U of S | Mailing List Archive | alt-photo-process-l | Re: OT: 16 bit editing myth or reality?

Re: OT: 16 bit editing myth or reality?

Title: Re: OT: 16 bit editing myth or reality?
I have a few comments on this topic, then I’ll retreat back to my lurking/loitering status (hehe).

> I
forgot, if as I think PS works with real number behind the scene, then it should be obvious that only
> when the original as more then 8 bit, will those extra bit will be maintained all the way through the editing
> process (16 bit mode only) but when those real numbers are converted back to 8 bit integers it will be
> possible that a few pixel value will fall 1 level higher or below its 8 bit counter part.  

It seems to me that the original question is becoming somewhat convoluted.  There’s no doubt in my mind that this is the case, but this has little to do with the advantages/disadvantages of editing in 16-bit mode.  The following web page gives a nice description of how PS (and I assume other image apps) handle contrast adjustments:


Here’s what happens in 8-bit mode (copied from the above article):
> If an image is a bit underexposed and lacking in contrast, it may only use values between 0 and 199
> perhaps (200 out of the possible 256 values). We can help fix things by spreading those values further
> apart so that they range between 0 and 255 instead of only 0 and 199, but we have no way to invent
> new values to fill in the resulting gaps. We still only have 200 discrete values; we've just moved them
> around to stretch all the way to 255. Worse yet, they are no longer evenly distributed. They can't be, in
> fact, since all we have are integer values and not fractional ones (we can use, for example, 128, but not
> 128.5). We'll still have 56 unused values just as we did to begin with, so about every fifth possible value
> will be left unused (56 out of 256 is about one fifth).

These “unused” values are what often cause posterization in prints.  On the other hand, 16-bit has 65,536 values per channel.  Granted, PS is only 15-bit, but the numbers are still much, much higher than 256.

Eric Neilson’s coating analogy nailed it.  If you’re making your image adjustments in 16-bit mode, you have a lot more tones to spread around, and are therefore much less likely to end up with gaps in your histogram which will produce a posterized print.  This is the real advantage to working in 16-bit mode.

Camden Hardy