The so-called Mode 3 is an undocumented Nikon DSLR feature which I came across on the internet when looking for noise reduction techniques in astrophotography. There’s some information on it here but nothing specific to the D700. It’s possible that Mode 3 is irrelevant for later Nikon DSLRs.

Briefly put, the consensus is that Nikon’s raw NEF format isn’t really raw, i.e. it isn’t a pristine dump of the sensor, with some amount of noise reduction being performed using a median filter. This can possibly reduce the signal content from faint stars. To circumvent Nikon’s median filter and to obtain the unprocessed sensor data, Mode 3 is performed by enabling long-exposure NR from the DSLR menu. Long-exposure NR takes two photographs instead of one; the second one is used for dark frame subtraction. If the camera is powered off during the second photo, then the camera panics and writes the first (untouched) photo to the memory card.

To test this out, I took two photos of Alnitak (Zeta Orionis, or the left-most star on Orion’s belt). The first one is a 120-second exposure at 200mm f/4, ISO 800. The second one was taken with long-exposure NR enabled, but with the camera powered off during the second photo. A side-by-side comparison is shown below (click on it for the full-sized version):

For reference, Alnitak is the brightest star in the image, with the Flame Nebula directly above and the Horsehead Nebula showing as a faint red cloud below it, towards the left. It’s a very interesting part of the sky, but more on that later.

The photo on the right was taken with the Mode 3 technique. If one looks at it long enough, one can immediately see additional stars (e.g. right next to Alnitak itself). But what’s more interesting is that all the additional stars appear to the right of their reference points on the image on the left.

The fact that the additional ‘stars’ appear with a bias to the right of the image leads me to believe that they aren’t in fact stars, but an artifact of the imaging process. There’s obviously something going on with the sensor and/or Nikon’s image processing but my understanding of that process is limited. Whether that’s particular to my D700 or with all D700s, I don’t know. That aside, I’m not able to see a significant improvement in SNR using the Mode 3 technique. I also have to constantly hover around the camera, ready to power it off and on repeatedly, which isn’t fun when you’re taking a stack of photos (I’d much rather be looking at the skies).