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09/20/2002 04:57:46 PM · #1 |
I was trying to take some low light pictures last night and discovered many pixel hot spots or burn out, what ever you call them. I know there was a thread about that a while back but I can't find it.
Is this fixable? I have a Nikon Coolpix 775. |
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09/20/2002 05:30:37 PM · #2 |
Originally posted by Frank Beckman:
I was trying to take some low light pictures last night and discovered many pixel hot spots or burn out, what ever you call them. I know there was a thread about that a while back but I can't find it.
Is this fixable? I have a Nikon Coolpix 775.
Frank, I don't know if this is the problem that you are having, specifically, but: a couple of challenges back, I was taking low light pics and discovered that I got a lot of pixel spots when the camera was at certain "angles" to the subject/lighting. I adjusted the height of the tripod/angle of camera, and the pixel spots went away.
Linda
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09/20/2002 06:01:32 PM · #3 |
Originally posted by lhall:
Originally posted by Frank Beckman:
[i]I was trying to take some low light pictures last night and discovered many pixel hot spots or burn out, what ever you call them. I know there was a thread about that a while back but I can't find it.
Is this fixable? I have a Nikon Coolpix 775.
Frank, I don't know if this is the problem that you are having, specifically, but: a couple of challenges back, I was taking low light pics and discovered that I got a lot of pixel spots when the camera was at certain "angles" to the subject/lighting. I adjusted the height of the tripod/angle of camera, and the pixel spots went away.
Linda
[/i]
Mine show up on every shot, same place, every picture in the dark areas.
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09/20/2002 06:32:41 PM · #4 |
I get that junk, but only under certain circumstances. Near total dark and long exposures. If I increase the light or decrease the exposure legnth, it helps. I am pretty certain this is a limitation of the camera technology. Some manufacturers do better than others.
The pixel issue that is a "problem" is when you can see "bad" pixels in every shot, all lighting conditions. This would be more indicitive of a dead pixel(s). The only around this is to have the CDD replaced at nearly the cost of the camera, unless under warranty.
Does that help or hurt?
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09/20/2002 08:50:35 PM · #5 |
Originally posted by Swashbuckler:
Does that help or hurt?
[/i]
That helps, I only get the little red and green dots in low light against something black, I think I will call Nikon anyway because it about out of warranty, that is if it has a year warrenty.
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09/20/2002 09:19:02 PM · #6 |
There is a technique/tutorial addressing this issue. I found it via this sites tutorial links, but I can't remember which one. I'll try to find it again and post it here...
JD Anderson
Here is the link...
//www.dpreview.com/learn/Image_Techniques/Night_Spots_01.htm
This is a way to correct without any service to the camera...
* This message has been edited by the author on 9/20/2002 9:18:45 PM. |
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09/20/2002 10:38:07 PM · #7 |
| If your camera has a "noise reduction" feature, make sure you turn that on. And also use a low ISO 100, anything higher usually produces hot pixels in low light digital photos. |
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09/21/2002 02:37:35 AM · #8 |
You have either what's known as a "stuck" pixel, or a "hot" pixel.
Your camera contains an array of thousands of sensitive photo sensors (the CCD) that are responsible for actually capturing the light that enters the iris of the camera, and converting that light into a digital image.
Some sensors are designed to capture blue light, some are designed to capture red light, and the remainder are designed to capture green light. The light values these sensors read are then combined to form the pixels that comprise your final digital image.
In a perfect world, each sensor would be exactly as sensitive to light as all of its neighbors. Unfortunately, this is not a perfect world and so you have some sensors that are much more sensitive than others.
A "hot" pixel is when you have a sensor that is significantly more sensitive to light than is normal. What this means is that this one sensor will report there to be more colored light than there really is at its location (red if the sensor is a red sensor, green if the sensor is a green sensor, etc.) The end result is a pixel (or two, always in the same location) whose color is slanted more towards the color the sensor was capturing: either red, blue, or green.
With short-exposure times and large apertures, the color shift of the pixel may not even be noticed. However, the longer your exposure, the narrower your aperture, or the dimmer the lighting, the more color shift you will see in that pixel. In many exposure settings, this pixel will actually be seens as solid color of either red, blue, or green.
A "stuck" pixel is an extreme example of a hot pixel-- it is caused by a sensor that is so sensitive (or outright faulty) that even in no-light conditions it will report having received colored light. Hence, you will see the symptom of a stuck pixel-- a solid pixel of red, green, or blue-- in every photo you take.
Depending upon on how many bad/hot pixels you have, some manufacturers will replace your CCD. However, in most cases they will likely tell you that hot and stuck pixels are par for the digital photography course. In some cases they may be able update the camera's software to have it ignore the stuck/hot pixel so that you will no longer see the speck of solid color in all of your images.
In any case, you can easily remove most remnants of hot and stuck pixels by running the DPChallenge-legal Photoshop Despeckle filter as the first step you do when post-processing your image.
For non-DPChallenge photos, an even better way to remove hot and stuck pixels is to use Photoshops Noise > Median filter with a radius setting of 1. There is also a very effective means of reducing all sorts of CCD noise from your final image by taking a "dark" shot: you take a photo with your lens cap on using the same exposure settings as your final image was, and you use this photo to "subtract" out the areas of noise on your final image. There are plenty of tutorials out there on how to do this that do a better job than I could, but it relatively simple and quite effective.
One final tip for reducing the effect of stuck/hot pixels is to use as little JPEG compression as possible when you are taking your photos. The more compression you use, the more likely that the single solid red, green, or blue pixel has bled into five or six neighboring pixels. |
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09/21/2002 03:07:44 AM · #9 |
| Thanks James for the information above. THis is very useful.I tried using the median filter with radius 1 but the compromize on the sharpness of the image is too much. I will try the digital subtraction technique and get back.Al the nize reduction filters some how soften the image and even by increasing the sharpness later, I am unable to get a result (sharpness) as good as the original image. Further increasing the sharpness blows out the highlights. How do you handle that? |
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09/24/2002 01:55:01 AM · #10 |
Sohr was right on the money.
There's no cure for these. You can treat the symptoms though.
If you shoot in JPEG the bad pixel has already propogated to nearby pixels because of the compression used. You can't even recover them completely, but you can usually use the clone tool to remove any visible trace.
If you shoot in RAW or TIFF you can open the file pre-compression and usually the stuck pixel is a single pixel only, instead of larger defect as in JPEGs. Fix the single pixel, usually only on a single channel (R,G, or B) too, and then save and work with the file.
I don't know which cameras have firmware upgrades to block these pixels, but I do know the high-end Canon line (anything that shoots RAW) has software floating around on the net to map these out before they even interpolate to the final image, thus making the smallest impact on the final image. This (aside from the firmware) would also be the easiest way. Stuck pixels are always in the same spot so it's inherently scriptable.
Unfortunately, most of these projects are in the early alpha stage and only have source available (though they seem to build correctly) and aren't for the non-programmer, yet. I'm actually working on an automated hot-pixel remover but it'll be a while before it's to where just anyone can use it. (That's my goal though, my grandmother just bought a G2 and I want it to eventually be where she can use it.)
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09/24/2002 06:08:17 AM · #11 |
Originally posted by sanandan:
Thanks James for the information above. THis is very useful.I tried using the median filter with radius 1 but the compromize on the sharpness of the image is too much. I will try the digital subtraction technique and get back.Al the nize reduction filters some how soften the image and even by increasing the sharpness later, I am unable to get a result (sharpness) as good as the original image. Further increasing the sharpness blows out the highlights. How do you handle that?
Sanandan - Your observations are correct. Anytime you make use of a filter, or more specifically, a noise reduction filter, you definitely impair the integrity of the image. This is simply the characteristic of these sorts of processes that transpire when filtering an image. The subtraction technique may yield a better result for you. I use a program called Genuine Fractals on some of my JPEG images to try to recoup lost image details. Then I try the processes mentioned. By the time I print the image it is often much more usable because of this software. Spot editing is a very good solution to this problem too. However, some of these ideas may or do disobey the DP Challenge rules.
So, the best recommendation is if you are using a D1x as I am, then go back to Nikon to see how they can help you. I have two D1x cameras and Nikon's (in Canada) support has been superb to date. I also understand that these cameras do have some pixel correction firmware that should be useful in reducing this problem. I wonder if it can be configured to deal with the specific pixels that you are having trouble with in your D1x camera.
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09/24/2002 09:11:40 AM · #12 |
Originally posted by WNight:
Sohr was right on the money.
There's no cure for these. You can treat the symptoms though.
If you shoot in JPEG the bad pixel has already propogated to nearby pixels because of the compression used. You can't even recover them completely, but you can usually use the clone tool to remove any visible trace.
If you shoot in RAW or TIFF you can open the file pre-compression and usually the stuck pixel is a single pixel only, instead of larger defect as in JPEGs. Fix the single pixel, usually only on a single channel (R,G, or B) too, and then save and work with the file.
I don't know which cameras have firmware upgrades to block these pixels, but I do know the high-end Canon line (anything that shoots RAW) has software floating around on the net to map these out before they even interpolate to the final image, thus making the smallest impact on the final image. This (aside from the firmware) would also be the easiest way. Stuck pixels are always in the same spot so it's inherently scriptable.
Unfortunately, most of these projects are in the early alpha stage and only have source available (though they seem to build correctly) and aren't for the non-programmer, yet. I'm actually working on an automated hot-pixel remover but it'll be a while before it's to where just anyone can use it. (That's my goal though, my grandmother just bought a G2 and I want it to eventually be where she can use it.)
For the G2, the noise reduction on anything over a 1 second shot is
pretty good - for every frame you take, it takes an equivalent 'dark
frame' of the same exposure time, straight after the shot, and uses
this to subract any sensor noise from the image - works pretty well.
The advantage to doing it at the same time is that the sensor noise
varies with the temperature of the camera/ sensor so it isn't something
you can accurately do once and use repeatedly.
The simplest way to fix these kinds of stuck or hot pixels if you only
have a few, and they only appear on certain types of long exposure shots,
is to use the clone brush with a small brush size and clone them out.
If you have clumps of stuck pixels, and they show up in 'normal' shots
under your typical usage of the camera (depending on how you use it)
then it may be worth talking to the manufacturer about a replacement.
But _all_ consumer level digital cameras will have some amount of stuck/
hot pixels. You will not get a perfect sensor for your current camera,
no matter how many times you replace it. Its the same for LCD screens,
there are always a small number of defects/ inactive pixels on those too.
You can drive yourself mad trying to get a perfect version, but it is
better to just learn how to deal with it and move on, as long as it
doesn't impact a lot of the pictures you take (which is only something
you can decide)
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09/24/2002 10:54:12 AM · #13 |
The loss of sharpness generally isn't an issue for me when using the Median filter or the Despeckle Filter... in fact, I had some hot pixels in my current submission that I removed with the Despeckle filter and you probably wouldn't ever know if I hadn't told you.
The keys to ensure this works well (DPChallenge-wise) is:
1) Setting your JPEG compression to the highest-quality setting as possible (you don't need TIFF mode, or even HI-QUALITY mode on a Nikon; a Normal quality should be sufficient)
2) Make sure you're using your camera's entire CCD (i.e. leave the camera's Image Size setting at Normal-- not VGA, SVGA, 3:2, or anything else).
3) Run the despeckle filter on the image as your very first post-processing step.
4) You will notice your image may have lost a wee bit of sharpness, but that's OK... you'll get that back later when you resize, resample, and sharpen. If you camera is at least a 2 megapixel camera, you'll be just fine. For now, just do any color correction (levels, curves, etc).
5) Do any cropping you want, ensuring you are maintaining a legal ratio (640 by 480, 480 by 640, etc.)
6) Now resample and resize your image using the Resize Image command... set your final image dimensions to be a legal dimension size, and choose 'Bicubic' as your sampling method used.
7) Sharpen your image using the Unsharp Mask filter.
Like I said, assuming you started with a large enough image you'll never know that there was ever a hot pixel there, nor will you notice any lack of sharpness.
The same method works quite well for the 1-pixel despeckle filter for non-DPChallenge photos. An even better bethod is the black frame subtraction method.
I even have some actions (not DPChallenge legal, though) that automate the clean up process for various Nikon digital cameras-- including noise removal, hot/stuck pixel adjustments, etc. Let me know if you'd like a copy for your particular camera. Fred Miranda has wonderful collection of photoshop actions for various digital cameras-- but he does charge a small amount for them, and they are not DPChallenge legal. |
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