#Nikon d200 review ken rockwell iso#

But nevertheless I still think it helps to understand the inter-relationships of exposure time, ISO, f/ratio, and lens focal length, to get the shot you're after. so there are limits as to how much a longer focal length lens will help you. Since the elevation angle (above the horizon) of the celestial pole is equal to the latitude of your location, it's not as simple as backing away from your desired foreground object(s) and using your longest focal length lens, assuming you still want the celestial pole in the shot. But artistically, the challenge of composing a photograph of star trails with terrestial objects in the foreground becomes more difficult with a longer focal length lens. So while it's true that for TRACKING the stars the gain in collecting area (as focal length squared, for a given f/ratio) directly translates into ability to detect fainter stars, for star TRAILS the gain is only proportional to focal length itself (not focal length squared) because time a star spends on a given pixel is inversely proportional to focal length.Д«ack to Bert's original question, the up-side is that (in addition to recording fainter stars relative to the sky background at a given f/ratio and ISO) a longer focal length lens will also yield star trails of longer length, all other things being equal. While sky brightness is independent of lens focal length (it depends ONLY on '1 over f/number' squared, ISO, and exposure time), the signal from the stars themselves (recorded as point sources) depends on the collecting area of the lens aperture: an 18mm lens at f/4.0 has only one quarter the collecting area of a 35mm lens at f/4.0 so at a given f/ratio, fainter stars can be recorded with a longer focal length lens, and collecting AREA scales as lens diameter squared (or as focal length squared, with f/ratio, ISO, and exposure time held constant).Д«ut in the case of star TRAILS (as opposed to TRACKING the stars during the exposure to counter-act for the earth's rotation), lens focal length also determines how long a star's image remains on a given pixel, before earth rotation moves the star's image to the next pixel. But the down side is that the faintest stars recorded will be twice as bright (i.e., much fewer in number at the slower f/ratio), because the collecting area of the lens aperture is also cut in half at f/4.0 compared to f/2.8 (for a given focal length lens).

#Nikon d200 review ken rockwell iso#

Square of the lens f/ratio, meaning at a given ISO speed, you can expose longer at slower f/number and achieve the same sky background surface brightness: e.g., the you can expose twice as long at f/4.0 than at f/2.8, and have equal surface brightness in the sky background. The brightness of the sky background is inversely proportional to the (As an aside, it will also remove the purple haze from the upper two corners in Bert's example photo - see also the link Tito provided to Ken Rockwell's web site). 'Long Exposure NR' in the D200 subtracts a shutter closed (dark) exposure of equal duration from the shutter open (star trail) exposure, and this effectively removes 'hot pixels' and pixel-to-pixel variations in CCD 'dark current', but not the dark current shot noise. higher ISO makes fainter stars AND the sky background brighter, along with increased noise.

(I'd also be interested to know these details for Bert's 'Not Nearly Long Enough Star Trails' example image at the top of this thread). But I'd also like to raise D200 ISO setting and lens f/ratio and focal length as other factors that can greatly influence star trail photos.

Regarding Darren's question, I certainly agree with Bert about a dark site being important.