THE EFFECTS OF SCANNING IN 14 BIT MODE VS 8 BIT MODE

The bit depth of an image controls the number of shades that can be defined for a color. The difference between an 8 bit (per channel) image and a 16 bit (per channel) image is not discernable to the human eye if the image is not manipulated but when the levels of an image's histogram is heavily manipulated, low bit depth images will suffer rounding errors of pixel values resulting in posterization. In this test, we will use an extreme example of a heavily underexposed slide to study the effects of working with 14 bit scans vs 8 bit scans.

The methodology used involves scanning the same slide twice in the sRGB color space and at 4000 dpi resolution using a Nikon Super Coolscan 4000. One scan is made in 8 bit mode and the other is made in 14 bit mode. The images then have their levels adjusted in Photoshop where the white point is brought down to 50 and the gamma up to 3.0. The 8 bit image is adjusted in Photoshop's 8 bit mode, while the 14 bit image is adjusted in 16 bit mode (since Photoshop converts all images with bit depths greater than 8 bits to images with 16 bits per channel). The images are then resized and saved out as high quality (Q = 90) jpgs in Photoshop to make them viewable on the web. This system is not perfect due to the compression of the images and other factors but I believe it accurately shows the effects of the different bit depth settings.

The 8 bit image shows much more posterization and subsequent loss of detail than the 16 bit image.

CONCLUSION: In this test, the results are very dramatic in showing that the 8 bit image suffers from far more posterization and subsequent loss of detail than the 14 bit image. Because this is such an extreme example though, the differences seen from adjusting most images will not reflect the differences seen here. However, for the sake of preserving as much data as possible and enhancing image quality if heavy adjustments are made, it is recommended that all scans be made in 14 bit mode as default.

Many people downplay the significance of bit depth because they don't realize just how much images are adjusted behind the scenes. Remember that every image, even ones that will never be touched by Photoshop, have heavy gamma adjustments made (~2.2 for PCs) to compensate for your monitor's non-linear response. For many scanners such as the LS4000, this operation is performed on 8 bit data if you scan in 8 bit mode, resulting in significant loss of data. By scanning in 14 bit mode, you can ensure the gamma adjustments and color management are performed on 14 bit data to minimize posterization. If you want to save disk space or need to use tools that work only in 8 bit mode, you can convert the files after you import it and still preserve most of the data.