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Photo noiseTuesday 10th September 2013
I recently wondered what is better when taking RAW photos in dark places. Set a high ISO in-camera or boost the image in post-processing. Generally, I don't like shooting with high-iso as I don't like the noise that accompanies it - but it is often required in dark scenes without good lighting.

In digital cameras, ISO is just the gain on the sensor - or basically, how much you overdrive the light detected. The more you amplify the more noise you get. Surely a professional image editor like Photoshop on a powerful PC should be able to produce better gain than the camera can do on the fly?

Well - sort of. Doing things in post-processing means you can boost the brightness of select bits of the image, which potentially leaves much of the noise you'd get from bumping the ISO in-camera in the shadows. But the other thing to consider is "read-noise", this is dependent on the sensor and can vary massively from sensor-to-sensor. As the ISO increases in-camera (on some cameras), the read-noise decreases.

I don't know why this is the case, and I guess it doesn't really matter as there is nothing you can do about it. So you just have to consider it when shooting dark scenes. Sensorgen.info has a list of sensors and their rated specs, so you can check to see what yours will do.

For instance, my camera the read noise reduces up to about ISO-1600, so in theory if I need to boost the brightness in the image then I should do it with the in-camera ISO as it will produce less "read-noise", that doesn't mean there will be less noise, just a certain type of it. The web-site pbase.com has an example of an ISO-1600 picture and a ISO-400 picture that has been boosted in post-processing. The former is better.

There is further complication though - looking at the sensor web-site we see that the dynamic range of the sensor also drops as the ISO increases - this means there is less room from what the camera sees as black, and what it sees as white (blown out). I imagine this is because the image is being pushed up against a maximum sensitivity.

My hunch from this is that in normal conditions you should let your camera use auto-ISO, but limit it to the point that the read noise stops reducing (ISO-1600 for me). If you know the image is more about range than pulling out the dark details (e.g. I would hazard a guess that sunsets come into this if you don't want to highlight the foreground), then lock the ISO at minimum and work with the rest in post-processing.

As it happens, this tends to be how I shoot anyway based on experience from what comes out of my camera - it's just nice to confirm my own experience/findings with some numbers.



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