Canon 40D Long Exposure Noise Reduction

One if the really great features of the Canon 40D for sunset and night photography is Long Exposure Noise Reduction. This is enabled by selecting C.Fn II:Image -1 and setting it to 1 (Auto) or 2 (On). This is an interesting feature and according to Canon it works as explained below.

Noise reduction will be automatically activated when noise is detected in any exposure over one second. Noise reduction is subtractive, so after the initial exposure, a second exposure of identical length is made with the shutter closed. Noise from the second exposure is subtracted from the first image leaving a pristine, noise-free image. The only downside is the exposure takes twice as long.

Post Sunset Blues

Deep Blue Night
Copyright © 2008 Jeff Lynch Photography
Shot taken with a Canon 40D , 17-40mm f/4 L at 22mm, f/16, 30 sec at ISO 200 with C.Fn II-1 set to (2:On) on SanDisk Digital Film.
Click on the image above for a larger version.

As you can see in the shot above, even the deep blue and totally black areas in the image are virtually noise free. Just to see how really noise-free this image was, I exported the raw file from Lightroom as a tiff and opened it in Noise Ninja. Even Noise Ninja’s superb engine could only find a few pixels in the entire image with any color noise at all. Very cool indeed!

9 thoughts on “Canon 40D Long Exposure Noise Reduction

  1. Duh, I’ve been staring at this screen too long. I read 1 sec as 1 minute. Sorry I can’t count, but neither can Obama.

    Seriously, Thanks for the info. I now think I understand when LENR works on RAW and also the fact that High ISO NR has no effect on RAW files.

  2. Jeff:
    I’ve been researching the noise reduction capabilities iin my 40D and the more I read, the more confused I get.
    I feel like a left-wing Democrat watching Fox News. 🙂

    You stated that when noise is detected in any exposure over one second

    Your Deep Blue Night Shot shows you shot it at 30 sec.
    Thus the “long exp. NR” should not have kicked in….???

    Can you clarify this?

    Also, what is the difference in noise generated by High ISO vs long exposures?

    • Don,

      Thanks for reading.

      Long exposure noise reduction is a great little technology that eliminates noise in exposures over 1 second by taking two exposures; one with the shutter open and one with the shutter closed. These two exposures are then compared and any digital noise found (usually in the shadow areas) in the first exposure that is not present in the second exposure is “subtracted” from the final image. A neat little trick that almost completely eliminates any noise from your image.

      There are three settings; OFF, AUTO & ON. Selecting Auto lets the camera decide when to apply long exposure noise reduction to an image WITH an exposure of 1 second or longer. If you set it to ON, long exposure noise reduction is ALWAYS applied to an image WITH an exposure of 1 second or longer. A 30 second exposure is longer than 1 second so LENR will be applied to the image.

      High ISO Noise Reduction is entirely different. It is applied to the image by the camera’s processor when it converts from RAW to JPEG. This noise reduction is similar to what software like Noise Ninja and Dfine does in post capture processing except its done “in-camera”.

      Jeff

  3. Actually, it’s not EXACTLY the same on the 5dII. It offers not just on and off, but four settings: Standard, Low, Strong, and Disable.

    I can’t figure out exactly what the 5DII is doing — isn’t a subtraction a subtraction? — unless its firmware multiplies the dark frame by some coefficient before subtracting it.

    • Mark,

      I think you’re confusing the Long Exposure Noise Reduction settings with the High ISO Noise Reduction settings on the 5D2. LENR’s settings are “OFF”, “Auto” and “ON”.

      Jeff

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