photo concerns!

It seems seems I have "burned" out pixels. Today I took a two, two minute exposure and all of a sudden I'm seeing little dots on the image when I zoom in allot. I can't recal them being there when I have taken long exposures What could be occuring? I haven't used the camera in a few weeks because of school and finals and now I go out to relax after finals and I find this! This is of a pitch dark room to bring them out. They're not noticable on short shots.
pixel.jpg

Have these been here before but have I just not noticed them. I have taken lots of night pics before though. Anyone have this problem, do I have camera going bad???
 
not freaking out as much... it seems to be an effect of the long exposure function on the camera. It's usually suppose to help with excess noise but I guess it has it's on issues. Well, atleast I can go to bed now. Atleast this is a message/warning about the long explosure setting on the D70.
 
Robert, I had this very same problem after I bought my canon 300d and took a few extended exposures. These spots are referred to as "hot pixels". Doing a web search on that term will bring up all the technical jargon/info related to the it. In short, basically all digital cameras suffer from this to varying degrees. (EDITED OUT THIS LINE BECAUSE I STATED IT WRONG---SEE BELOW) The only real solution is the clone out those spots using photoshop or some other similar program in your post edits. OR, don't take extended exposures.......lol which is out of the question for lightning shots.

Do the web search on on hot pixels. There's a few topics which explain how to test your camera for excessive hot pixels, and what should be the norm for a given time etc. (some of which is subjective I might add) Obviously the longer the exposure the more noticeable it will be. But if you're seeing a bunch in just a few seconds exposure (in total blackness) you might want to have it checked out. From the pic you provided, it doesn't look any worse than what I'm getting on mine however, so I don't think its anything to loose sleep over. Certainly had me worried until I found out what it was....then I was just annoyed about it :blink:

Just one of the downsides to digital I guess. The upside to digital still rocks though IMO.


Hope that helps some

Heres a brief description about what hot pixels are from: http://webpages.charter.net/bbiggers/DCExp...hot_pixels.html

What are hot pixels:

Hot pixels are individual sensors on the CCD with higher than normal rates of charge leakage. They can appear as small pixel sized bright points of light on longer exposures. Every pixel on the CCD has some charge leakage, and if you expose long enough, any pixel would light up. On a long exposure, you will see pixels ranging from just barely visible to possibly bright hot starlike points. There might be a few bright hot pixels, more intermediate one, and lots of very faint ones, an entire spectrum of brightness. At the low end, the faint hot pixels contribute to the noise in a picture. All cameras on the market today have "hot pixels"
 
Some cameras (don't know about Nikon) have long exposure noise reductions functions... you may want to see if it has that. I know on my 20D that exposures at ISO100 @ 4min don't have nearly that many hot pixels. Noise is also a function of ISO (more as you increase) and temperature (more as it becomes warmer) The cameras you see on telescopes often have chilled CCDs to prevent noise.

Aaron
 
Some cameras (don't know about Nikon) have long exposure noise reductions functions... you may want to see if it has that. I know on my 20D that exposures at ISO100 @ 4min don't have nearly that many hot pixels. Noise is also a function of ISO (more as you increase) and temperature (more as it becomes warmer) The cameras you see on telescopes often have chilled CCDs to prevent noise.

Aaron
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It seems like the noise reduction function is what caused it. Why, don't know. I turned it off and the camera went back to normal. I'll try again tonight to see if that's still true.
 
My canon 300D always had them, at least that bad on longer exposures. It's pretty normal I think. Photoshop has a function that helps with them if you don't want to fix them by hand. I'm not home and don't have photoshop here but I think it is Filter/noise/dust and scratches....or something like that. I'll have to look later. But anyway, some settings on that really work well and makes most of them vanish. If there are stars you want to keep it will tend to soften those or make them go away. Fixing them by hand with the clone tool sure gets old fast when you have lots of them.
 
I've been thinking about ways to use Photoshop to get rid of hot spots with some type of action that could be applied to any image quickly and easily. Here's what I came up with: With the lens cap on, take a shot with the exposure time you normally use that gives you the hot spots. The idea is to get a reference photo of all of them with absolutely no real light information. After that, use the Desaturate command (Shift+Control+U) to change it to black and white. This should basically give you a drop dead accurate mask for all of your hot spots, which could presumably be used with any image where they are a problem. If any Photoshop users out there have a big interest in this, but aren't sure exactly how to do it, I could probably whip up an automated droplet that does it on relatively short order.
 
Some cameras (don't know about Nikon) have long exposure noise reductions functions... you may want to see if it has that. I know on my 20D that exposures at ISO100 @ 4min don't have nearly that many hot pixels. Noise is also a function of ISO (more as you increase) and temperature (more as it becomes warmer) The cameras you see on telescopes often have chilled CCDs to prevent noise.

Aaron
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Yup, the 20D compensates for this by taking a second photo with the shutter closed and then subtracting the hot pixels out of the original long-exposure. The downside is that this doubles the exposure time -- not a big deal for a 10 second exposure, but very much not cool if you just did a 1 hour exposure of the stars!
 
Yup, the 20D compensates for this by taking a second photo with the shutter closed and then subtracting the hot pixels out of the original long-exposure. The downside is that this doubles the exposure time -- not a big deal for a 10 second exposure, but very much not cool if you just did a 1 hour exposure of the stars!
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The D70 has this feature as well, and it works in the same way. I've done some of those hour-long exposures with noise reduction on, and the battery actually lasted and I got the shot, but it takes FOREVER. I'd turn this off if I were chasing in low light under the base of a gigantic meso; I'd prefer getting twice as many lightning shots with a little grain that I can remove later in PSP to missing half of the good ones (or maybe all the good ones!).
 
The bad news is that just about all commercial cameras have hot pixels. (This is the sort of thing that never seems to get much coverage in the "Newer!" "Better!!" hype driven media...) Furthermore, the sensor will only get worse as it ages. My 5 yr old 3MP Oly has dozens of 'warm' pixels, and shows around 10 full-tilt pink (red), green, or blue spots on anything but the shortest exposures. I don't use the camera very often, so it usually has a few new spots to show off each time I drag it out. Your higher quality DSLR sensor will also degrade, but probably not nearly as quickly.

The good news is that that are several freeware dark frame subtraction programs floating around. You can also use any layer capable editing program to easily remove the hot pixels.

-Greg
 
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