RAW File Sizes

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Today’s Question: I recently bought a Fuji X-T1 camera. It has a 16 megapixel sensor. The file size in RAW (RAF) is approximately 33.3 MB. My Nikon D4 which also has a 16 megapixel sensor has a RAW file (NEF) of about 20 MB. When I convert the RAF file to a DNG, the file size goes down to about 21 MB (from 33MB). What’s going on here?

Tim’s Quick Answer: The size of a RAW capture certainly has a relation to the resolution of the image sensor, but it is also affected by the bit depth of the analog-to-digital conversion for the camera, compression applied to the RAW capture data, and other factors. In short, the sizes here are not unusual considering the amount of information being gathered.

More Detail: An image that contains around 16 megapixels of information (as is the case with the two cameras referenced in today’s question) will result in a file size (without compression) of about 92 megabytes if the image is saved in the 16-bit per channel mode. If the image is instead saved in the 8-bit per channel mode, that value will be cut in half to about 46 megabytes.

In other words, any file size for the RAW capture file that is less than the 16-bit uncompressed value for the image based on pixel dimensions is not too surprising. The bit depth of the analog-to-digital conversion plays a role, compression (even if lossless) plays a role, and “special” proprietary data saved by the camera will also affect the RAW file size.

In the specific example cited here, the information related to the NEF file size suggests that the captures are being processed at a lower bit depth, are being captured at a lower resolution, or have compression applied (all of these options are available for the Nikon D4). Based on the specifics here, I suspect you have compression enabled for the RAW captures on the Nikon D4. If you capture at full resolution and the full 14-bit per channel depth without compression, you can expect file sizes for the NEF capture that are about the same as those with the Fuji camera of the same resolution.

Converting a RAW capture to a DNG file will generally produce a file that is around 20% smaller than the original RAW capture, and so your experience with the RAF files does not stand out as being unusual. When you convert to DNG lossless compression is applied, which will reduce the file size compared to the original capture. The specific degree to which file sizes are reduced varies based on the original capture data.

So, you’re capturing the same amount of information, more or less, but producing files of a different size based on how that information is actually recorded. In general this relates to overall resolution, bit-depth, and compression, though in this case I suspect compression is the only real factor involved.