Why are there dots with long exposure?

[Cliffy]

Hello, I have taken a 30sec pic with my new E-500, but there are lots of dots...

full resolution crop of the top left


is it normal?

[tao]

1. Have you read the user manual yet? Please do or you will not understand what I typed below.
2. Do a pixel mapping first
3. Enable NR when doing such long exposures

[drumma]

woah.. i'm hoping those AREN'T dead pixels..

[RtOaNn]

dun think so la..since it's cropped from the pic..
otherwise jialat for us liao....& him..

[Expiredyoghurt]

iirc dead pixels go BLACK ... stuck pixels can be any color ... but honestly i think it's just sensor noise due to the long exposure ...

[OlyFlyer]

Originally Posted by tao 1. Have you read the user manual yet? Please do or you will not understand what I typed below. 2. Do a pixel mapping first 3. Enable NR when doing such long exposures

Why pixel mapping? Isn't that only to kill dead pixels? To me it looks like just noise. Anyway, I usually just enable NR to get rid of these ugly dots.

[u2nofear]

Activate Noise reduction and low ISO

[Cliffy]

OH WOW!!! The noise reduction works wonders!!!!

[u2nofear]

Originally Posted by Cliffy OH WOW!!! The noise reduction works wonders!!!!

remember to turn off noise reduction; if you want to take multiple/burst shoot

[Hacker]

Originally Posted by Cliffy OH WOW!!! The noise reduction works wonders!!!!

Post post post!

[OlyFlyer]

Originally Posted by Cliffy OH WOW!!! The noise reduction works wonders!!!!

Don't forget, it takes time. And I mean TIME. First time I took a 5-minute shot the red LED indicating buffer flushing was on and flashing for several minutes. I thought I broke my camera with that long shot. But no, this is normal. NR tends to take very long time some times, but works wonders.

[zaxh81]

in the first place,why was it so?
My new cam also no give me such problem with NR turn off.

[zaxh81]

Originally Posted by tao 1. Have you read the user manual yet? Please do or you will not understand what I typed below. 2. Do a pixel mapping first 3. Enable NR when doing such long exposures

So meaning when taking firework have to enable NR?
If take 8 sec exposure, am i right to say must wait another approximate 8 sec before the image is written to the card?

[OlyFlyer]

Originally Posted by zaxh81 So meaning when taking firework have to enable NR? If take 8 sec exposure, am i right to say must wait another approximate 8 sec before the image is written to the card?

First, I have to say that I ony take in RAW format, so if you take JPEG than I don't know how it is.

Noise processing takes time, specially if it has to be done well. I think it takes more time when the explosure is longer since the longer the explosure the more noise you get. Also, I don't see much noise on my few seconds pictures, no noticable increase (at least it is not disturbing) in buffer flush time, but in a several minute long explosure the time is significantly longer (i.e. alarming if you don't expect it). The only way to stop NR from working if you get alarmed is to turn off the camera. Then it flushes at once, but NR is not completed in that case.

[OlyFlyer]

Originally Posted by zaxh81 in the first place,why was it so?

It is normal, if you increase ISO sensitivity noise will also increase. That was always the case, even with film cameras. Only there, it is called grain. That is one part. The other is that due to the fact that the longer the CCD is on and exposed for light the more sensitive it is. Not all pixels are exactly and equally sensitive to all compared to each other. Remember, there are almost 9 million individual pixels on an E-500 CCD. All these are tiny individuals, about a million of these are not used since the CCD has 8 million actually used pixels only. Still, a lot of individuals, not all of them behaving equally, not equally sensitive to the same wavelength.

Some links for more info about noise:
http://www.wrotniak.net/photo/tech/noise.html
http://micro.magnet.fsu.edu/primer/d...ts/ccdsnr.html
http://www.astrophys-assist.com/educate/noise/noise.htm

Originally Posted by zaxh81 My new cam also no give me such problem with NR turn off.

I guess that depends a lot on the format you look at the pictures. Are you shoure that looking at a nightshot at 100% with highest resolution and no NR would not give you some noise? In that case your camera is unik I would say. I think there are people who speak about noise and there are people who eigther don't recognize, don't see or don't admit that there is noise. I mean, I did take 8 sec pictures with only 2 'noise' pixels (checked manually) that without NR, but also took pictures like Cliffys without NR. It also depends on the number of pictures you take after each other. The more you take the warmer the CCD get and therefore the more noise you get. Maybe even the ambient temperature has an effect, definitly high temperatures will have effect. I also read that switching off the LCD helps also to reduce the effects of heat, which is actually logical. Generally, I think the less power you use the less heat will be generated around the camera's CCD, that includes also the batteries. The more power you use the warmer the battery will be. Cameras with full metal bodies have an advantage compared to E-500 which has only a metal frame under the plastic.

Originally Posted by zaxh81 So meaning when taking firework have to enable NR?

I would not worry about taking pictures at fireworks, but yes, you should have NR on and expect a longer than ususal buffer flush time.

[A.T.]

I can't remember the term. Hot spots, I think. Most digital cameras' image chip is prone to this and it's more visible when long exposure is used. When the chip ages, more will appear. There's no repair. Manufacturers brush it as noise and that's way NR comes in to clear it. Simply put it, it's normal.

[A.T.]

Ah... it's called Hot Pixels.

Have a read on this article.

[plastic]

There is something that you guys should take note in "stickies"...

[wind30]

to put it more simply, there are two kind of noise

1) hot pixels casued by long exposure which is what the thread starter is seeing. This noise are NOT random and can be removed easily by taking a second shot (dark frame) with the same exposure setting, which is why the NR takes as long as the first exposure. This is the reason why it takes so long. It is not the camera doing NR, but rather it is taking another shot with the same exposure setting.

2) random noise caused by high ISO settings. This noise are random and the only way to remove them is by noise reduction which inevitable removes details. This kind of NR is very fast.

[OlyFlyer]

Originally Posted by wind30 to put it more simply, there are two kind of noise 1) hot pixels casued by long exposure which is what the thread starter is seeing. This noise are NOT random and can be removed easily by taking a second shot (dark frame) with the same exposure setting, which is why the NR takes as long as the first exposure. This is the reason why it takes so long. It is not the camera doing NR, but rather it is taking another shot with the same exposure setting. 2) random noise caused by high ISO settings. This noise are random and the only way to remove them is by noise reduction which inevitable removes details. This kind of NR is very fast.

This is not completly right. Hot pixels are indeed random. The reason is just as the name says is the heat. When CCD exposed to light it is heated up due to electric current in the CCD. The longer the exposure the more hot pixels you get. But, it has to do with heat generated around the CCD also. That is also random. The reason for NR to take time is not that the camera does an other exposure with the same time, but because the camera does an image processing, calculating which pixel is noise, which is not and how to correct that. It does that for each pixel, so taking a RAW, takes a lot of calculation. Maybe in JPEG also, I havn't tested that. This can be seen if the red LED is observed, it is flashing for a very long time after a long exposure, still only one image is taken. It is however possible to calculate which pixels are hot, based on some advanced mathematical statistics. Taking a black image after the correct one and then subtracting this from the correct image is just old technique and according to many people should cause more demage than do any good. It is however used by many people, old habits are hard to kill.

Noise due to high ISO is the same as grain in film. The higher ISO, the more grainy picture. If you want to reduce this kind of noise, use lower ISO and let the camera NR handle it. It is doing a better job (at least in E-500) than you can do manually in PS or some other software.

Please follow the links I posted, really good information. There is nothing 'simple' in noise, except that it is 'normal' with todays CCD, even the most modern and expensive ones.