# Noise Reduction in Long Exposures



## JoeKerslake (Mar 11, 2015)

Anyone got any tips for reducing the noise produced in long exposures?

I took this yesterday on a 10 minute exposure (just missed the light), ISO 100, F/18
If you crop there's lots of specs of green and red.


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## Coldhands (Mar 11, 2015)

Try looking into dark frame subtraction. This amounts to taking an image of complete darkness (e.g. with lens cap on) at an identical temperature and for and identical exposure time to the previous photo, and then subtracting it from said photo. This works because the noise you see in very long exposures is mainly caused by dark current in the sensor and is consistent across exposures made under the same conditions.

It is possible to have the camera do this automatically, however this renders it unusable following the shot for however long that exposure was, while it integrates the dark frame. I believe the feature is simply called long-exposure noise reduction.


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## mackguyver (Mar 11, 2015)

Here's what Coldhands is talking about and yes, it's annoying because the exposure time is essentially doubled and your camera is tied up the whole time, but it does work, in RAW and JPEG mode alike.


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## JoeKerslake (Mar 11, 2015)

Ah yes, does this mean the camera needs to stay in the same place? Or can I pack up and move?


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## privatebydesign (Mar 11, 2015)

You can pack up and move after the initial open shutter exposure. But you can't, obviously, turn the camera off.


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## neuroanatomist (Mar 11, 2015)

Coldhands said:


> This works because the noise you see in very long exposures is mainly caused by dark current in the sensor and is consistent across exposures made under the same conditions.



Hot pixels are consistent across exposures and are effectively removed by long exposure NR (LENR). But also, many RAW converters detect and remove them effectively even without LENR. However, there is shot noise in the dark current and there is also thermal noise introduced as the sensor heats up during a long exposure (that's why my microscope camera sensors use Peltier cooling), and those noise sources are random, not fixed. When you subtract a dark frame, the random noise actually _adds_ in the resulting image. Not a big deal if you're shooting at low ISO, but something to note.


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## privatebydesign (Mar 11, 2015)

The main problem is your choice of exposure settings, you used 100iso, f18, 600 sec exposure.

I would use 400iso and f13 and 75 seconds, take 2 to 8 shots and then average the images in a layer stack. http://www.cambridgeincolour.com/tutorials/image-averaging-noise.htm


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## Coldhands (Mar 11, 2015)

neuroanatomist said:


> Coldhands said:
> 
> 
> > This works because the noise you see in very long exposures is mainly caused by dark current in the sensor and is consistent across exposures made under the same conditions.
> ...



Right. Looks like I got my noise sources confused. After checking my facts: it's the fixed pattern noise that remains consistent and can be isolated in a master dark frame and subtracted.

As I now remember reading on Clarkvision, dark current itself can be suppressed, but not the noise resulting from it, which is random (as you rightly state).


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