5.Long Exposures

The last stand of film has been the long-exposure domain, in which digital simply hasn't been competitive. The arrival of Nikon D2H made a serious dent in the film stronghold, and now D2X finally gets the walls tumbling down.

Nikon D2X has the ability to create virtually noisefree, long-time exposures, and of as long duration as you care to have (or can provide battery support for). The camera tracks the exposure time even on a "B" setting with high accuracy, and uses this information to run a second "blank" exposure internally to cancel out noise accrued within the first exposure. The second stage should last exactly as long as the first, actual image-taking stage, to the 1/10 second in fact, and the duration is duly logged in the EXIF header. D2H used a 16-bit counter to count time steps of a "Bulb" exposure and eventually this counter would overflow. Since the noise-reduction circuitry depends on knowing the exact exposure time, this meant D2H would cease to do NR after 109 minutes 13.5 seconds (= 65,535 steps each of 1/10 sec duration). I'm happy to report that this flaw has been corrected in D2X, so this camera will do its NR magic well beyond the 16-bit "barrier". However, the EXIF data field still is limited to 16 bits so you wont't get recorded any exposure beyond 6,553.5 seconds.

You might object to the idea of a camera spending half of the time in a shooting session running its noise reduction program. However, since unlike film there is no reciprocity error involved, the practical importance of the NR delay is less significant than you might think. This simply means that you will get twice the output for twice the exposure duration, something film cannot match at all at long-time exposures. In fact, reciprocity error will cut the effective speed of a 100-125 ISO film to a level below that of the D2X for any exposure longer than a few minutes. So, if you set a D2X and a film-based camera side by side and tripped the shutters simultaneously for a long exposure, the chances are that you would finish first with D2X, even with its NR delay.

The really nice part here is that D2X generates noise having an intrinsically random nature for the long exposures. There is no such thing as banding or patchily distributed blobs of colour noise involved. I could not find night skies sufficiently dark for any exposure beyond a few minutes, so this time I ran the long exposure tests with body cap on the camera (the eye-piece shutter also closed). Admittedly this is a less harsh test than exposing for dim subjects, but you have to do whatever is feasible for a given situation. I did this series of tests outdoors at ambient temperatures ranging between -5 and +2 ºC, mainly during late evening or night. I feed the D2X from the mains to avoid depleting the battery pack whie testing. The last image of the test series is shown here,

Black as Black Can Get, or The Proverbial Black Cat in a Coal Cellar This is the output from Nikon D2X, set @ 100 "ISO" with noise reduction on, after an exposure of 3 hours.	© Bjørn Rørslett/NN

The perfect test result should be 100% perfectly black pixels, i.e. with a zero value. As shown in the graph below, this occurred for exposures up to nearly 30 minutes, a tremendous result indeed. Small traces of colour noise started to creep in from there and noise peaked at 109 minutes, to drop back at 180 minutes. I surmise it's quite difficult to keep ambient conditions equal over such long time spans and this might impair the noise reduction to some extent. The 3 hour exposure was run in the deep of the night so conditions should be optimally stable for this test, again borne out by the low noise recorded.

Long exposures with street lights or similar strong point light sources inside the frame, indicated that D2X gracefully handled the overexposure bound to happen at these highlights. In fact, the inevitable clipping of the highlights seemed better and better controlled as exposure extended. I'll have to do more research in this field, but there seems to be a remarkable progress here compared to film (which will blow out highlights and the glare from them will diffuse into the surrounding shadows as well).

However, the price to be paid for all this long-time wizardry is power, or rather battery, consumption. Lots of it in fact. Thus at least under a sub-zero Norwegian winter climate, 30 to 60 minutes exposures seem to be an upper practical limit because anything longer will completely drain even a fully-charged battery pack. I have tried so should know. Do remember the need for conducting a noise-reduction stage afterwards. However, if you can provide stable mains power (with the EH-6 unit), only your patience sets the limit. Film might possibly be even better suited for doing exposures longer than say 3 hours, but we are talking about a problem only for a minority of the users here. Virtually all exposures taken with any camera are shorter than 3 hours in duration.

Given the superb low-noise performance of D2X, I wonder whether this is just a simple dark-frame subtraction, or whether D2X has more advanced and clever tricks up its sleeve.

Nikon D2X Reviewed