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long exposure noise reduction feature

Eric Merrill , Nov 25, 2011; 06:00 p.m.

How does the long exposure noise reduction feature work?

I've poked around long enough to learn that it used dark frame subtraction. The idea being that if you expose a dark frame for the same length of time, then you can subtract away all the static noise. That makes sense.

It also makes sense as to why the camera spends as long processing after the shot as the initial exposure. If I made a 30 second exposure, it spends another 30 seconds so it can find the noise in a 30 second exposure. That makes sense.

If what I've described above is correct, how can I make another exposure while the camera is processing the noise from the dark frame? How can it be reading the sensor for noise while the sensor is being used to record the next scene? As long as I have buffer, I can take more pictures. I just can't get into the menu during the processing time.

I have a Canon 1D4, if that matters. I tried my hand at star trails last night. First time ever. It was lots of fun. More fun than results. So I'm trying to learn more about other techniques.


Eric

Responses

G Dan Mitchell , Nov 25, 2011; 06:33 p.m.

If you have LENR working correctly you cannot make another photograph until after the dark frame exposure completes.

Dan

Cinto Brewer , Nov 25, 2011; 09:48 p.m.

I believe you can take one 30 second dark frame image and use it to remove the noise from a number of 30 second shots taken around the same time. I can't remember if you can use Photoshop or need some specific software, but if you search around you should be able to find out. Eventual the non-random noise will change so you will need to re-shot a refrence shot.
http://www.mediachance.com/digicam/blackframe.htm
something like that.
Cheers,
Cinto

Eric Merrill , Nov 25, 2011; 10:19 p.m.

Dan:

If you have LENR working correctly you cannot make another photograph until after the dark frame exposure completes.

Are you positive about this? I have it enabled, and I can continue to make photographs. Doing so appears to postpone the dark frame exposure.

Eric

G Dan Mitchell , Nov 26, 2011; 11:57 a.m.

Eric, I am not 100% positive regarding how LENR works on your 1D4. But the way I described it is how it works on my 5D2 and other Canon cameras with which I am familiar. A quick Google search - less than exhaustive, I admit - did not turn up anything to confirm this one way or the other.

On my camera, there are three settings for LENR, basically off, auto, and on. (The exact labels may be different - I'm working from memory without the camera in front of me.) The "off" and "on" settings do what you would expect - either completely disable the use of LENR or force it to be used on every shot. However, if you are using the AUTO setting, this could be another explanation for your ability to keep shooting. In AUTO, the camera automatically enables LENR for exposures beyond a certain threshold length (which I would have to look up). Let's say that it is 5 seconds. If your exposure is less than that the LENR will not be used; it it is longer it will.

There is an approach to long exposure noise reduction that uses a manual black frame exposure and then does the "subtraction" in post, but this would not involve turning on the LENR feature on your camera at all. Basically, after determining the exposure time and aperture and ISO that you will use for a series of long exposures, you make a single exposure using the same settings with the lens cap on. Then you use this single dark frame sample during post-processing where you do the subtraction process manually in software. But that is a very different - and somewhat limited - situation and it does not use the camera's LENR feature.

Do you have the manual for your camera? Can you look up the information about LENR in the manual to see what it says?

Dan

Oscar van der Velde , Nov 27, 2011; 09:28 p.m.

I once had long exposure noise reduction on while shooting lightning with a 5D. If you do a series of images with say 10 second exposures and keep the cable release locked, it will shoot continuously for a number of shots, then pauses to do the dark frame, and continues with more shots, and so on. This would cause interruptions in a star trail series. In super wide angle star trail shots perhaps so small it would not be noticed. But if long exposure noise reduction is off, the hot pixels can be overwhelming. In cases when I forgot to turn it on, the Pixel Fixer program (http://www.pixelfixer.org) saved me, but for optimal results it is better to take a dark frame during the shoot for ingesting into this program.

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