NEF (RAW) Compression

There are 3 different types of method of recording RAW files.  These are selectable from the shooting menu in the camera.

Lossless Compressed
This method uses a compression algorithm that is reversible.  This means that the images are compressed to save space on the card but the type of compression used can be reversed so that the image is restored to its full quality when opened. 
Lossless compression allows the best image quality to be recorded whilst saving space on the memory card. This method of saving RAW files is ideal for users of NX Studio.

Compressed
This method also compresses the data to reduce its size on the card but this method uses a non-reversible algorithm.  Therefore data that has been removed from the image cannot be restored later.
Compressed is most useful where available space is an issue on cards but the NEF format is still required.

Uncompressed
This file format does not apply any compression to the images when recorded on the card.  This means that files will be larger and take longer to write to the card than when using compression.
Uncompressed offers the best image quality without applying any compression to the file.  This method is useful for customers who wish to use 3rd party software to open their images.

The file size that is recorded in the camera is dependent on the type of RAW compression used.  The chart below shows the percentage of reduction in file size depending on which method of storing the file is used.