On Mon, Oct 11, 2010 at 6:35 PM, Larry Colen <l...@red4est.com> wrote:
> Alternatively, is there a good/easy/cheap way to stack a bunch of (for example > 10 second) exposures to get a much longer exposure with less noise and short > star trails? It would be preferable for me to do this in lightroom, but I > also have > photoshop. I don't think that's a winning strategy with film. To begin with, there's the technical problem of registering the images for stacking, since they won't inherently be in pixel-perfect alignment. I would imagine that the same kinds of feature-matching programs that people use for panoramas could help with this issue. But the problem with film for long exposures in reciprocity failure--the film is effectively slower than its ISO speed would suggest. But it's not REALLY a problem with long exposures; it's a problem of low photon rate. My understanding is that the chemical reaction for exposing a grain of film requires multiple photons (2 or 3). If a photon hits a grain, it's in some temporary, unstable state, and if a second photon doesn't come along quickly enough, it goes back to its initial state, and you're gained nothing from the first photon. Shortening the time of an individual exposure does nothing to help this issue, and probably makes it worse, because you're down in the toe of the characteristic curve, instead of the linear part. (As a side note, the effective exposure time for star trails depends on how long the star's motion allows it to illuminate a particular spot on the film, not how long the shutter is open. I'm sure there's web pages that do the math for you.) -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
