Weather often plays a part, sending clouds through the scene during capture. If the sky conditions were changing during the course of the image capture session, any single frame will not be so close to the average glow of the final image, so will either be over or under correcting the sky glow. While this is much improved, there are still some limitations with this technique. The final star trail image with the gaussian blurred sky glow subtracted All too often the sky glow is not uniform, but in fact a gradient from top to bottom of the frame, meaning the results of curve adjustment show up the gradient The problem with this is that the corrections will apply to the background sky, the stars themselves and any foreground objects uniformly. A simple way is play with the curves/levels to reduce the intensity of the background glow and/or use colour balance corrections to try and make it less noticeable. There are number of techniques for removing sky glow, with varying pluses and minuses. When taking images in London though, there is a major problem with sky glow from the ever present light pollution. The startrail application will merge all the dark frames and then subtract the result from the light frames, removing the hot pixels from the final image. These frames will record any hot pixels or general sensor noise that may be present. The first thing people do is to add in dark frames, which are images captured with the lens cap on. This is all very straighforward if you’re capturing images under ideal conditions, but life doesn’t always work out that way.
The basic star trail image with no sky glow adjustments applied
0 kommentar(er)
