I'm a few days into getting to grips with my new D800 and am starting to appreciate the trade-offs associated with the high resolution images that it can produce. For some time now I've been shooting solely in RAW format, mainly due to the extra flexibility it gives you when post processing. RAW files on the D800 are huge compared to my old D300 - at least 4 times the size (circa 40MB compressed instead of 10MB).

I was prepared for this before I bought it and take the view that it will help me to be more selective in what I shoot and more ruthless in culling the dross in post-processing. But a few things have suprised me:

  • Low ISO RAW images generally produce smaller sized files than corresponding high ISO ones
  • Performance in Adobe Lightroom (and I expect other RAW processors) is marginally better with Low ISO images than High ISO ones, all other things being equal

Thom Hogan touches on this in his excellent D800 ebook, but it wasn't until I started to compare D800 RAW images downloaded from dpreview.com with my own that this really sunk in. By default the D800 comes set up to record RAW in 14-bit lossless compressed mode - in effect, applying a PNG like compression algorithm to the image data. Both my own test images (at ISO 6400) and the one from dpreview (at ISO 100) were recorded in 14-bit lossless, but my image is 48MB versus 42MB for the dpreview one.

Now the images are not identical of course because the scene is different, but the image size (both 36 MP) and RAW format is. The biggest difference is ISO. So I took some more images at ISO 100 myself, and these come out at about 42MB, consistent with the dpreview download.

You may be thinking "big deal", but a 6MB difference in file size just because of a higher ISO surprised me. Also, it seems to me that in processing these images in Lightroom (in the Develop module) that the high ISO ones were dragging the system down more than the low ISO ones. For instance, dragging around a developed high-ISO image at 100% magnification I was seeing more appreciable tearing as the image was moved around than with low ISO images.

Again you may say big deal, but as a techie this stuff interests me. It's also another good reason to try to shoot at as low an ISO as possible, despite what modern DSLRs are capable of in terms of noise handling.

Thinking this through, I expect what I've seen is largely explained by two factors:

  • the lossless compressed algorithm has more work to do when crunching a noisy image into a RAW file, so the compression is less efficient;
  • corresponding rendering of a noisy image on the PC would therefore be more demanding, especially in the develop module where adjustments are being overlayed in real time.

I expect that if I shot a low ISO and high ISO image in uncompressed RAW that the file size would be the same, but I haven't tried it yet (and I've no intention of trying - 50MB files are quite large enough!).

P.S. another plug for Thom Hogan's excellent eBook - it contains a huge amount of background, much more than just a user manual for the camera, but at 800 pages you have to persevere.