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Author Topic: Purple Artifact  (Read 3426 times)

maddogmurph

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Purple Artifact
« on: July 22, 2015, 08:23:56 pm »

Hello all,

I've had some issues with my 10 stop B&W filter:

The purple blob is fairly consistent across photographs.  Very few taken over 30" exposure don't have them.  It seems it's from the angle of the sun, basically a glare spot that's been diffused by time lapse.  

First is there any way to remove the purple tinge and fix these photographs?  I'm looking for someone who's likely done or experienced the same thing and fixed it in post processing.  What techniques did you find useful, or tutorials would you suggest?

Second, is anyone else having problems similar to this with their 30"+ time lapse.  Is this the filter?  I had issues with a Vari ND, and this doesn't seem to be doing much better, just different artifacts.  If I can't open this up for 45" in direct sunlight what's the point of owning it... ?
« Last Edit: July 22, 2015, 08:26:28 pm by maddogmurph »
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stamper

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Re: Purple Artifact
« Reply #1 on: July 23, 2015, 04:28:54 am »

The B&W 10 stop - I have two in different sizes - has a reddish cast. I think you were too near the sun. I don't often see the effects of the cast because I mostly shoot sunsets which means the cast blends in and sometimes adds to the sunset. If you use LR try the eyedropper tool or in PS desaturated the image - not much colour in it - inverse the layer and paint back the colours you wish to retain. There will be other members with suggestions I'm sure.

SanderKikkert

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Re: Purple Artifact
« Reply #2 on: July 23, 2015, 05:31:05 am »

There are many ways to get rid of this afterwards. I am sure there are members here more experienced than me who can point you in the right direction.

However, as I noticed from the example: you pointed your camera downwards quite a lot with a wide-angle. (It doen's look like you used shift here to place the horizon higher in frame, right ? ) which is, certainly in these very harsh light conditions (close to midday ?) literally asking for all sorts of trouble with flare, glare and what not.

EDIT: Forgot to mention that the above relates to situation where you have a large part of the frame filled with water which reflects the sunlight coming from above. YMMV if you are shooting architecture midday for example.

So a different sort of advice imho would be: Keep the camera 100% horizontal and crop out sky afterwards or more drastic: don't try to take these long exposures this time of day, unless the sun is shielded by a thick layer of clouds. Using the 10 stop closer to dawn or dusk will even give you 300" exposure times ;-)

my 2 cents, Best Regards, Sander

PS you díd cover the viewfinder right ? that also causes problems, yet those are usually more red-ish.
« Last Edit: July 23, 2015, 06:06:24 am by SanderKikkert »
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spidermike

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Re: Purple Artifact
« Reply #3 on: July 23, 2015, 05:41:24 am »

It may be infra-red coming through - some lenses are more prone to it than others.
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Slobodan Blagojevic

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Re: Purple Artifact
« Reply #4 on: July 23, 2015, 10:33:22 am »

It appears at the same spot as in your other photograph (The Fringe). So, two further options: 1. Filter is unevenly coated at that spot - easy to check by orienting it differently or 2. It is a sensor issue that demonstrate itself only during long exposures.

maddogmurph

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Re: Purple Artifact
« Reply #5 on: July 23, 2015, 01:08:44 pm »

It appears at the same spot as in your other photograph (The Fringe). So, two further options: 1. Filter is unevenly coated at that spot - easy to check by orienting it differently or 2. It is a sensor issue that demonstrate itself only during long exposures.

Well, it's happening in about 80% of long exposures, but only with the B&W filter.  I have probably 20 examples in different locations, some more apparent than others.  Typically it's showing up when shooting in direct light (daytime) and appears to be a glare artifact of sorts.  I'd like to try another 10 stop from a different brand to see if it's the filter, so I'll try this I suppose.  Quite frustrating really, I'd rather not sit around post processing out giant purple blobs on my photographs.
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davidh202

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Re: Purple Artifact
« Reply #6 on: July 23, 2015, 10:37:10 pm »

Since there is no other Purple in the photo the best you can do is open in ACR or Lightrooms HSL panel and completely de saturate the Purple slider, and in the Hue slider move the Purple slider to the blue side. You can also play with the lightness slider a little. it wont remove it completely but will make it a lot less obvious. Then you have to be a bit of an artist...
I go into the adjustment brush choose a very light  tint that will hide the remaining offending color in that area ( screen shot) and blend it in lightly. In this case I chose to add some green to match the green area below the purple. The only other way would be to completely de saturate the image and adjust the luminance in that area. Oh, I also straightened the horizon ;-) 
« Last Edit: July 23, 2015, 10:38:58 pm by davidh202 »
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Arlen

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Re: Purple Artifact
« Reply #7 on: July 24, 2015, 12:50:06 pm »

As Slobodan suggested, you should consider the possibility of a light leak if you haven't already checked for that, particularly if it shows up in the same area of the frame in different scenes. It is very common for light to leak through the viewfinder window in a DSLR, and it may become noticeable only at long exposures. Are the other (non-B&W) filters you compared with it also 10-stops? In other words, are the exposure times with those filters comparable to what you are using for this B&W filter?

An easy test for light leaks: Using the same exposure time under which you saw this effect, make another exposure in the same conditions, but keep the lens cap on the lens. If you see anything other than pure black across the resulting image, it's due to a light leak. Now do the same test again, except this time by covering the viewfinder during the exposure--with the tab designed for that on some (Canon) camera straps, or with tape etc. That will probably eliminate the light leak. If not, cover the whole camera and lens with a nearly opaque black cloth, like a towel, and repeat the  lens-cap-on exposure. This time the frame should come out black. From there, you can selectively uncover parts of the setup to narrow down where the leak is occurring.

Also, see this link for more about light leaks during long exposures:
Tips & Tricks - Light Leakage/
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