Matthew Hunt has nailed it. We've discussed this before. (Would be helpful to know what lens we are talking about here. </obvious>) Odds are, even if your lens is not the Tamron SP 90mm f2.5 that it IS a design that has a very flat rear element (facing the sensor). In certain light conditions, at certain apertures, you get light bouncing off the sensor and back off the rear element.
Keep track of the aperture you are shooting at, in what lighting conditions, and you may be able to avoid that particualar combination of circumstances. But in any case, you won't get that issue to a noticable degree with a lens that does NOT have a flat rear element. On Thu, Apr 9, 2015 at 10:46 AM, Matthew Hunt <[email protected]> wrote: > 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. -- Life is too short to put up with bad bokeh. -- 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.
