Olympus XZ-1 팁과 요령 매뉴얼 - 페이지 28

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XZ-1 Tips

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the size of the file goes up dramatically, using far more storage space and working

memory; and making image processing software work much harder, and therefore take

longer, for any image change we make when using it.

Indeed, when we get to 16-bit numbers, each pixel can be described as any one of 65,536

brightness values, and that seems like total overkill, doesn't it? So why is 16-bit (or

anything above 8-bit) an absolute necessity for a fine monochrome image?

If we didn't alter brightness values in Photoshop (or other software) it wouldn't matter, as

long as we had a perfectly-exposed 8-bit image in the first place. But we DO alter

brightness values by stretching them hither and thither in Photoshop. Increasing or

decreasing brightness. And contrast. That is precisely why we use Photoshop!

If we manipulate an 8-bit file in Photoshop, we often squash some parts of that 256-step

scale together (so that gray brightness values that were different are now the same), and

we stretch others apart. Now, between each of the just noticeable difference steps, spaces

open up. These get filled by the same gray as one of the adjoining pixels. Our eye can tell

it should be a different gray though.The smooth transition from one gray to another has

just become an abrupt step. A visible step.

When it becomes obvious enough to show, this effect is known as 'posterisation'. It is

unpleasant to see, and it is very easy to provoke in monochrome. In an 8-bit image, with

only minor manipulation, it is almost inevitable. There are real limitations to how much

you can alter the brightness levels of an 8-bit JPG.

But, if you start with a 16-bit RAW file, and adjust it in RAW software like Olympus

Viewer BEFORE saving it as an 8-bit JPG... the outcome will be much, much better.

Why adjust the 16-bit RAW file? Well, here you are working with 65,536 brightness

values, not 256. If you now save this 16 bit RAW file to 8 bit JPG, the reduction from

thousands of brightness values (even if some of the 65,536 were missing) fills up the 256

JPG-brightness-levels nicely. No matter how drastically you altered the brightness and

contrast of the RAW file! After 8-bit output, you get a continuous range of grays -- no

'white spikes of doom' on the histogram. So the lesson for fine monochrome digital