I'll do a couple of tests but I have a busy weekend ahead so might not be for a few days.
Here is what Nikon have to say in the tail end of a piece regarding JPEG v's RAW and compressed RAW. Read all they way through as it add more weight and information regarding compressed NEF toward the end.
Compressed NEF
Compressed RAW format available in Nikon cameras (and which can be processed by Nikon Capture) employs a strategy that uses the subjective nature of human perception to reduce the overall data volume while maintaining its quality.
Compressed RAW format algorithm claims to be visually lossless because it treats data differently according to the areas there effects of reduction are masked. It therefore applies compression only to areas which will not be critically visible in the final RGB result. In this respect it resembles part of the compression concept of the widely accepted MPEG layer 3 (MP3). Using Compressed NEF, the data to which the process is applied has reduced significance to the naked eye – Whereas JPEG compression, on the otherhand reduces data based on the orientation and degree of detail, and therefore produces effects that are immediately clear.
The NEF compression strategy is based on the fact that the human visual system has differing sensitivity to tonal variations depending on whether they are highlight, mid-tone or shadow.
Compression is applied selectively so that the effect is minimised and in most cases undetectable to the human eye at normal viewing distances.
In extreme cases the effect of compression may become visible, particularly in areas of an image where there is a combination of high levels of detail and brightness, or extreme saturation.
As the compression process is a mathematical operation carried out on subjectively accessed data, it is impossible to predict with accuracy the few types of image that might produce unfavourable results.
Nevertheless the compressed NEF format will offer the benefit of reduce data size with minimal sacrifice of quality compared with JPEG, and allow maximum flexibility to be maintained.
As a useful tip, it may be helpful to note that, as it is only areas of high saturation or brightness that may be affected, shooting one stop under will reduce the amount of data in the sensitive areas.
The decision on when and where to use the compressed NEF format is therefore dependent upon the experience of the user.
So far, Nikon’s tests* have failed to reveal any clear trend in subject type that maybe affected by the NEF compression. In fact it has been impossible in practical use to identify any artefacts when using the compressed format.
The main identifiable disadvantage is in the longer processing time required to create and decode a compressed NEF image, compared to an uncompressed NEF. However in cases where storage space or transmission bandwidth is at a premium compressed Raw remains a valuable option in the armory of today’s versatile photographer.
Reference
'NEF: No loss of data', Nikon Pro, April 2005, Cedar Communications.
It seem uncompressed is only to be used when blown highlights are to be expected in Y or any R, G and B channels.
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