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User955306757 reddit dark noise2/26/2023 This is a converted ORF, at default Lightroom settings (Sharpening 25, Luminance NR 0, Color NR 25), with only the white balance corrected. As should be expected, there is no long exposure noise. This first image shows the scene taken with long exposure noise reduction (dark frame subtraction) turned ON. Roll over the image to get a 1:1 view of the pixels. The image comparison that follows shows this lower right corner after four different types of processing. 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. It was a 60 second exposure taken at f5.6 and ISO 200. I took this image in the early morning whilst it was still dark. 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. There is, however, an alternative to Dark Frame Subtraction, which is the careful use of Photoshop’s ‘Dust and Scratches’ filter. Detractors of the E-M1 have been quite vociferous in making this point. 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). The downside of this process is, of course, that every photo takes twice as long to take. It does a very good job of this in practice. 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. In the same conditions (that is, length of exposure and external temperature) this noise will produce exactly the same pattern for each exposure. 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. The long exposure noise created by digital sensors is essentially caused by the sensor heating up and is known as ‘thermal noise’. Long exposure noise reduction uses a technique known as Dark Frame Subtraction. 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). Many commentators have suspected that the on-sensor phase detection is the culprit here. The camera’s predecessor, the E-M5, does not produce long exposure noise to anything like the same degree. The criticism has some justification unpleasant artefacts begin to appear after about 15 seconds. The Olympus OM-D E-M1 has received a fair amount of criticism online for producing very noisy long exposures. E-M1: Dark Frame Subtraction vs Photoshop for Long Exposure Noise Reduction February 27, 2014
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