cmos noise?

[CouncilmanDoug]

I shot this at iso 400, f8, and 20 minute exposure. Why did all of these crazy pixels pop up? I don't think they detract from this picture too much, but I can reshoot it and I'd like to get rid of the damn things if possible.

 

[rexbobcat]

Do you have long exposure noise reduction? Long exposures have very high noise because those little discrepancies in the pixel color are basically stacked on top of each other and are accentuated.

 

[CouncilmanDoug]

Is that in camera?

 

[Solarflare]

Yes.

 

[Garbz]

All cameras exhibit this kind of noise. It's due to the thermal effects of the CCD / CMOS. The noise is constant with temperature / shutter speed and ISO. This is why long exposure noise reduction will take a second shot exactly as long as the first one but with the shutter closed and then subtract the two frames.

The work around is to reduce the shutter speed. This can be done by taking multiple shorter shots and then overlaying them on top of each other with some software that adds the frames together. It's quite common in groups who point their camera at the night sky. Software like Deep Sky Stacker will do wonders for you.

 

[KmH]

Yep! With a 20 minute exposure a ot of it is thermal noise.

When power is applied to electronic circuits, they heat up.

The image sensor in a camera has millions of pixels. Each pixel is actually a small array of transisitors and a photoreceptor. If you have a 10 MP camera and the pixles are 4 transistor arrays there are 40 million transitors. Each transistor generates heat when power is applied to it. The longer the power is applied the hotter the image sensor gets. Cameras that have magnesium alloy chassis are better able to disipate some of that heat than cameras that have plastic chassis.

CMOS requires less power than CCD does, but camera makers have adopted CMOS mainly because it is less expensive to make CMOS image sensors than it is to make CCD image sensors.

Astronomical digital cameras usually have a method to cool the image sensor during long exposures. Methods range from a simple fan in an amateur level astronomical astrophotography camera, to the pros using super cold liquified gases to cool the image sensor. Astrophotography Cameras | Orion Telescopes: Shop

 

[AaronLLockhart]

Yep! With a 20 minute exposure a ot of it is thermal noise.

When power is applied to electronic circuits, they heat up.

The image sensor in a camera has millions of pixels. Each pixel is actually a small array of transisitors and a photoreceptor. If you have a 10 MP camera and the pixles are 4 transistor arrays there are 40 million transitors. Each transistor generates heat when power is applied to it. The longer the power is applied the hotter the image sensor gets. Cameras that have magnesium alloy chassis are better able to disipate some of that heat than cameras that have plastic chassis.

CMOS requires less power than CCD does, but camera makers have adopted CMOS mainly because it is less expensive to make CMOS image sensors than it is to make CCD image sensors.

Astronomical digital cameras usually have a method to cool the image sensor during long exposures. Methods range from a simple fan in an amateur level astronomical astrophotography camera, to the pros using super cold liquified gases to cool the image sensor. Astrophotography Cameras | Orion Telescopes: Shop

Ok, read that, now answer this: I thought that CMOS sensors were supposed to be more effective, sharper, and more accurate than CCD? So, why would anyone want a sensor that does that (the thermal noise), when you can avoid it using a CCD?

Sent from my iPhone 4S

 

[Infidel]

Anyone ever try to cool the back of the camera with something? Would this even come close to being able to pull heat away from the sensor? I'm guessing there is way too much hardware between the lcd screen and the sensor for this to work.

 

[unpopular]

^^ likely not. The heat is distributed through the body of the camera. Imagine trying to cool a large, hot area with a single ice cube. I suspect that eventually sensors and amplifiers will be actively cooled, especially now that video is being implemented. I almost predict this feature in the next two years on high-end DSLRs.

 

[rexbobcat]

Ok, read that, now answer this: I thought that CMOS sensors were supposed to be more effective, sharper, and more accurate than CCD? So, why would anyone want a sensor that does that (the thermal noise), when you can avoid it using a CCD?

Sent from my iPhone 4S

CMOS uses less power which means less heat.

 

[CouncilmanDoug]

so is it worth it to go try to use the noise reduction? and will it do another 20 minute exposure?

 

[rexbobcat]

so is it worth it to go try to use the noise reduction? and will it do another 20 minute exposure?

It is worth it. It doesn't take an actual second exposure, you'll sill only have one image.

It'll just take the same amount of time as the exposure to process.

So, if you have a 20 minute exposure, after you close te shutter it will take another 20 minutes for the image to process.

In manycases the noise without the in-camera reduction is soooo bad that it can't be easily corrected even with aggressive noise reduction in post.

 

[northierthanthou]

neat effect

 

[Josh66]

so is it worth it to go try to use the noise reduction? and will it do another 20 minute exposure?

It is worth it. It doesn't take an actual second exposure, you'll sill only have one image.

It'll just take the same amount of time as the exposure to process.

So, if you have a 20 minute exposure, after you close te shutter it will take another 20 minutes for the image to process.

In manycases the noise without the in-camera reduction is soooo bad that it can't be easily corrected even with aggressive noise reduction in post.

You can do the same thing in post, if you forgot to turn noise reduction on or something.

This was a 20 minute exposure ... so you need a 20 minute dark frame. Just put the lens cap (hell, body cap even) on and set it to bulb. Take a 20 minute exposure. Anything that shows up in that is the noise that is showing up in your picture.

You can load this dark frame into your PP software and subtract it from the image you have.


That is basically the same thing the camera is doing when you turn long exposure noise reduction on.

 

[Josh66]

One major benefit of the above method that I forgot to mention is that you don't have to sit around for an extra 20 minutes waiting for your camera to process the picture. You can do the dark frame later, at home. Just make sure the exposure time is the same.