Photography - Here's two revelations about noise-reduction on the Sony Alpha camera system.

First, shooting long exposures in bub mode on my now severely-outdated Sony α 100 DSLR has always provided a slight annoyance for me when the LCD screen would just display "Processing..." for seemingly forever after the shutter closed. If I took a 2-minute exposure of the night sky, the camera would just sit there for an additional 2-minutes afterwards. But the annoyance hasn't been enough for me to seek out some answers, until now.

The delay is caused by the camera performing dark-frame subtraction. I've always thought of this as a technique used in post-processing, and didn't realize the camera was doing it on it's own. This is a noise-reduction technique that tries to subtract out thermal noise in the digital CMOS sensor. Basically, heat in the sensor can produce phantom photons, so that even in a completely dark environment (such as with the lens cap on) the sensor will not output a completely pure black image. This noise gets proportionally worse with ambient temperature and with exposure time. With normal everyday photos, this noise is rarely noticeable, but with long exposures in low-light settings the noise can really add up and ruin a photo. So the idea behind dark-frame subtraction is that two (or more) photos are taken with the same settings, but with one of them having all light blocked so that it's resulting image is therefore only the result of any thermal noise. You can then take that dark-frame and mathematically subtract it from the other photo to effectively eliminate the noise.

So on the Alpha during that extra delay the camera is actually taking an additional photo, but with the reflex mirror lowered and shutter closed to block incoming light; which the camera then uses to do dark-frame subtraction on the first exposure. This second dark-frame is taken with the same settings, including exposure time, which is why it seems so long when using bulb mode. By the way, if you'd prefer to do this dark-frame subtraction manually in post processing (or to skip it entirely), you need to disable the "Noise Reduction" feature deep in the camera settings menus.

A second revelation is how the Sony does hot pixel removal. A common problem with all digital sensors is they can occasionally get a "stuck" pixel. This is a single pixel that, regardless of the actual light exposure, will always register either as off (black) or fully on (white). This same kind of defect also occurs with LCD displays too. The Alpha automatically tracks these stuck pixels and minimizes their effect. The first part is the correction, so if a pixel is known to be stuck (malfunctioning) that corresponding pixel in the saved image will actually be an average of the neighboring pixels (though it may be more sophisticated with anti-aliasing, etc.). But the more interesting part is how the camera detects these stuck pixels to being with. It turns out that on the Alpha, the first time you power OFF the camera each month, the camera will actually silently take a picture (a few-second dark frame just like in the dark-frame subtraction above), and will then hunt for pixels that show up as white, or perhaps even those that are perfectly black despite expected thermal noise in working pixels. The camera tracks these defective pixels in it's own internal persistent memory so that those bad pixels will be corrected in the future for the rest of the month.

So here's the trick: if you discover that you have a hot pixel (a white pixel that always appears in the same place on all your pictures), you can trick the camera into re-doing it's stuck-pixel detection by simply changing the camera's date to a month in the future, turning it off, and then turning it back on again and changing the date back.
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