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12-08-2015, 11:38 PM   #16
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QuoteOriginally posted by stevebrot Quote
I may be off-base, but the hot spot in that photo looks like a back reflection (hot spot) off the sensor reflected back off the diaphragm blades. Translation? Not your garden variety flare. As for using the burn tool, good luck on making that one go away. The image is toast for the most part, IMHO.

Nice theory, but a back reflection at f/11 on a f/1.8 modern lens? Looking at the excellent f/11 reference in #10, I'm not convinced, but certainly it is flare, and I can't imagine matte black aperture blades reflecting back much of anything. I like your idea though that it could be a just a backward flare off the sensor.

The burn tool isn't going to completely "fix" the image, but it should go a long way in helping.

12-09-2015, 04:12 PM   #17
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QuoteOriginally posted by Alex645 Quote
Nice theory, but a back reflection at f/11 on a f/1.8 modern lens?
Ummmm...yes. Why not? My comment is based on personal experience with the reflect-back anomaly. The relative size of the aperture outline is a clue, though the anomaly is fairly hard to characterize except that it is most common with lenses having a fairly exposed, wide diameter rear element. The Sigma 18-35 fits that description. As for blade reflectance...I don't have the Sigma 18-35 in hand, though if the blade finish is anything like that on my Sigma 17-70/2.8-4 (C), it would be a medium gray anodized brushed surface with albedo comparable to that of blades of most of the lenses on my shelf.

I know, it is a apples vs. beefsteak comparison, but here is a sensor reflect hot spot at f/11 from a K10D. The image has not been cropped and the taking lens was a vintage M42 Auto-Rikenon 55/1.4 (single coated) with a deep hood fitted. The Rikenon has a notoriously wide and prominent rear element and while it is not real evident in this shot, there is a halo of reduced contrast extending several degrees concentric to the aperture spot, but which does not veil over the full field.

I had some other shots taken of a scene with bright highlights oblique to the lens axis and out of frame that resulted in a hot spot almost identical to the example posted here. Unfortunately, those were bit-bucketed some years ago.

Anyway, I am not married to that explanation.


Last edited by stevebrot; 12-09-2015 at 04:26 PM.
12-09-2015, 05:57 PM   #18
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Everybody is talking about how and why the flair happened without speaking the obvious.

Where you trying to have your subjects back lit? If not, you could have turned everybody including yourself 60 to 90 degrees away from the direction of the sun so the light would be coming from the side and not behind your subject.

Of course that would not work if you were trying for the back lighting.
12-09-2015, 06:37 PM   #19
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QuoteOriginally posted by virtualjsk Quote
Thanks... Is there any explanation for this?
If the hot spot is a back-reflection off the sensor, prevailing theory is that it is caused by light bouncing back off the sensor. This light may reflect off bright surfaces in the mirror box or the rear of the lens causing an overall drop in contrast. That which strikes the rear element may reflect directly back off the rear element surface and/or reflect off surfaces in the lens interior as sort of a reverse-direction flare.

The anomaly is poorly understood and is not usually a problem with most lenses. It is most commonly seen in situations that also cause ordinary flare. These include:
  • Strong backlighting
  • Strong specular highlights
  • Bright light from out-of frame, striking the front element
Avoid the first two by planning your setup and use a hood to remedy the last.



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