Shouldn’t 16 bit mode display more than 256 shades/channel?

CK
Posted By
cathy_keifer
Apr 4, 2004
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1095
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4
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I’m not sure that I fully understand the difference between Photoshop’s 8-bit mode and 16-bit mode.

I think I’m correct by saying that 8-bit provides 256 (2 to the 8th power) shades in each of the red, green, and blue channels, for a total of 16,777,216 (2563) colors. And that 16-bit provides 65,536 (2 to the 16th power) shades per channel, for a total of 1.E+77 (65,536 to the third power) colors.

If I’m correct, then why am I limited to only 256 shades per channel when selecting colors from the color picker when in 16-bit mode? Is it because the color picker isn’t setup to handle 16-bit color, even though I have Photoshop CS, which supports 16-bit for most core operations? My thinking is that providing more shades in the color picker would require decimals, so the Photoshop designers decided that allowing more choices in the color picker would be unwieldy. I’m assuming black would still be represented as 0,0,0 and white as 255,255,255.

If I make color correction in 16-bit and then switch to 8-bit, there should be some color shifting and loss of detail. Yet, I have not been able to see this loss even when looking at individual pixels at full zoom. Photos seem to have more details when shot in 16-bit mode, so why don’t those details become lost when converting them to 8-bit? Perhaps I’ve been watching the monitor too long and am not picking up on subtle differences.

Any information you can give me on color differences will be greatly appreciated. Thanks.

Cathy Keifer

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L
LenHewitt
Apr 4, 2004
Cathy,

Your display (monitor) is only 8-bits/channel. What you see is an 8-bit ‘proxy’ of the 16-bit image.
JS
John_Slate
Apr 5, 2004
I learned this from Chris Cox recently:

There are only 32769 levels in Photoshop 16 bit data.

So thats 15bit plus 1, in order to make 50% fall directly on a value instead of between them as 50% does in 8bit.
MM
Mac_McDougald
Apr 5, 2004
Printers don’t print in 16 bit either.

Here’s the deal with 16 bit. Its main (sometimes) strength is to allow for less destructive editing, like preventing banding, posterization etc.

Once you have finished editing an image, it doesn’t matter if you output the 16 bit or 8 bit finished file, they will look identical. I run a film recorder, and there is no diff even output to film, still the highest rez output media there is.

Histograms prove that many editing functions done in 16 bit is preferable. But human eyeballs prove that it’s a vast minority of images that will actually produce a discernable result.

There is a wealth of information on the web re 16bit editing, or you can spend a week or so perusing the archives of comp.periphs.scanners (just to mention one pertinent UseNet group) with Google Groups. 16 vs 8 bit is one of those digital jihads and there are vehement arguments for both sides.

One area in which 16 bit images ARE quite important is in the world of measurement imaging, where instruments other than human eyes analayze imagery (like astrophysics).

I don’t know the science of why there are still only 256 shades in 16 bit grayscale images, but I suspect it has to do with the fact that we can’t even perceive all 256 as it is. Or let me rephrase that: we can SEE them all, we just can’t DISTINGUISH between several steps of the nearest neighbors. I suppose that there are actually more shades there, but if you can’t discern between them to select, it’s simply an extended scale without practical use.

Same with color: we certainly can’t see (or rather, distinguish) the entire 16.7 million possible in 8 bit either.

Mac
HL
hanford_lemoore
Apr 5, 2004
Cathy writes:

then why am I limited to only 256 shades per channel when selecting colors from the color picker when in 16-bit mode?

Good question. I think only Adobe can answer this for sure.

If I make color correction in 16-bit and then switch to 8-bit, there should be some color shifting and loss of detail.

As far as I know, there is no computer video card that outputs color at that bit depth. It is limited to print. So even when you’re editing in 16 bit mode, you’re only seeing 8 bits per channel. Photoshop saves the data internally at 16 bits so that you can print it accurately.

When you have an image with 16 bits per channel, an RGB image is 48 bits. If you check your Windows monitor settings it is either set to 16 bits (5 bits per channel, one left over) or 32 bits ( 8 bits per channel). Both settings are significantly lower than a 48 bit image.

~Hanford

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