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PostPosted: Mon Feb 11, 2019 7:38 am 
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Hi together,
perhaps this bit of information can help: The first generation of the Tamron 90mm 1:2,5 Macro lenses had a blue spot problem when used on a DSLR. Reason was the completely flat rear element that didn't work well together with the flat sensor. The problem became only visible when the aperture was closed beyond 8.

Bob


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PostPosted: Mon Feb 11, 2019 12:12 pm 
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Location: Estepona
Quote:
The first generation of the Tamron 90mm 1:2,5 Macro lenses had a blue spot problem when used on a DSLR. Reason was the completely flat rear element that didn't work well together with the flat sensor. The problem became only visible when the aperture was closed beyond 8.

That is a very good point.

With an afocal setup the lens should be focused at infinity (real infinity - not what the dial says*) and the aperture wide open.

*Place camera on a tripod - focus carefully on the farthest object you can see clearly on the horizon and then tape the lens so that it can't move.

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Zeiss Standard WL on sick leave, Standard 18 & Wild M8
Olympus E-P2 (Micro Four Thirds Camera)


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PostPosted: Mon Feb 11, 2019 12:54 pm 
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Joined: Mon Apr 10, 2017 8:24 am
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Location: NorthWest England
MicroBob wrote:
Hi together,
perhaps this bit of information can help: The first generation of the Tamron 90mm 1:2,5 Macro lenses had a blue spot problem when used on a DSLR. Reason was the completely flat rear element ...

That's optically similar [albeit not identical] to the Pentax problem that I mentioned here:
http://www.microbehunter.com/microscopy-forum/viewtopic.php?f=9&t=7027&hilit=pentax+filter

In the Pentax case, the reflection was bounced between the rear of a flat filter and the flat face of a lens element. ... so they produced a 'watchglass shaped' filter.

MichaelG.

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