>I read in many places people saying that recording RAW >pictures in 12 bit or in 14 bit do not make any difference in >the quality of the picture. However, I am not certain to >understand if this is true by just viewing RAW pictures with, >for example, Lightroom or Photshop.
You can't see a 12-bit or a 14-bit color gamut on a monitor that only displays something between a 6-bit or 10-bit color gamut. Forget about that aspect of it unless you're going to invest in a very, very expensive graphics monitor.
>My question is : when converting RAW into JPG a 12 bit RAW >file compared with a 14 bit file, will the JPG quality be >exactly the same also ?
The JPG quality will be slightly better - as long as you process the RAW properly. Just as Rick noted, the more color gamut you work with (14-bit) the smoother and cleaner your shadow detail is liable to be. As well, gradations such as high overhead sky fading to a lighter or darker horizon benefits from greater bit depth because there are more and finer tonal steps. When a conversion to TIF or JPG is then done, everything looks better. Just as important, the more color depth you've got in the RAW file, the more accurately the converter can work when you create your TIF or JPG.
IMO, I think the most important reason for making photos at the greatest possible bit depth is to improve the quality of your prints. The greater the gamut, the better the print. Todays multi-ink, wide format printers are capable of outputting a huge gamut. Starting the image selection and post-processing workflow with a 14-bit RAW file makes the best sense and will eventually result in the best possible print output.
Mario Georgiou (one of the genuises at Harrod's graphics design group in London, one of the internal store key ad design managers, and the Harrod's in-store brand guardian), has taught photographers, graphic artists and designers for years to forget about space concerns and to forget about trying to predict what something is going to be used for at some point in an undefinable future. Always work with original digital photos and scans created at the highest possible resolution and greatest possible bit-depth. Once you have all the data that can possibly be captured with a DSLR, video camera or scanner, you can then create anything you need - from giant billboards, large ultra high res posters or art prints for viewing in high detail at only arm's length, down to the 8"x10" framers, down to small web graphics. If you don't have all the color depth and resolution in the first place, you can't recreate it or put it in later.
Make sure your monitor isn't letting you down. Spend $75-$125 on a good monitor calibrator such as the ColorSync Spyder. It's fully automated and it takes only 10 minutes to fully calibrate your monitor in a darkened room. You'll then be looking at the best it can do for you.