Convert 8bit to 16bit before editing

I was inspired by a PM earlier today to explain a bit about what you can and can’t do with a 16bit file, and how Photoshop actually works with images when you set them at 8bit or 16bit. This hopefully will answer questions such as “is there a point to working in 16bit if the image was 8bit?” The usual answers start an argument whereas the correct answer as always is It depends.

First a background for those who don’t know. The bit depth of an image determines it’s dynamic range. By that we mean the number of discrete possible values a single pixel can have. The end points are still the same and some colours directly map to each other. Black can be represented as RGB8(0,0,0) or as RGB16(0,0,0). White is represented as RGB8(255,255,255) or RGB16(65535,65535,65535), middle grey is represented as RGB8(128,128,128) or RGB16(32768,32768,32768) etc. Every 8bit value can be converted to a 16bit value by simply multiplying by 256.

But you don’t get any extra detail! They say. And this is true. If you convert an 8bit file to a 16bit file you don’t get any extra detail, You’ll get discrete steps in the colour values such as the next level below white will be RGB8(254,254,254) or RGB16(65279,65279,65279). Therefore all the values between 65279 to 65535 will be basically wasted when you convert an 8bit file to a 16bit file. So the conversion is pointless, maybe.

But suppose you start with 16bit file and convert to an 8bit file after making an adjustment. Suppose you take a really dark picture with a couple of the RGB8 values: 0,1,1,2,2,3,3 and you now double the brightness values you get the values 0,2,2,4,4,6,6.
Now lets do the same with a 16bit file: RGB16 0,128,240,440,512,660,768, which if we convert to RGB8 would be: 0,1,1,2,2,3,3. Suppose we take this 16bit file and double the brightness you get: 0,256,480,880,1024,1320,1536. Ok I’m going somewhere with this. Now convert it to 8bit and the result is RGB8: 0,1,2,3,4,5,6 EXTRA DETAIL.
Ok but we knew that already. Working in 16bits has more data in the in between values than 8bit. And what I mean by in-between, I mean that 8bits can display every discrete visibly discernable colour in the sRGB gamut. So it really all comes down to working with 16bit files that were recorded as 16bit files avoid rounding errors from causing a loss of detail.


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