Long exposure noise reduction.

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Lemaildetom

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thomas
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Hi all,

I have never use the long exposure noise reduction function on my canon camera. The function which take two consecutive shots to cancel the noise.
-Firstly I don't even understant the principle. I thought the noise would be random, why would the noise be similar (I mean at a pixel level) in two succesive shot?
-Secondly would you use this function for stars photography (milky way, northern light,...) where light point source will move slightly from one shot to the next? Will this consider stars as noise?
-Thirdly in which circunstances would you use this function?

What do you guys think about this?
 
The second shot is done with shutter closed, no image. It gives the basic dark pattern noise of the sensor at that time, temperature, etc., and simply subtracts it from the image shot. The result at higher ISOs is a much cleaner image.
 
Use it all the time for astro photography. As Chris says, the second shot is done shutter closed for the same length of time as the original. My 350D can be set to do it automatically on any exposure over 30 sec. I presume my 550D is the same but I've not used it on that yet.
 
Thanks I wasn't sure it was good for astro-photo. Still don't understand why there is a noise pattern? Should this not be random?
I'll definitly try using it next time.
 
Thanks Frank, we posted at the same time. I'll read your link to understand.
 
Actually that link give me a bit of an headache, i need to read it again. It seems that the substraction remove only hot pixel and no noise. So how much of the noise is actually hot pixel?
Are the hot pixel not the easiest bit to process out using the colour slider in the noise reduction bit in lightroom?
 
If you're shooting star trails then LENR is a bad idea - it makes your trails turn into dotted lines.

If you're shooting images with plans to stack them then you're better off not using LENR. Instead you shoot a few dark frames (same exposure settings and temperature as your light frames - but with the lenscap on). The stacking software will use these to perform a similar function to LENR - but with a couple of advantages.

First, you save a lot of time while shooting.

Second, you'll get a better result. When the LENR routine operates it takes a single dark frame - which will have bright pixels of systematic and random noise. It will subtract all of those pixels from your image. But you really only want the systematic noise removed. Your stacking software will sum all of your dark frames. Because the systematic noise always happens in the same place those pixels become really bright, but the random noise will rarely be in the same place. The software decreases the amount of random noise that it subtracts from your image.
 
Its "mainly"sensor noise. Rather than random noise.
Hot pixels etc
 
Actually I haven't done a great deal of astro or long exposure shots with light trails.

But this subject is of significant interest as I am looking to venture out to cold London nights and catch some long exposure shots with light trails.

So LENR turned on will turn a light trail into dots? How does that work?

Also I have always been under the impression noise from Long Exposure is notoriously difficult to eliminate if not done at camera level. As a matter of fact recently I took a couple of astro shots with NR turned off to save shooting time and had a horrid time during post process in terms of noise reduction. So is there a prescribed process for post noise elimination on images?
 
Last edited:
ricky1980 said:
So LENR turned on will turn a light trail into dots? How does that work?
Click to expand...

I guess not but when you do stars trail you stack some picture together so if you do 30 minutes trails stacking 30 secondes exposures with LENR ON. You will get 30sec exposure - 30 sec black - 30sec exposure - 30 sec black - and so on.... So you would get dotted line created by the stacking. I actually have never try so that is just my guess.
 
No. Don't use it if your planning on doing stacking or trails. You should use proiper astrophotography software and then use dark frames rather than trying to do it in one go in-camera. Registax or DeepSkyStacker are what you need.
 
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