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colour problem
- Thread starter biglou
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Tom Mann
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Set the histogram in CS5 to display each of the three color channels separately. I'll bet you several (figurative) steak dinners that you will see the red channel is blown in each of them. Select the signs and the red histogram will look even worse.
Not withstanding the possibility of recovering some of the blown highlights, the only real way to fix this problem is to cut the camera's exposure until the red channel isn't blown.
HTH,
Tom M
Not withstanding the possibility of recovering some of the blown highlights, the only real way to fix this problem is to cut the camera's exposure until the red channel isn't blown.
HTH,
Tom M
hi tom
i dont really understand what you are saying the histogram looks ok the picture looks fine in the raw interface its only when i open it in cs5 the colour changes but not all reds just certain signs . i cant recall ever seeing this before looking forward to hearing from you .
lou
i dont really understand what you are saying the histogram looks ok the picture looks fine in the raw interface its only when i open it in cs5 the colour changes but not all reds just certain signs . i cant recall ever seeing this before looking forward to hearing from you .
lou
Tom Mann
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If you look carefully, my guess is that there will be a narrow spike at the extreme RHS of the red histogram for your image. I've attached an example: a capture of the ACR screen for an image of neon lights I took using a little point and shoot camera. The sharp spike in the area circled in green in the upper right corner is what I suspect you will also see in your image - ie, pixels in which the red channel is maxed out.
In the case of this example image, the spike comes from the pixels representing the cores of the long, thin, tubular neon lights (and similar areas). For example, look at the horizontal neon tubes just above and below the word, "Casino". The core of the neon tube is yellow, but as you get nearer the edges of these neon tubes the pixels are red, as is the glow these neon tubes cast on the surrounding areas. This shows that the true color emitted by these tubes is red, and the yellow core is an artifact of the photography.
In fact, this is a very mild example of "blown red channel". I anticipated the problem and manually reduced the exposure considerably for this shot. If you let the camera determine the exposure, because of the large dark areas and high contrast, the camera will almost certainly overexpose these areas.
This problem is especially common around Halloween when people try to take pictures of pumpkins lit from the inside by candles. They tend to crank up the exposure to allow the surroundings to be seen, but this blows out the red channel where the inside of the pumpkin can be seen. This problem also occurs frequently with highly saturated red flowers going either yellow or magenta, and with over-exposed blue skies turning cyan.
There's lots written on the web about the causes and fixes for this problem. Google {"blown red channel"}.
HTH,
Tom M
PS - Just to make sure I'm not full of hot air
, it would be very helpful if you could post a representative image.
In the case of this example image, the spike comes from the pixels representing the cores of the long, thin, tubular neon lights (and similar areas). For example, look at the horizontal neon tubes just above and below the word, "Casino". The core of the neon tube is yellow, but as you get nearer the edges of these neon tubes the pixels are red, as is the glow these neon tubes cast on the surrounding areas. This shows that the true color emitted by these tubes is red, and the yellow core is an artifact of the photography.
In fact, this is a very mild example of "blown red channel". I anticipated the problem and manually reduced the exposure considerably for this shot. If you let the camera determine the exposure, because of the large dark areas and high contrast, the camera will almost certainly overexpose these areas.
This problem is especially common around Halloween when people try to take pictures of pumpkins lit from the inside by candles. They tend to crank up the exposure to allow the surroundings to be seen, but this blows out the red channel where the inside of the pumpkin can be seen. This problem also occurs frequently with highly saturated red flowers going either yellow or magenta, and with over-exposed blue skies turning cyan.
There's lots written on the web about the causes and fixes for this problem. Google {"blown red channel"}.
HTH,
Tom M
PS - Just to make sure I'm not full of hot air
, it would be very helpful if you could post a representative image.Attachments
hi tom
i now understand, what i was seeing in raw interface was in fact the clipping warning for blown highlights making the signs look red .i will send you the photo ,ballys flamingo and ceasars palace should be red
many thanks for your time and knowledge
lou
i now understand, what i was seeing in raw interface was in fact the clipping warning for blown highlights making the signs look red .i will send you the photo ,ballys flamingo and ceasars palace should be red
many thanks for your time and knowledge
lou
Tom Mann
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Hi Lou -
You are quite welcome.
Yup, its exactly what I thought. As soon as you mentioned the words, "neon sign", I knew the chances of the problem being due to a blown red channel were extremely high.
BTW, it's a lot easier to see what's going on if you do a crop in ACR / LR (centered on one of the signs), and look at the histogram of the cropped version -- see attached. A histogram of the entire image hardly gives any indication of the problem because the number of pixels with the red channel blown is a very small fraction of the total number of pixels in the image, so the size of the red spike at the extreme RHS of the histogram will be very small. However, once you crop in, the fraction of bad pixels increases significantly and it's very easy to see the red spike on the RHS of the histogram.
Avoiding this problem with neon signs at night can be difficult. The obvious solution is to decrease the exposure by a few stops to that the brightest pixels don't have blown channels. The problem with this is that when you go to brighten up the shadow areas to get a nice looking image, you bring up all sorts of noise out of the mud.
By far, the best approach is to shoot earlier in the evening when there is a bit of ambient sky light still around. This reduces the contrast ratio to a level that the camera can handle.
Another approach is to try HDR techniques with a large number of narrowly spaced (ie, 1 stop) exposures that cover an extremely wide range (say, 10 stops). Done with care, restraint and previous HDR experience, this can look quite good. However, if you space the component exposures further apart or don't cover a wide enough range, this approach can look worse than the single exposure approach.
I hope this helped.
Sincerely,
Tom M
You are quite welcome.
Yup, its exactly what I thought. As soon as you mentioned the words, "neon sign", I knew the chances of the problem being due to a blown red channel were extremely high.
BTW, it's a lot easier to see what's going on if you do a crop in ACR / LR (centered on one of the signs), and look at the histogram of the cropped version -- see attached. A histogram of the entire image hardly gives any indication of the problem because the number of pixels with the red channel blown is a very small fraction of the total number of pixels in the image, so the size of the red spike at the extreme RHS of the histogram will be very small. However, once you crop in, the fraction of bad pixels increases significantly and it's very easy to see the red spike on the RHS of the histogram.
Avoiding this problem with neon signs at night can be difficult. The obvious solution is to decrease the exposure by a few stops to that the brightest pixels don't have blown channels. The problem with this is that when you go to brighten up the shadow areas to get a nice looking image, you bring up all sorts of noise out of the mud.
By far, the best approach is to shoot earlier in the evening when there is a bit of ambient sky light still around. This reduces the contrast ratio to a level that the camera can handle.
Another approach is to try HDR techniques with a large number of narrowly spaced (ie, 1 stop) exposures that cover an extremely wide range (say, 10 stops). Done with care, restraint and previous HDR experience, this can look quite good. However, if you space the component exposures further apart or don't cover a wide enough range, this approach can look worse than the single exposure approach.
I hope this helped.
Sincerely,
Tom M
