At least on the Tamron 90mm macros, the bluish spot near the center of the frame is from light reflecting off the sensor, then off rear optical element of the lens back toward the sensor. It's a known issue with older Tamron 90mm macros, because of a flat surface on the sensor-facing side of the rear element. It's apparently more of a problem on digital than it was on film, due to more light being reflected off the digital sensor (back toward the rear element of the lens) than was the case with film.
I'm not sure what you can really do about it; Tamron improved the coatings of later versions so it's not as much of a problem. I haven't heard about it on the Pentax 50mm macro; maybe the coatings have always been good enough that it's rare to see. I also can't find an optical diagram of the Pentax to see if it has a flat rear element like the Tamron. My only advice would be that since it's a rarer problem on the Pentax, try to be attentive to it when setting up your shot, and if you see it try recomposing the scene or the lights to see if you can get it to go away. (You probably need to check wtih Live View or an exposure, in case it manifests differently with light bouncing off the sensor vs. the viewfinder.) If it makes you feel better, the 4-meter telescope at Kitt Peak National Observatory has the same problem. (See Figure 9.) http://www-kpno.kpno.noao.edu/glaspey/4m_WF_corrector.pdf On Thu, Apr 9, 2015 at 9:56 AM, Jens Bladt <[email protected]> wrote: > Hi guys > I sometimes get a blueish spot, when I use a macro lens. > Please look at: > > https://www.flickr.com/photos/bladt/16900869850/in/photostream/ > > Any suggestions and help is very much appreciated :-) > > Thanks > Jens > > -- > PDML Pentax-Discuss Mail List > [email protected] > http://pdml.net/mailman/listinfo/pdml_pdml.net > to UNSUBSCRIBE from the PDML, please visit the link directly above and > follow the directions. -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
