. Did you pay attention to two small perpedicular stripes attached around the exit pupul? They could have something to do with the measurement. Meanwhile the lateral hole is puzzling. As it is off the optical axis, which purpose it could serve for?

And I might be wrong, but isn't the rear glass of the lens mat instead of being transparent?


That hole has been puzzling me as well. Almost as if something is missing that is supposed to protrude through there. Perhaps actuated by the front dial, which clearly must have three operating positions + aperture lever). Without seeing the whole thing disassembled it is hard to know what other mechanisms may be lurking inside...
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