Yes Darren, he did :-)
The porblem is that I buy most stuff on-line - so I don't see if the
rear element is flat. An of cource I didn't know the reason until just
now :-)
I was using a Pentax-FA 2.8 50mm Macro. The problem only occurs at
F25-32 - and it seems more likely to occur, when using flashlight.
So now I try to stay below F 25 - or use other lenses - like the 43mm
Limited. Also the 645-FA 120mm macro doesn't have this issue at all. But
the 645D is tiresom to use, when many shots are needed for fdocus
stacking - 'cause it's quite slow, processing files.
Regards
Jens
Den 09-04-2015 kl. 18:17 skrev Darren Addy:
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
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