One feature that people often say they need in Photoshop is 16 bit color support.
What is 16 bit color?
Color information (as with any other information) is encoded as numbers in the computer. The more bits you use to code, the more choices you can have. Right now the possibilities are 8 and 16 bit, with 32 bit coming in for HDR. Technically, this is 8 or 16 bits per channel, with 3 channels: red, blue, and green. Thus you can also see these called 24 and 48 bit color.
An 8/24 bit image has more colors than the eye detects, so it seems that it should be enough. If you are shooting jpeg images, this is enough because that’s all jpeg supports.
When you might need 16 bit color
But what if you are doing a lot of edits to your photos? The edits are computations with the color number codes in computer memory. These are subject to round-off error. Colors that are slightly different could get rounded to the same value, and the color becomes a uniform mass instead of subtle variations. The photo has become posterized.
16/48 bit images have plenty of gradations so that this is not a problem. There are billions of possible colors instead of about 16 million for 8/16 bits. So even with a lot of computations and rounding, there are still plenty of possible values and much smaller chance that different colors get rounded to the same value. If you are shooting jpeg but will do a lot of edits, it may be a good idea to convert to 16 bits and work with a tiff.
OK, do you really need 16 bits? If you are doing a lot of heavy edits to the photo, then yes. I use it because I look to make prints that go on display for people who don’t know me to enjoy, and I want the best possible color rendition, even though I rarely do heavy editing. But I don’t think I represent the majority of people who take pictures. If you are taking pictures for personal and family use, you aren’t printing larger than 4×6, and you capture images that look good with little editing, you don’t need to worry about 16 bit.
16-bit color without photoshop
The other question is, do you need Photoshop in order to get 16/48 bit color support? The answer to that is also no. The programs I use heavily, Lightroom and Picture Window Pro, both support 16/48 bits. So does Photoline, a program I have seen good reviews for but do not know personally (yet). GIMP is supposed to have this in the next full version.