This is something that just last year I wouldn't have attempted with a digital camera. Standing in my kitchen Sunday evening I noticed that Venus was really (SUPER) bright in the sky right above my next door neighbor's house. I know that if I grabbed my camera and ran downstairs - I'd loose the angle to view Venus and it'd get lost in the trees. I decided it'd be easier to remove the screen from my kitchen window and shoot out into the sky (my house is built off the ground - being on the side of a mountain - and this window is on the high side of the house, 2 stories off the ground below). I slapped my Nikon AF-S 17-35mm f/2.8 lens on my Nikon D4s and I did the following (which is what I've always done when I shoot film exposures of star spin (the image below). Set focus manually to infinity and set the aperture to f/8. I disengaged the D4s' Auto ISO feature and manually selected 100 ISO for the exposure - hoping to minimize noise on a 15 minute exposure. I was testing 4 things with this shot. 1) Battery Life - one big drawback to digital cameras vs. film cameras is the amount of energy required to keep the sensor engaged and gathering light data as opposed to a film camera that only requires enough power to hold the shutter open. Which is very little power compared to holding it open and running a digital camera's CMOS sensor.
2) Artifacts & Pixel Errors - When you keep a digital sensor running for long periods of time they tend to develop artifacts and if there are any burnt-out pixels on the sensor they will become very difficult to ignore - especially against a dark background. 3) Noise Levels and Noise Reduction - Again, noise is kicked way up with a long exposure. I set the camera to the highest level of auto-noise reduction for the shot.
4) The BULB feature of the new remote shutter release I purchased to use for shots like this.
I have to say that the camera outperformed my wildest expectation on all four point... but it was notable that the NR feature - in camera - was VERY slow. It took 10 minutes after I ended the exposure for the camera to process through the noise reduction. But to the camera's credit, this is one super-clean final image. Very little - almost no noise or grain from the long exposure. It's as good as any 35mm shot I've ever done of star spin - and I was honestly just playing around - not sure if I'd get anything at all.
Nikon F5 - 35mm film
24mm Nikon Lens (AF-D f/2.8) 20 minute exposure on Ektachrome T320 reversal stock Aperture was set at f/8 Focus set to infinity.