Nikon D7000 manual 241

Chapter 8 ■ Setup: Playback and Shooting Menus 241

14-bit depth. The default values for type (Lossless compressed) and color depth (14- bit) work best for most situations, but there are times when you might want to use one of the other choices.

Compression is a mathematical technique for reducing the size of a collection of infor- mation (such as an image; but other types of data or even programs can be compressed, too) in order to reduce the storage requirements and/or time required to transmit or transfer the information. Some compression algorithms arrange strings of bits that are most frequently used into a table, so that a binary number like, say, 1001011011100111 (16 digits long) doesn’t have to be stored as two 8-bit bytes every time it appears in the image file. Instead, a smaller number that points to that position in the table can be used. The more times the pointer is used rather than the full number, the more space is saved in the file. Such a compression scheme can be used to reproduce exactly the original string of numbers, and so is called lossless compression.

Other types of compression are more aggressive and actually discard some of the infor- mation deemed to be redundant from a visual standpoint, so that, theoretically, you won’t notice that details are missing, and the file can be made even more compact. The Nikon D7000’s RAW storage routines can use this kind of size reduction, which is called lossy compression, to reduce file size by up to about half with very little effect on image quality. JPEG compression can be even more enthusiastic, resulting in images that are 15X smaller (or more) and which display noticeable loss of image quality.

Under Type in the NEF (RAW) recording menu, you can select from:

■Lossless compressed. This is the default setting, and uses what you might think of as reversible algorithms that discard no image information, so that the image can be compressed from 20-40 percent for a significantly smaller file size. Typically, you’ll get up to 200-250 RAW images on a single 8GB memory card at this setting. The squeezed file can always be restored to its original size precisely, with no effect on image quality.

■Compressed. Use this setting if you want to store more images on your memory card and are willing to accept a tiny potential loss in image quality in the highlights, after significant editing (I’ve never been able to detect any effect at all). The D7000 can achieve from 40-55 percent compression with this option, giving you 330 or more RAW exposures on an 8GB Secure Digital card. It uses a two-step process, first grouping some very similar tonal values in the mid-tone and lighter areas of the image together, and then storing each group as a single value, followed by a loss- less compression scheme that is applied to the dark tones, further reducing the file size. The process does a good job of preserving tones in shadow areas of an image, with only small losses in the midtone and lighter areas. The differences may show up only if you perform certain types of extensive post-processing on an image, such as image sharpening or some tonal corrections.