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xaprb
Joined: 28 Jan 2021
Posts: 170
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 Posted: Fri Apr 26, 2024 1:22 am Post subject: Improving Mirror Lens Contrast |
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xaprb wrote:
Here is an article on a way to reduce stray light in catadioptrics and reflecting telescopes, which improves contrast markedly.
https://retrofocal.com/articles/improving-mirror-lens-image-quality/
The synopsis is that mirror lenses and reflecting telescopes show the sensor a reflection of itself, which hazes the image, but the spot that forms that reflection doesn’t participate in making the image, so it can be blackened. |
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Alun Thomas
Joined: 20 Aug 2018
Posts: 632
Location: New Zealand
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| Posted: Fri Apr 26, 2024 5:09 am Post subject: |
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Alun Thomas wrote:
An amazingly simple idea, it is surprising it hasn't been addressed by designers. It would seem that the effectiveness of the manufacturers sensor coatings would also be a factor. However, as noted, the best idea could be that shown in the link. |
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RokkorDoctor
Joined: 27 Nov 2021
Posts: 1277
Location: Kent, UK
Expire: 2025-05-01
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| Posted: Fri Apr 26, 2024 8:56 am Post subject: |
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RokkorDoctor wrote:
Interesting concept.
Almost seems too obvious to be true; given that some mirror lenses already contain internal baffles and cones to block stray light and reflections, one would have thought something as simple/obvious as this would not have been overlooked. But then that sometimes does happen.
Also, reflection of film was less of a problem than digital sensor reflections are, so maybe it wasn't considered as much of a problem then.
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connloyalist
Joined: 22 Jul 2020
Posts: 253
Location: the Netherlands
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| Posted: Fri Apr 26, 2024 9:24 am Post subject: |
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connloyalist wrote:
Very interesting!
Regards, C. |
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Himself
Joined: 01 Mar 2007
Posts: 3216
Location: Montreal
Expire: 2013-05-30
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| Posted: Fri Apr 26, 2024 11:00 am Post subject: |
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Himself wrote:
Anyone with a mirror eager to give it a try?
I don't have one but I'm curious.
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connloyalist
Joined: 22 Jul 2020
Posts: 253
Location: the Netherlands
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| Posted: Fri Apr 26, 2024 11:31 am Post subject: |
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connloyalist wrote:
| Himself wrote: |
Anyone with a mirror eager to give it a try?
I don't have one but I'm curious. |
Eager, yes. But sufficiently confident of my skills, no.
Regards, C. |
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marcusBMG
Joined: 07 Dec 2012
Posts: 1305
Location: Conwy N Wales
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| Posted: Fri Apr 26, 2024 5:04 pm Post subject: |
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marcusBMG wrote:
Very detailed article, I am interested to try this out, I have more than one tamron 500mm so can compare.
It's a bit curious as to who wrote this piece, the webpage/website has no info....
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Ernst Dinkla
Joined: 30 Nov 2016
Posts: 380
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| Posted: Sat Apr 27, 2024 9:05 am Post subject: |
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Ernst Dinkla wrote:
| RokkorDoctor wrote: |
Also, reflection of film was less of a problem than digital sensor reflections are, so maybe it wasn't considered as much of a problem then. |
Still there is a way longer period of mirror lenses used with film, on a variety of tasks and with film emulsions that have different reflection values. Possibly the film reflection was less visible while more uniformily spread than the hot spot reflection of a sensor is. Anyway contrast loss would still be measurable. Maybe a lower T-stop number had better marketing value than enhanced contrast.
Digital sensors were already used in astronomy before the general photography sector could buy them. Mirror lenses more common there.
All together it is a mystery why nothing on this subject has been published before.
I do hope the writer archived a list of his disc measurements for the popular mirror lenses
he tweaked. I guess with a 3D printer at hand it would even be possible to make the proposed cones to fill in the voids between the light paths. Black 3.0 painted for low reflection.
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jamaeolus
Joined: 19 Mar 2014
Posts: 2935
Location: Eugene
Expire: 2015-08-20
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| Posted: Sun Apr 28, 2024 4:25 am Post subject: |
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jamaeolus wrote:
Fascinating. I have a cheap Tokina I might try this with.
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xaprb
Joined: 28 Jan 2021
Posts: 170
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| Posted: Mon Apr 29, 2024 12:03 am Post subject: |
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xaprb wrote:
Regarding film's differing reflectivities, I thought the same thing about film. But I was reading an article about Maitani's design process for the OM-1, which metered off-the-film. If I recall correctly, in the interview he stated that he got samples of every film stock available on the market, and metered off them, and found them all within 10% of each other, validating that off-the-film metering would work OK. Then he went ahead and designed the system and it worked well. There weren't very many cameras that metered off the film, but the ones that did (several of the OM, as I recall, as well as the Minolta CLE) metered quite well. So maybe film stocks aren't as different as one would think? |
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Ernst Dinkla
Joined: 30 Nov 2016
Posts: 380
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| Posted: Mon Apr 29, 2024 7:19 am Post subject: |
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Ernst Dinkla wrote:
| xaprb wrote: |
| Regarding film's differing reflectivities, I thought the same thing about film. But I was reading an article about Maitani's design process for the OM-1, which metered off-the-film. If I recall correctly, in the interview he stated that he got samples of every film stock available on the market, and metered off them, and found them all within 10% of each other, validating that off-the-film metering would work OK. Then he went ahead and designed the system and it worked well. There weren't very many cameras that metered off the film, but the ones that did (several of the OM, as I recall, as well as the Minolta CLE) metered quite well. So maybe film stocks aren't as different as one would think? |
That the OM system worked is already an indication film reflected a measurable amount of light. I was thinking whether the film emulsions gloss differences could shift the reflection between diffuse to more directed light.
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Slalom
Joined: 10 Dec 2017
Posts: 152
Location: Stourbridge
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| Posted: Mon Apr 29, 2024 8:28 am Post subject: |
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Slalom wrote:
I have an OM2N and it has two sensors at the bottom of the mirror box, only effects exposure when film is fully visible and static.
Now thinking it through, it is getting difused light at the bottom of the box, So reflections are also difused. Less trouble forthe unused bit of the mirror. |
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kds315*
Joined: 12 Mar 2008
Posts: 16553
Location: Weinheim, Germany
Expire: 2021-03-09
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| Posted: Tue Apr 30, 2024 11:14 am Post subject: |
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kds315* wrote:
Interesting as I have quite a few such lenses and some do have lower contrast...
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