Long Exposure Noise Reduction?

[Paul Hadfield]

I was tinkering with the Canon just a little while ago when I discovered the long exposure noise reduction feature. Just wondering if any of you have used it and if so, is it helpful and at what point does it kick on? Mine was originally set to "off" whereby I changed it to "auto".

Coloring makes me tired too; apologies for the band aid lol

 

[Robert Edmonds]

I have a nikon with the feature too... What occurs with CCD is there is a thing called dark current. With long exposures you may have areas of the image which glow. With my camera they show up as a "pink" like glow in the corners. This feature helps at removing it. I would say probably for exposures greater than a second I would likely use it. However, this may vary from camera to camera. The only downside to it being on, is that it may take longer for the exposure to be processed within the camera. It becomes more noticeable with my camera for very long exposure times (greater than 30s).

 

[Dennis Sherrod]

I am sure others can answer this better, but on my Nikon, it takes as long to process as the length of the exposure with the Noise Reduction on. It does help, but I do not use if I am trying to do several photos in a row.

 

[Greg Campbell]

You can also subtract the noise with software. (No need to waste half the night taking a seperate DF for each photo.)
Dark noise is more-or-less proportional to time and temperature. You save a series of dark frames taken at increasing exposure times. Subtract one of these, the one that best fits, from the photos you've taken.

This is easily done with any layer-capable editing software. Google for "Dark Frame Subtraction" for more info and links to techniques and software.

 

[Paul Hadfield]

Naturally as everyone knows, at low ISO noise is virtually a non-issue so I wasn't sure if maybe this wasn't something meant to aid higher ISO as is typically used in low light situation. I have used the median noise filter in Photoshop for correcting noise without too much softening but generally, I try to stay away from noise reduction and just avoid it altogether. Not much to look at tonight but perhaps later i'll have a try at ISO1600 and post the results.

 

[Steve Miller]

I use it on my XSi for long nighttime exposures. You won't be disappointed with the results.
Be aware though that it takes about half to three-quarters as long as the actual exposure to process the image. A 10-sec bulb might net you 7 seconds or so of post shot processing. At least this is the case on my Canon.

 

[Mike Deep]

I have a nikon with the feature too... What occurs with CCD is there is a thing called dark current. With long exposures you may have areas of the image which glow. With my camera they show up as a "pink" like glow in the corners. This feature helps at removing it. I would say probably for exposures greater than a second I would likely use it. However, this may vary from camera to camera. The only downside to it being on, is that it may take longer for the exposure to be processed within the camera. It becomes more noticeable with my camera for very long exposure times (greater than 30s).

This won't apply for Paul's camera - Every Canon DSLR since the D60 (2002) uses CMOS sensors, not CCDs.

What long exposure noise reduction does in Canon DSLRs is attempt to remove hot pixels via dark frame subtraction. Since this requires a second exposure of equal length, it's not something I would do the in the field, especially shooting something like lightning. It will not remove the "grainy" noise you typically see in a digital image, only hot pixels.

See this page from DPR's XT review for a (Less than exciting) example: http://www.dpreview.com/reviews/canoneos350d/page20.asp

 

[Robert Edmonds]

This won't apply for Paul's camera - Every Canon DSLR since the D60 (2002) uses CMOS sensors, not CCDs.

What long exposure noise reduction does in Canon DSLRs is attempt to remove hot pixels via dark frame subtraction. Since this requires a second exposure of equal length, it's not something I would do the in the field, especially shooting something like lightning. It will not remove the "grainy" noise you typically see in a digital image, only hot pixels.

See this page from DPR's XT review for a (Less than exciting) example: http://www.dpreview.com/reviews/canoneos350d/page20.asp

The concept to dark current doesn't change between CMOS and CCDs. There is still the thermal excitation of electrons. In my camera it shows up as a glow... in others hot pixels or both. Either way the same consequence of removing this effect is the same... it takes more time.

 

[Christine Hudnall]

Even at low ISO, if you are taking a long exposure, especially where it is warmer, it is an issue, because of thermal heat/signal. Astrophotographers have to deal with this all the time (at least those that use a non-dedicated astro CCD camera). Some have made homemade peltier cooling devices to help thwart this problem.

Yes, you can use the function in your camera, as others have already said though, it takes twice the amount of time, since your camera is taking the original exposure, and then closing off the shutter to take the dark frame exposure.

What some astroimagers do is, as long as ambient temp is roughly the same the whole time, is take their dark frames (calibration frames), at the end of imaging for the night. And then either use editing program or special astro program (like, Images Plus), to do the dark frame subtraction. Just make sure temp is the same, length of time is the same, and same ISO is used. As well as taking more shorter exposures and stacking them, which is a good way to do it, whether taking astroimages or not. You get more signal that way... the good kind.

Either way - dark subtraction does help, unless of course you are using a Starlight Express, SXV-M25. And then, you don't need dark subtraction!