Image Processing – 16 Versus 8 Bit.

WHY WORK IN 16-BIT?

JPEG is a lossy 8-bit format. An 8-bit image has 256 levels of brightness, from 0 (black) to 255 (white). A 16-bit RAW image has 65,536 levels of brightness or, put another way, 256 mini levels of brightness in between the 256 levels (65,536 divided by 256 = 256). This represents a huge increase in captured information which means you have plenty of leeway for making adjustments.

You don’t need to work in 16-bit all the time, especially if storage space, software or computer processor speed are issues. But if you are working on an image that requires adjustments, especially to the brightness of the image, 16-bit will give you maximum headroom. Process your image as a RAW, TIFF or PSD (Photoshop Document) file – if you are starting with a JPEG fie, convert it to a TIFF file first. You can convert back to 8-bit after you’ve finished editing.

8-bit adjusted

 

16-bit adjusted

Look at the histograms of images that have had the Levels adjusted and you can see that the diagram of the 8-bit file has lots of spikes just like a ‘comb’, which indicates some brightness levels have been lost. The spikiness occurs because the 256 levels of tones have been stretched out leaving gaps. This means that some tones in the image will be missing, degrading the image – known as posterization, showing noticeable jumps in tonal values – known as artifacts.

Make the same adjustment to a 16-bit image and you’ll end up with a much healthier looking histogram which is characteristic of an image with fine, smooth tonal transitions.

  

Convert to Black and White using the Channel Mixer

I find this one of the simplest and most effective ways of converting a colour image to black and white.

First, open your photo and then add a Channel Mixer adjustment layer by clicking on the icon at the bottom of the layers palette.

Select Monochrome and adjust the percentages for RGB.

There is a common misconception with this technique – the RGB percentages must equal 100%. The truth is they do not need to equal 100%, they can actually equal whatever you want them to be, since the main goal is to make sure the photo looks the way you want it to look.

When I have used this method before, I usually start with the following settings:

R: 70% G: 30% B: 5%

From there I adjust the settings until the photo looks the way I want it to.

In this case, I used R: +88% G: +29% B +4%

Adjust the percentages until the photo looks good, don’t worry about great, we’ll get to that in a moment. You can also adjust the constant percentage to lighten or darken the photo. In this case I darkened the photo by setting the constant to -2%.

Now you’ve got it looking as good as you can, use a Levels adjustment layer to check where your lightest and darkest tones are in the image. If it’s looking rather flat, tweaking the levels can give the image more punch by increasing the contrast.

Create the layer by choosing from the same drop-down list as before, shown in the first diagram above.

The Levels diagram above shows the tonal range of an image with 255 different tones between 0 – pure black and 255 – pure white.

The tones in my image were well distributed so I didn’t make any changes at this point but I did some fine tuning using a Curves adjustment layer to increase the contrast.

I pulled upwards in the centre of the line to brighten the mid-tones, downwards at the bottom to further increase the darkness of the darker tones but I pulled the top of the line back a little to avoid blowing out the highlights which are bright enough.

The final image

LEVELS

Read more: http://www.digital-photography-school.com/digital-black-and-white#ixzz1Ob07PnYz