Olympus XZ-1 Tips And Tricks Manual page 27

XZ-1 Tips page 27 of 29
But there are no formulas here. We all have different tastes, and the joy of an XZ-1 or
Leica Monochrom is that you have choices, wonderfully traditional choices. And the
amazing ability to see that black-and-white image on the LCD screen!
Why RAW?
...condensed from an essay by Barry Thornton
Normally, the human eye will see an image of 300 pixels per inch (ppi) in its final
reproduction size as smooth continuous tone. Having more pixels per inch isn't necessary
because we won't be able to see any difference. But below 300 ppi, we increasingly begin
to detect those separate 'tiles' -- pixels -- so that the quality of the image begins, literally,
to break up.
In monochrome, each of those pixels needs another piece of information - what shade of
gray each one is -- its brightness value. This can shade between absolute black and
absolute white. Monochrome needs only a brightness value... color needs the
hue/saturation of each co-ordinate as well.
An 8-bit number lets us describe 256 brightness values (normally shown as 0 - 255).
Because of JNDs, 256 is a critical number. What's a JND, and why is it important? A
JND is a 'just noticeable difference'. If we have a 256-step brightness value scale from
black to white with evenly spaced adjoining steps of gray running from the darkest just
off-black gray to the lightest just off-white gray in sequence, the average human being
can just perceive -- barely -- the difference between any two adjacent shades.
But if we couuld put an extra step in between any two of the steps, the human eye would
be unable to detect the difference in brightness between that injected step, and the steps
on either side.
If each pixel is assigned a brightness value using 8-bit numbers, the human eye will
perceive the overall image to be a continuous tone image, just like a conventional
monochrome photograph. If we put any more brightness values in by using, say, 12, 14 or
16-bit numbers, we human beings just wouldn't be able to see a difference. What's more,