Why To Use Long Exposure Noise Reduction

A late afternoon tweet from a friend on the east coast snapped to to attention: 2010 Perseid Meteor Shower was peaking tonight!  That was Thursday afternoon…denying the fact that I had a morning meeting in San Francisco on Friday, I headed out at 11:30pm Thursday night to the closest dark spot I know of to see how lucky I could get.

Not that lucky…  The below photo is an 11.5 minute exposure from the Altimont Wind Farm east of Livermore.

2010 Perseid Meteor Shower over Altimont Pass

2010 Perseid Meteor Shower over Altimont Pass

At first glimpse it looks like a suitable photo.  Yes, the light pollution from Tracy (20 miles over the hill and behind the wind turbines) limits the impact of the night sky, but otherwise it looks acceptable except for the lack of any meteors.

Now, let’s zoom it to full resolution (click thru to view full size):

Full Resolution Portion of Alimont Wind Farm

Full Resolution Portion of Alimont Wind Farm

See all those colored dots?  That is noise from created by the sensor of the camera becuase it was turned on for 11.5 minutes. This is the down side of digital photography.  But what the technology taketh away, the technology giveth in return.  Most DSLRs have a feature buried in the camera menus called “Long Exposure Noise Reduction”.  In a nut shell, this feature (when on) removes that digital noise from your photos when taking long exposures.

My unlucky part is that I was spooked when setting up my camera and forgot to turn on that feature.  (more on the spooked bit in a second)  So all my photos from Thursday night have pretty red, green, and blue stars!

How Does Long Exposure Noise Reduction Work?

When you turn on long exposure noise reduction, all long exposures (over 1 second on my Canon 5D Mark II) have the actual exposure capture with the shutter open followed by a second image capture with the shutter closed.  Then the camera subtracts the noise captured in the second shot, from the photo captured in the first shot and writes the cleaned up image to the memory card.  Bye Bye red, green, and blue stars…

On the Canon DSLR’s that I’ve used, when you capture a frame the card access light turns on for the duration of the second exposure.  If you don’t realize that you have this feature turned on, it can be confusing why the photo exposure is complete but you’re camera is not responding.  For this reason, I typically keep this setting to Off and turn it to On only when shooting at night with long exposures.

Remember: this feature will double all your exposures.  So my 11.5 minute exposure above would have been 23 minutes long had I turned this feature on.  Something you need to keep in mind when doing the math of how long you’ll be out and how many exposures you plan on capturing.

How Do I Turn on Long Exposure Noise Reduction?

On the Canon DSLR’s this feature is located in the mysterious land of .  On the 5D Mark II it is under Custom Function II: Image menu. Refer to your owner manual for details on where to find this on your specific camera.  (Yes, the Owner’s Manual…that little book that was also in the camera box with the camera when you first opened it…  🙂  )

Getting Spooked on Thursday

I mentioned that I got spooked and forgot to turn on this feature…it was one of those close encounter moments that makes you want to call out for Mulder from the X-Files.  I was parked on the side of the road with my car perpendicular to the road to block the wind (the wind turbines are there for a reason).  This road is isolated every night except for the few locals who know about it and use it for watching meteor showers or submarine races, so it is pitch dark and isolated. I was pulling my 5D Mark II out of the back seat of the car and turned around toward the tripod when suddenly the entire area was light up by a light streaming down from above.  I could clearly see the windmills and cows grazing in the fields as I tried to put my heart back in my chest….

At the exact moment I turned around, an airplane coming in from the east to land at the Livermore private airport had turned on their landing lights and they just happened to hit me dead on.

I think you can now understand how I forgot to turn on the long exposure noise reduction…

2 thoughts

  1. No that it really helps – but it happened to me too when we were shooting northern lights in Alaska for a first time a few years back.
    I’m not sure if it doubles all exposures, I believe it only doubles the downloading time to the chip, the second part is for rendering, while the cam is closed.
    I’m not a tech freak at all, but I think the exposure stays the same.
    Great blog – not many know about this feature!

    [Reply]

    latoga Reply:

    @Rolf Hicker, This is definitely an unknown feature of most cameras. The trick is remember to turn it on! 🙂

    [Reply]

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