Adobe Photoshop CS5 for Photographers - Martin Evening [141]
My second point is that you never know what the future holds in store for us. On pages 338–341 we shall be looking at Shadows/Highlights adjustments. This feature can be used to emphasize image detail that might otherwise have remained hidden in the shadows or highlight areas. This feature exploits the fact that a deep-bit image can contain lots of hidden levels data that can be further manipulated to reveal more detail in the shadows or highlights. A Shadows/Highlights adjustment can still work just fine with 8-bit images, but you'll get better results if you open your raw processed images as 16-bit photos or scan in 16-bit per channel mode first.
Comparing 8-bit with 16-bit editing
1.
Here, I started out with a full color image that was in 16-bits per channel mode and created a duplicate that was converted to 8-bits per channel mode.
2.
With each version I applied two sequential Levels adjustments. The first (shown here on the left) compressed the output levels to an output range of 120–136. I then applied a second Levels adjustment in which I expanded these levels to 0–255 again.
3.
The outcome of these two sequential Levels adjustments can clearly be seen when examining the individual color channels. On the left you can see the image histogram for the 8-bit file Green channel and on the right you can see the histogram of the original 16- bit file Green channel. As you can see the 16- bit version retained a nice, smooth histogram.
Photoshop also offers extensive support for 16-bit editing. When a 16-bit grayscale, RGB, CMYK or Lab color mode image is opened in Photoshop you can crop, rotate, apply all the usual image adjustments, use any of the Photoshop tools and work with layered files. The main restriction is that there are only a few filters that can work in 16-bits per channel mode, such as the Lens Correction and Liquify filter. You may not feel the need to use 16-bits per channel all the time, but I would say for critical jobs where you don't want to lose an ounce of detail, it is essential to make at least all your preliminary edits in 16-bits per channel mode.
Camera Raw and bit depth output
If you use Camera Raw to process a raw camera file or a 16-bit TIFF, the Camera Raw edits will all be carried out in 16- bits. If you are satisfied with the results obtained in Camera Raw and you have managed to produce a perfectly optimized image, it can be argued there is less harm in converting such a file to an 8-bits per channel mode image in Photoshop. However, as I mentioned in the main text, you never know when you might be required to adjust an image further. Keeping a photo in 16-bits gives you the peace of mind, knowing that you've preserved as many levels as possible that were in the original capture or scan.
In the tutorial shown opposite, I started with an image that was in 16-bits mode and created a duplicate version that was converted to 8-bits. I then proceeded to compress the levels and expand them again in order to demonstrate how keeping an image in 16-bits per channel mode provides a more robust image mode for making major tone and color edits. Admittedly, this is an extreme example, but preserving an image in 16-bits offers a significant extra margin of safety when making everyday image adjustments.
16-bit and color space selection
For a long time now Photoshop experts such as myself have advocated editing in RGB using a conservative gamut color space such as Adobe RGB (if you want to find out more about RGB color spaces then you will need to read Chapter 12 on color management). Although 16-bit editing is not new to Photoshop, it is only since the advent of Photoshop CS that it has been possible to edit more extensively in 16-bit. One of the advantages this brings is that we are no longer limited to editing in a relatively small gamut RGB workspace. It is perfectly safe to use a large gamut space such as ProPhoto RGB when you are editing in 16- bits per channel mode because you'll have that