"How many bits does a native .psd file have?
PSD plays by the same rules as Tiff. If it’s an 8 bit file – then 8×3=24bit composite.
If it’s a 16bit file – then 16×3=48bit composite.
If it’s bitmap(1-bit), then 1=1bit.
Jason covered (1), I’ll have a crack at (2) …
Essentially the greater dynamic range will be useful to correct for significant exposure adjustments or for white balance [which is really just an exposure adjustment, which is different on different channels].
You should note that, until PS CS, the possibilities for working on 16 bit data were very limited. Now there is more freedom, but there are still some restrictions.
Now I’ll wait for the real experts to correct me …
Thanks guys, can you also explain to me why there are only 3 channels in the channels palette for all images? Is this because it is being displayed on screen in RGB? Is that why it is a r,g,b, channel and not a c,m,y, k?
CMYK images have four channels in the Channels palette, and Grayscale images will have only one . . . not counting any Alpha channels that have also been saved with the image.
RGB is the model used for most photographic work, unless you are having prining done by a commercial print-shop.
Exingo,
Is that why it is a r,g,b, channel and not a c,m,y, k?<<
No, it is because the image you are looking at is in RGB Mode. Change it to CMYK mode and the channel palette will show you 4 image channels
So what does it actually mean to have an 8 bit tiff? Is this just the # of colors that the image can store?
Exingo.
Is this just the # of colors that the image can store?<<
8-bits per channel will allow 255 levels or ‘shades’ in each channel.
So with an 8-bpc RGB file you can have 255 levels of red, 255 levels of green and 255 levels of blue
exingo – don’t add up the bits/channel. It leads to confusion.
Is a "32 bit" document organized as 2*16, 4*8, 8*4, or 1*32 ?
If you have an 8 bit/channel RGB image — that is how it should be described.
And PSD stores exactly what you have in the document: 1, 8 or 16 bits/channel.
Len…
Just an aside out of sheer curiousity….
How does one do the maths to arrive at 8 bits = 255 shades?
Grateful for a response…
Thanks… Cheers…
JJ
Is a "32 bit" document organized as 2*16, 4*8, 8*4, or 1*32 ?
Chris…
So how does 8 bits per channel calculate the shades? Have asked Len the same question.
Cheers…
JJ
How does one do the maths to arrive at 8 bits = 255 shades?
An 8 bit number has the range 0-255. In all channels 0 will be black, so it’s not a shade of that colour. Hence there are 255 left over.
Interesting, I would always have said 256, but this made me think about it …
An 8 bit number has the range 0-255. In all channels 0 will be black, so
it’s not a shade of that colour. Hence there are 255 left over.
Interesting, I would always have said 256, but this made me think about it
….
Colin…
That’s not quite what i meant…. figured anyway it started from zero (0). What i meant was how does 8 bits translate to 256 ( or 255 shades if we take the stepping from zero for black instead of from 1).
It’s 2 to the power of 8, right? Sorry if i’m being a bit dumb here… but is the value(?) of a bit 1 or is it 2? If so, why 2 and what’s smaller than a
bit? This was never my forté 🙁
Cheers…
JJ
Jay
Let’s try counting in binary:
00000000 = 0
00000001 = 1
00000010 = 2
00000011 = 3
00000100
00000101
00000110
00000111 etc.
Each bit has 2 states, 0 and 1, hence the use of powers of 2. Getting any clearer?
Jay,
Just expanding a little on Colin’s post (you may need to view via a browser rather than NNTP to see this table properly)
An 8-bit byte can be anything from 00000000 (no bits set) to 11111111 (all bits set)
Converting to decimal that is 0 to 255:
Decimal equivalent ‘Decimal bit value’
128 64 32 16 8
4 2 1
0 0 0
0 0 0 0 0
0
1 0 0 0 0 0 0 0 1
2 0 0 0 0
0 0 1 0
3 (2+1) 0 0 0 0
0 0 1 1
4
0 0
0 0 0 1 0
0
5(4+1) 0 0 0 0
0 1 0 1
255 (128+64+32 etc)
1 1 1 1 1
1 1 1
An 8 bit number has the range 0-255. In all channels 0 will be black, so it’s not a shade of that colour. Hence there are 255 left over.
Interesting, I would always have said 256, but this made me think about it …
Well if we’re going to think about it that way, 255 (white) isn’t a shade either, so you’re left with 254.
Like Colin said, 256 shades is 2 to the eighth power.
But why is that?
It is because it is convenient to use a byte of data to store the shades of tone.
And a byte contains 8 bits.
Each bit has 2 possible values.
256 is the total number of possible combinations of 2 values in 8 slots.
Put another way, if you thought of a byte as being 8 light switches in a row, there are 256 different ways you can arrange the on/off configuration of those 8 switches.
255 (white) isn’t a shade either
It’s only white when all 3 channels have this value. But the 0 is only black similarly. I would have said 256 shades, but I was trying to rationalise what Len had said before.
—
I’ll give in to temptation an wheel out an old joke that still makes me smile: There are 10 kinds of people in the world: those who understand binary and those who don’t. 🙂
Colin, Len….
Thanks for the explanations… 🙂
The fog has cleared a wee bit… but am not too sure if the Dutch has improved 🙂
Cheers….
JJ
