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Archive 2017 · What causes this

  
 
lighthound
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p.1 #1 · p.1 #1 · What causes this


I had once read something about this long ago but for the life of me I can't recall what this nasty light deformation is called.

I intended on researching it, to figure out what causes it, but it's kinda hard to look up when one is too brain dead to know what to call it.

So I guess as long as I'm here asking what it's called, I might as well ask what causes it and how do I eliminate this on future images. Is this a function of my lens? Sensor?

Image info if needed.
The sun was just about to rise over the horizon but not up yet and was located on the right about where I made the crop.
Which btw is a massive crop for illustrative purposes. This light defect is very significant on the full frame image as well and on every shot I took with various exposures it shows on anything above the horizon.
This is a sooc raw image file with no pp at all.








Edited on Feb 22, 2017 at 07:13 PM · View previous versions



Feb 22, 2017 at 06:22 PM
Michael White
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p.1 #2 · p.1 #2 · What causes this


Isn't it chromatic abbreviation?


Feb 22, 2017 at 06:40 PM
lighthound
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p.1 #3 · p.1 #3 · What causes this


Michael White wrote:
Isn't it chromatic abbreviation?


I don't think so is it? If it is, then this is the worse case of it I have ever seen.

I always understood chromatic aberration to be a very thin purple or green fringe around the edges of objects such as tree branches in the sky and such.

If that's what this is, then something must be wrong with my lens. Which is supposed to be very good with chromatic and spherical aberration.

Could it be caused by or amplified by the 2 2/3 EV ? I see it on the 0 EV images as well though, just not as extreme.



Feb 22, 2017 at 07:12 PM
Lauchlan Toal
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p.1 #4 · p.1 #4 · What causes this


It's the pixel receptors overflowing (really rough analogy and imprecise language, sorry).

But basically the sky is exceptionally bright, much brighter than pure white. It looks no different to us than barely pure white, but it bleeds into nearby pixels because it's just so bright.

If you expose it right at rgb 255,255,255 you won't get any of this bleed, but if you then expose it more than that you'll still get the same rgb 255,255,255 on the sky since it can't get brighter than pure white, but all that extra light will affect nearby pixels.

This is why commercial photographers shooting things against white backdrops tend to underexpose the backdrop a tiny bit, to say 250,250,250, and then boost it in post a little. Otherwise you risk getting artefacts like this.



Feb 22, 2017 at 07:20 PM
Scott Stoness
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p.1 #5 · p.1 #5 · What causes this


I think its caused primarly caused by defraction of light around the object. Light bends around slit due to the quantum nature of light beams.

And it is exacerbated by the effect described by Lauchlan Toal, Because the energy in the light is really really strong a modest defraction will create lots of effect. Thus filling up the adjoining pixel wells.

And its exacerbated by imperfections in your lens (CA distortion and reflections). But likely the smallest effect.



Feb 22, 2017 at 07:31 PM
didierv
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p.1 #6 · p.1 #6 · What causes this


Act of god !


Feb 22, 2017 at 07:34 PM
lighthound
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p.1 #7 · p.1 #7 · What causes this


Ah, gotcha! Thanks for that info. That's what I'm now recalling I had read long ago.

As soon as I read that I went in and checked my sooc images and even my very lowest (under) exposures which were -1 1/3 EV have a blown Red channel. The G/B channels are way down to about 100/80 respectively.

So apparently due to the incredible red sky that morning it pushed the R channel through the roof. I need to shoot much lower exposures and blend me thinks.

Thanks again for the info folks!

Dave



Feb 22, 2017 at 07:52 PM
AJSJones
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p.1 #8 · p.1 #8 · What causes this


I just found a neat site called Physics, explained by cats http://physicswithcats.blogspot.com/2016/02/things-that-waves-do-part-2-diffraction.html

It shows how diffraction works at an edge - we are familiar with it when the edge is the aperture in the lens and diffraction limits et. but this looks like the light got "bent" around the cross...




Feb 22, 2017 at 08:19 PM
EB-1
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p.1 #9 · p.1 #9 · What causes this


The exposure is excessive. Also there is flare and possibly sensor blooming.

EBH



Feb 22, 2017 at 08:32 PM
AJSJones
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p.1 #10 · p.1 #10 · What causes this


It's all happenin'


Feb 22, 2017 at 08:37 PM
Jeff Nolten
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p.1 #11 · p.1 #11 · What causes this


It happens to our eyes too in some circumstances. You usually put you hand up when it happens.


Feb 22, 2017 at 08:45 PM
EB-1
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p.1 #12 · p.1 #12 · What causes this


didierv wrote:
Act of god !


I assumed the cross was physically present, if not...

EBH



Feb 22, 2017 at 09:32 PM
rattlebonez
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p.1 #13 · p.1 #13 · What causes this


Excessive overexposure


Feb 22, 2017 at 10:21 PM
daleg
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p.1 #14 · p.1 #14 · What causes this


taking nothing from the more technical suggestions, I would first look to the exposure settings.

I'm guessing the OP was using evaluative metering - in which the meter attempts to "normalize" the background to 18% gray.

good luck with that.

I'd suggest spot metering.



Feb 22, 2017 at 10:55 PM
howard
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p.1 #15 · p.1 #15 · What causes this


It's the second coming of chromatic aberration!


Feb 22, 2017 at 11:31 PM
Scott Stoness
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p.1 #16 · p.1 #16 · What causes this


daleg wrote:
taking nothing from the more technical suggestions, I would first look to the exposure settings.

I'm guessing the OP was using evaluative metering - in which the meter attempts to "normalize" the background to 18% gray.

good luck with that.

I'd suggest spot metering.


You are spoiling my fun - avoiding the technical discussion.

Less exposure will help but i suspect that this scene is really hard to capture and will require bracketing exposure and manually blending.



Feb 22, 2017 at 11:52 PM
15Bit
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p.1 #17 · p.1 #17 · What causes this


Lauchlan Toal wrote:
It's the pixel receptors overflowing (really rough analogy and imprecise language, sorry).

In x-ray detectors we would call that "blooming" - the overflow/spreading out of intensity when the detector saturates. Performance is there is described in terms of a "point spread function". Ideally the PSF would be 1 pixel, which would give no blooming at all.

I don't believe that much bleed is due to any diffraction effect. And in any case this can be easily tested, by just dropping the exposure and seeing if the effect remains.

It could be due to a nice smearing of oil/grease on the lens though....



Feb 23, 2017 at 04:26 AM
charlyw
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p.1 #18 · p.1 #18 · What causes this


15Bit wrote:
I don't believe that much bleed is due to any diffraction effect. And in any case this can be easily tested, by just dropping the exposure and seeing if the effect remains.



And yet it isn't blooming - that is something of the past ever since Canon introduced CMOS sensors with their much higher resistance to any charge overspill from one sensor cell to the next. Early CCD sensors were quite prone to blooming... Look at http://www.imaging-resource.com/PRODS/D80/D80A5.HTM - there are good examples how blooming in an imaging sensor looks like. The shown effect is a combination of diffraction, overexposure combined with lens coating efficiency to reduce lens flare and haze...



Feb 23, 2017 at 04:38 AM
lighthound
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p.1 #19 · p.1 #19 · What causes this


As I had mentioned above, I do see this on underexposed -1 1/3 EV images as well, just not as pronounced as seen in the OP image. However, I do see that the Red channel was blown on those underexposed images so I think that explains why.

And yes, the op image is one of the shots from a bracketed set of 3 which is why is overexposed. You will see why it was needed soon.

And nope, nope, nope. There is/was no oil or grease on this lens nor any others I own. I doubt there was much more than 3 specks of dust on it because I'm very anal like that.

Thanks again all for your thoughts on this.
I went back out this morning and did a reshoot of this location and made damn sure I didn't blow any channels this time. I had read the above comment about using evaluative metering just before I left the house and changed my setting to spot metering.

I hope I got something decent to share once I get home and take a look.


Oh... If my 5DII sensor blooms, will it become a beautiful 6DII this spring?
Answer to that is... You bet your sweet ars it will. That day cannot come soon enough.

Dave



Feb 23, 2017 at 12:46 PM
dmacmillan
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p.1 #20 · p.1 #20 · What causes this


Since we are on a religious tangent, there's a scene in Franco Zeffirelli's mini series "Jesus of Nazereth" where they show Christ's face with very strong backlighting. The combination of lens flare and film overexposing creates an effect very similar to this.


Feb 23, 2017 at 01:18 PM
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