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Re: blue in center of FOV
Posted: Mon Feb 11, 2019 7:38 am
by MicroBob
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
Re: blue in center of FOV
Posted: Mon Feb 11, 2019 12:12 pm
by 75RR
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.
Re: blue in center of FOV
Posted: Mon Feb 11, 2019 12:54 pm
by MichaelG.
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:
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.