Christian Buil who focuses on CCD detectors in the field of astronomy and spectroscopy has an interesting Nikon D70 test. Results can be translated into just about any photographic field that requires very long time exposures (say, rotten industry). Though the D70 is an excellent camera, it shows the same design issues owners of earlier Nikon DSLR models already have reported of. In his set of rather scientific tests Christian details what has assumed before:
- Nikon RAW is not RAW: a median filter gets applied to the sensor raw data, resulting in losses in sharpness and details
- Output circuits mounted right next to the sensor cause bright spots on long time exposures
While the latter is a no-issue below say 300...400 seconds exposure time on lower ISO numbers, the former is the reason why some Nikon owners complain about low sharpness no matter how expensive their lenses are. The example shows why: to the left, a standard Nikon RAW without darkframe substraction. A median filter is applied to all three layers: "The presence of this numerical filtering explains why the long exposure in the darkness in this mode do not show any sign of thermal signal, which is completely abnormal for CCD specialists." With darkframe substraction, the camera takes two successive frames: the first with shutter open, the second with shutter closed. Then, median filtering of the first frame, substraction of second frame, repeated median filtering, writing to disk. To the right, the same exposure unfiltered (true RAW):
Saving unfiltered RAWs requires a workaround: in noise reduction mode, switch the camera off for a tiny while after it has completed the first frame: "The firmware of D70 automatically saves the image stored in the buffer memory on the CompactFlash card. It is a safety measure if the user cuts off the power supply without taking guard there. Now, the D70 save an absolutely true raw image." Now this isn't exactly convenient or elegant, but seems to work. Firmware hackers, start your engines. Nikon, update your manuals and don't keep silent about the filter application.
Though differences seem to be dramatic on the test shots, keep in mind that we are talking about very long exposure times on higher ISO numbers in a special field. Most general D70 shots I've seen so far, including night shots, are superb. But this workaround might help you to push the limits once you've reached them. And while you're at it, take some infrared shots. Here, CCD sensors excel.
Comparative test: Canon 10D / Nikon D70 in the field of deep-sky astronomy >
