Wieland Wilker wrote, >Btw. 20s? Isn't this too long? The rule of thump is to open the shutter >400/f = 400/50 =8s to avoid strokes. What are your personal experiences? >Also note the typical vignetting. Reasons? How can one avoid this? >I am generally intersted in all astro tips (I don't have any tracking devices). Hi Wieland, Glad you enjoyed the picture. I love playing around with these starfield shots -- just wish I had a darker site with less "pollution" from the lights of nearby streetlights, shopping centers, etc. For stars, the rules for maximum exposure times are much more forgiving than the 400/f rule suggests. (I'll go into a bit more detail below on this.) Even my 4" x 6" prints of thirty-second exposures (tripod mount, 50mm lens) look OK to my eye, but 40- or 45-second exposures show a bit too much trailing for my personal tastes. So I try to keep my tripod-mounted shots to no more than about 30 seconds. The eight-second maximum exposure time you mention is a very good limit (at 50 mm) for *extended* objects at or near the celestial equator. For instance, imagine taking a picture of the moon -- it appears to revolve around you about once every 23 hours. The image on the negative is pretty small through a 50mm lens, but there is some detail there -- craters, lava planes, etc. By limiting your exposure to less than eight seconds, you ensure that every point in the image is constrained to the famous 0.03-mm "circle of confusion" that the depth-of-field folks always talk about. Expose the shot for longer than eight seconds, and the picture looks out-of-focus, and the sharp details now look blurry. With stars, the situation is different. You're no longer looking at subtle details and small contrast changes on an extended object. You're instead looking at nearly pin-point spots of very bright light against a very dark background. A *perfect* point source of light (approximated by a star) imaged by a *perfect* 50mm lens will give an image subtending 2.8 seconds of arc, or about 0.0007 mm, at the film plane of your 35mm camera. Of course, even the shortest exposure never records at this resolution -- your film's grain size is several times larger than this, the incoming starlight is "degraded" by thermal gradients in Earth's atmosphere, and the focused point of starlight gets scattered within the photographic emulsion. Thus, each star image recorded on your film is actually a blurry spot several microns (perhaps a few tens of microns?) in diameter, even for very short exposures. If you violate the 400/f rule, what you've done is taken this fuzzy spot of light and smeared it out a little bit, creating a slightly larger fuzzy spot of light. Your brain (or at least mine, anyway) doesn't interpret this larger fuzzy spot as being any more out-of-focus than the original fuzzy spot. From my experience, I've found that I can let the star trails reach a length of about 0.1 mm, and the stars in my 4" x 6" prints still look to me to be star-like. This corresponds to a 30-second exposure on a fixed tripod with a 50mm lens, for stars close to the celestial equator. If you're shooting stars away from the equator, just take this 30-second value and divide by the cosine of the star's declination angle. You can intentionally take out-of-focus pictures of starfields and get nice effects. Subtle differences in star colors show up great in severely out-of-focus pictures. You can't see these color differences as well if you're focused at infinity. For these types of shots, you can stretch out exposure times even more. I did this once while taking a shot of the "Keystone" asterism in the constellation of Hercules. It was an accident, but it gave interesting results. Light fall-off in the corners is a problem. Stopping down the lens should help improve this problem, but there's a limit to how far you can stop down if you're shooting from a fixed tripod mount. I suspect -- but am not sure -- that some of this brightening in the center may be due to "fogging" from the ambient night lighting in my suburban location. If this is the case, I might get better pictures by using some sort of hood. Perhaps a short length of large-diameter tube made from black paper would be sufficient? I'll have to try this next time I get a free evening with clear skies. Hope I didn't put anyone to sleep with all this rambling. Good luck with your night-time shooting! Bill Peifer Rochester, NY - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org.
