E-M1: Dark Frame Subtraction vs Photoshop for Long Exposure Noise Reduction

The Olympus OM-D E-M1 has received a fair amount of criticism online for producing very noisy long exposures. The criticism has some justification; unpleasant artefacts begin to appear after about 15 seconds. The camera’s predecessor, the E-M5, does not produce long exposure noise to anything like the same degree. Many commentators have suspected that the on-sensor phase detection is the culprit here.

Olympus have not commented on the cause but have made their solution very clear: when taking long exposures always shoot with long exposure noise reduction turned on (on the ‘Gears’ menu, section E). Long exposure noise reduction uses a technique known as Dark Frame Subtraction.

The long exposure noise created by digital sensors is essentially caused by the sensor heating up and is known as ‘thermal noise’. The temperature increase reveals minor variations and flaws in the silicon, which show up as a very different type of noise to normal colour and luminance noise. In the same conditions (that is, length of exposure and external temperature) this noise will produce exactly the same pattern for each exposure. Dark Frame Subtraction, then, involves taking a second exposure (of the inside of the shutter) for exactly the same period of time as the first exposure, such that the exact same noise pattern is generated, and this image is then ‘subtracted’ from the original to remove the noise. It does a very good job of this in practice.

The downside of this process is, of course, that every photo takes twice as long to take. This is not a problem most of the time but obviously is far from ideal when very long exposures are needed, or photographic conditions are changing rapidly (as they can easily do in the golden hour). Detractors of the E-M1 have been quite vociferous in making this point.

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There is, however, an alternative to Dark Frame Subtraction, which is the careful use of Photoshop’s ‘Dust and Scratches’ filter. I don’t claim to have invented this method, it has long been known to astrophotographers, but I have tweaked it a little to suit it particularly to the capabilities of the E-M1 in view of its automatic lens correction features.

I took this image in the early morning whilst it was still dark. It was a 60 second exposure taken at f5.6 and ISO 200. It probably wasn’t the best scene as the long exposure noise shows up worse against dark backgrounds, but fortunately we have a darker section in the lower right corner, which also gives an excellent example of just how sharp the M.Zuiko 12-40/2.8 is from edge to edge.

The image comparison that follows shows this lower right corner after four different types of processing. Roll over the image to get a 1:1 view of the pixels.

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This first image shows the scene taken with long exposure noise reduction (dark frame subtraction) turned ON. As should be expected, there is no long exposure noise. This is a converted ORF, at default Lightroom settings (Sharpening 25, Luminance NR 0, Color NR 25), with only the white balance corrected.

The rest of the images show different processing of the scene captures again with long exposure noise reduction turned OFF (so the camera has moved slightly while I was fiddling with its settings).

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This second image shows the ORF with default Lightroom settings (Sharpening 25, Luminance NR 0, Color NR 25), and only the white balance corrected (marginally differently). Ouch! There is a lot of prominent long exposure noise and some really nasty purple pixel clusters (and a few less obvious green clusters).

The image clearly needs some work doing on it and this is where Photoshop’s Dust and Scratches filter comes in. This filter picks up the white speckles in the image but doesn’t see all the purple and green blotches as dust (some are too big I think) so for an initial attempt we can pass the image to Photoshop with colour noise reduction pushed to the maximum to get rid of these blotches first (Sharpening 0, Luminance NR 0, Color NR 100). The image was then passed through the Dust and Scratches filter TWICE as one pass didn’t zap all the spots. On each occasion, and this is very important, the new Dust and Scratches filtered layer has the DARKEN blending mode applied; this ensures that only these bright pixels are affected. Finally, back in Lightroom the Sharpening slider was raised to 25 to broadly match the Dark Frame Subtraction image.

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This resulting (third) image is not perfect but is really pretty good I would say though slightly less sharp than the in camera Dark Frame Subtraction. I also note a colour shift in the oranges, and on further inspection of the intermediate stages in the process, I further noted that most of the blurring was an artefact of the maximum color NR setting. On reflection it occurred to me that there is more than one way to skin a cat, and particularly when using an Olympus lens. Olympus lenses are corrected for chromatic aberration in camera. Thus, Lightroom’s purple and green defringing sliders are going to have little further effect on edges, however they will knock out the purple and green pixel clusters.

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This final image shows this refinement to the process. In Lightroom it was processed with Sharpening 0, Luminance NR 0, Color NR 50 (turned down from 100 previously), Purple defringe 10, Green defringe 5. It then had the dust and scratches treatment, and finally it was brought back to Sharpening 25 in LR.

Apart from the slightly different white balance, I can see very slight edge degradation, and it clearly needs a slight amount of luminance noise reduction. However, I am not really seeing any SIGNIFICANT difference here at 100%. I certainly can’t see it zoomed out and I very much doubt that I could detect any difference on a print up to 16*12.

Here is a before/after slider that will let you compare dark frame subtraction vs the refined dust and scratches process:

  • Before-Long Exposure -|- Photoshop
    After-Long Exposure -|- Photoshop
    Before Long Exposure -|- Photoshop After

Obviously your mileage is going to vary if using uncorrected lenses on the E-M1 as pushing the defringing sliders to high values may have detrimental effects on high contrast edges in your image.

However, the result is good enough for me and I would now feel confident that if for any reason I didn’t turn long exposure noise reduction on in the E-M1 when I took the shot, I would be able to rescue the situation in post.

To summarise this process, here are the steps:

1. In Lightroom prepare the ORF with: Sharpening 0, Luminance NR 0, Color NR 50, Purple defringe 10, Green defringe 5.
2. In Photoshop apply the Dust & Scratches filter with: Radius 1,Threshold 0.
3. Blend this layer back in ‘Darken’ mode.
4. Repeat 2 & 3.
5. Back in Lightroom apply sharpening and luminance noise reduction to suit.