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Old Jul 20, 2007, 10:08 AM   #1
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On the 4th of July I took some pics of fireworks with my s9100 using a long shutter(4-15seconds). When I uploaded the pictures I noticed tiny white dots everywhere:


This was on all the pictures no matter the setting. Last night I tried to take pictures of lightning with long shutter, and had the same problem. I changed the ISO, the WB, the Shutter speed, Apeture, image size, RAW vs JPEG....nothing seemed to get rid of them. There are definietley less with shorter shutter speeds and a wide zoom, but they are still there. They aren't there, or visible, for daylight shots, or when the flash is used. Is my sensor broken, or am I messing something up?

- jason
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Old Jul 20, 2007, 12:20 PM   #2
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I think these are hot pixels that are becoming evident on the long exposures because the CCD is getting hotter during the long exposure. Is it normal? I don't know. It seems to me that there are more of them than usual. But then I haven't done any long exposures to find out.
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Old Jul 21, 2007, 1:09 AM   #3
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i have a s9500 only thing i can say is that i get a samll cluster of hot pixels on long exposure but by long i mean over 25sec if i was getting hot spots at just over 5 sec i would be getting in touch with fuji
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Old Jul 21, 2007, 8:22 AM   #4
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It's not uncommon to see hundreds of hot pixels on some newer camera models with exposures much longer than a second or two with noise reduction turned off, and it gets worse from there as exposures get longer. It tends to be more problematic as photosite sizes get smaller (higher resolution within the same size sensor will mean that each photosite must be smaller in order to fit more in). Higher resolution is not always better. ;-)

Many newer cameras have a built in dark frame subtraction noise reduction system now (that's the purpose of the noise reduction menu choice you'll see in these camera models). Some models engage it automatically on longer exposures (you can't turn it off), and some models don't have the feature (which is the niche I think your Fuji falls into).

It works by taking two photos when exposures are longer than about 1 second. The second (shutter closed) photo is designed to simulate a lens cap on exposure, using the same settings for shutter speed and ISO speed as the first exposure. So, if you take a 30 second shot, the camera needs another 30 seconds to take a dark frame shot.

The reason it works that way is because since both photos are taken at approximately the same time, at the same temperature (which will impact noise and hot pixels), using the same settings, the hot pixels are likely to show up in the same place in both images.

Then, the camera's processing maps out the same pixel locations in the actual exposure that it found hot pixels in the dark frame exposure (since these are the only pixels that would be much brighter in the dark frame). Basically, it replaces these pixel locations in the actual exposure with values interpolated from adjacent pixels so that you don't see them anymore. This process is know as dark frame subtraction.

But, I do not believe that your Fuji model has this feature.

So, you'd need to do it yourself (take a lens cap on exposure right after you take the actual exposure, using the same settings as the actual exposure, and use software to perform the dark frame subtraction). There are a number of ways to use a dark frame with an editor to help out with this process.

What ISO speed was that photo taken at?

It looks like you used PS 7 to edit/downsize it, and it stripped out the EXIF information (use save as versus save for web if you want to keep it).

If you're going to take night photos like that, set your ISO speed to the lowest value the camera supports.

Even though the exposure will be longer that way for a given night scene (and fireworks shouldn't need very long exposures), you'll have far fewer problems with hot pixels and noise. Of course, you'll want to use a tripod unless you want blurry photos.

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Old Jul 21, 2007, 8:41 AM   #5
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Here is an article on Dark Frame Subtraction using Photoshop, and the author also has some info on programs designed to do it for you:

Dark Frame Subtraction using Photoshop

P.S.

In addition to the techniques show in the above article to peform the subtraction yourself (and the programs the author mentioned that do it for you), here is more software you can use to help out. Chances are, the newer software mentioned in the above article would do a better job. But, you may want to give this one a try also.

http://www.tawbaware.com/pixelzap.htm

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Old Jul 21, 2007, 12:26 PM   #6
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Thanks for the replies and the great info. I had never heard of "hot pixels" before, but after a little research, I think that is exactly what is going on here. All the pics I took that night had these hot pixels, but that one was the worst, and it was the last one after nearly an hour of taking pictures. Plus it's Arizona and it was probably about 85-90 degrees outside. I took some pics with the lens cap on and it looks like I definetley have some dead pixels, but the hot pixels progressively fade with faster shutter speeds. I guess I'm going to have accept the fact that this camera is not a strong slow shutter performer...disappointing .

JimC, the info for this photo is:
ISO: 80
Shutter: 8sec
Aperture: F/11

Thanks Again

-jason
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Old Jul 21, 2007, 1:04 PM   #7
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Not dead, hot.

Hot pixels are when photosites record a higher value (brighter) than they should, usually with longer shutter speeds. When the problem occurs at faster shutter speeds, they're considered "stuck" (always bright).

Virtually all CCDs will have hot pixels in some conditions. The longer the shutter is open (and the darker the conditions since not enough photons are hitting the sensor) the more likely a photosite won't be linear in it's output when responding to light, and it will record a higher value than it should.

Temperature also impacts it (which is why you see cooling systems for sensors in digital cameras used in astronomy). So, the longer you test for hot pixels, the more you'll likely have, especially without any light hitting the sensor, since the camera will warm up while you're testing for them.

You just don't see hot pixels with most newer cameras because of built in noise reduction to subtract them out on longer exposures, since the ones that occur at typical shutter speeds have already been mapped out at the factory. But, it does not appear that your Fuji has a dark frame subtraction system to remove them (many newer cameras do).

If you have a pixel that is always hot (bright), even on shorter exposures in good light, then it's referred to as a "Stuck" pixel.

If you have a pixel that is not responding to light at all (always dark, regardless of lighting or shutter speed), it's referred to as a "Dead Pixel". That's the opposite of your issue.

When a camera develops a stuck or dead pixel (either always bright or always dark, regardless of lighting or shutter speed), a trip back to the manufacturer is needed. The manufacturer then updates a bad pixel table in EEPROM. When a photo is taken the image processing automatically checks the camera's bad pixel table and maps the pixel out (replaces it with values determined from adjacent pixels using sophisticated interpolation algorithms).

Some cameras allow the user to perform a bad pixel map themselves (a number of Olympus models have a menu choice that calls a procedure to locate and map out bad pixels).

KM DSLR models have an undocumented feature that checks and maps out bad pixels on a monthly basis that you can force by setting the date up one month if you don't want to wait after you see a bad one.

I've got software that can remap hot pixels for a number of Nikon and Olympus non-DSLR models. I've also got software that can update the bad pixel table in the D100 (one of Nikon's older DSLR models).

But, note that these remap utilities are *not* designed to remove hot pixels that occur on longer exposures (they're normal, and the dark frame subtraction noise reductions systems on cameras with that feature should get most of them if turned on). With your model, you'd need to use software to do it yourself.

IOW, hot pixels on longer exposures is not indicative of a sensor problem and you don't want them being replaced with values from other pixels (as a factory remap using the bad pixel table in EEPROM would do) when they are only hot on longer exposures.



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Old Jul 21, 2007, 5:39 PM   #8
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Jason, you didn't mention if your photo was cropped or not. I get the same effect with my S9100 at long exposure/high ISO but it's notreally visibleuntil I crop or zoom in. Even then, it doesn't seem as noticeable as in your picture. This shot is a 30-second exposure, ISO 200, 100% crop.
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Old Jul 22, 2007, 12:53 PM   #9
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toshi, that was cropped from the image at 100%.
Here is the original image:


Thanks for posting that picture, I can see that hot pixels are evident in your long exposure pic, as well. So that makes me feel better knowing that I my camera is probably not broken.

-jason
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