[LRflex] Re: Bluedot Frustration...The solution... I think!

Have you managed to reproduce the blue dot? (used to be Blaupunkt car steros in my youth BTW) 
Xavier also has an 80-200, perhaps he could try on his digital buddy?

Thanks for the updates
phx


David Young wrote:


Neil wrote:


Common to both of your examples is a diffused bright white background. Is
this the same with the other "blue dot" shots that you have? If so, it
would be interesting to see if you could get a blue dot when using a UV
filter. Beyond that, I think you have "ironed down" the nature of the
problem and can avoid it in the future.



Hi Neil!
Talk about having a firm grasp on the painfully obvious! Now at times the line above might be considered a veiled insult, but in this case, it's a compliment... for although I had "twigged" to the overcast sky problem, it did not occur to me that the background was almost uniformly white, giving it something in common with the indoor, in the shade, closeup shots.
And, yes, all the shots which show this problem have some sort of diffuse, white background.
Thus, I have added a second shot to the examples... at http://www.furnfeather.net/Temps/BluRose.htm
In this second shot, Rose is standing in front of the Eiffel tower, which is almost "grey'd away", with the low, light, cloud. But, of course, this means that the background is not the same diffuse white ... or, at least, not as much so. The blue-dot is considerably smaller. 

The largest and most pronounced "blue dot" is in the close ups ...

http://www.furnfeather.net/Temps/BluDot.htm
which are shot against a curved, white card, to eliminate background clutter and seams. Because it, too, is uniformly white, but brighter than the overcast sky, it creates an even larger "blue dot"!
By the way, all the outdoor shots were taken with a UV filter, in place. (OK, lets not get into the old filter/no filter debate!) So were some of the close-ups... especially those taken at a distance, using the zoom feature for convenience. The really close up shots have a Nikon T6 Achromat in front of the 80~200 Vario. They too show the "blue dot" phenomenon. OTOH, when they get close enough, that the shot only includes the camera, without any significant amount of the white background, the "blue dot" disappears!
Whether this is light reflecting off the sensor and then back again from the surface of the rear element (the most likely scenario, to me) or whether it is somehow being reflected off the surfaces of two or more of the rearmost lens elements is rather immaterial. The problem exists and key to it's solution is that the "blue dot" is lens specific. For, after encountering the problem with my Vario, I switched to the 50 Summi, with either Doug's Universal Extension Tube (a great device, btw) behind, or an ELPRO in front. Both combinations worked without any problems. 

Proof of this can be seen at
http://www.camerapedia.org/wiki/Debrie_Sept which has 16 shots taken with these combinations. No "Blue Dots" here!
I suspect that this "rear element reflection" is there whenever any 80~200/4 Vario is used with a digital body, for sensors are shinier and thus more reflective, than film. However, in "normal" use, there is enough light to overwhelm these minor reflections and no problem is seen. However, when the conditions are just right and a large proportion of the background is white, there is enough of this "rear element reflection" to show, to varying degrees, in the photos. (I'll bet the ray-tracing of this problem would be a lens designer's nightmare!) 

Dots it!

---
David Young, Logan Lake, CANADA
Wildlife Photographs: http://www.furnfeather.net <http://www.furnfeather.net/> Personal Web-pages: http://www3.telus.net/~telyt <http://www3.telus.net/%7Etelyt> 
Stock Photography at: http://tinyurl.com/2amll4

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