As you saw in the previous post vignetting in the individual frames and layers can be very obvious. Since the photo was shot at different exposures, I can use the HDR merge features of the stitching tool Autopano. After making some adjustments to the configurations to prioritize reduction in vignetting and a 2nd stitch yielded this pano. The vignette artifacts are still evident on the left side of the picture, and throughout the sky, but the majority of the foreground from the center to the right side is now clean enough to pass. At this point I will resort to a pre-processing filter that makes a lens and camera specific adjustment to all the pictures prior to stitching, unfortunately this process is not multi-threaded, so it will likely run in the background for at least 24 hours before I can do another stitch.
Vignetting is why gigapixel photographers take sunny days off, blue skies make bad pictures because of vignetting. That subtile darkening around the edges of a picture is rarely a problem with single shot pictures, and sometimes it's considered a desirable effect. But when you're stitching pictures together, that subtle darkening can become not so subtle. I have become the master of shooting when the conditions are favorable to avoid the problem, but sometimes you can't wait around for the perfect weather. When the problem does pop up it can be ugly, with 9 out of 10 shot's never reaching print. I have had varying degrees of success with different approaches to correcting the problem and I'm still learning. Recently, I took a beach sunset pano, on a nearly completely cloud free evening and the subtle changes in lighting and hue over the 13 minutes it took to take the picture have combined with what was probably a poor choice in ISO or aperture to make the vignette artifacts worse than usual. I thought I would take this opportunity to share what I've learned so far, and see what happens when I do everything I can to save a picture. Hopefully if you follow this series of blog posts you will gain some additional insight to the challenges of creating these pieces. If you're a fellow gigapixel artist, please feel free to share and comment on lessons you've learned.
Step1: Shoot Bracketed Exposures
The most effective way to deal with the lighting changes you see across a picture is to shoot exposure brackets. All of these pictures were shot on ISO640, at F13, and exposure was bracketed at 1/1,250, 1/640, 1/320, 1/168 1/80. After initial review, I discarded the 1/1,250 and 1/640 exposures as to dark. Here are the other 3 brackets as individual layers. The vignette artifacts reach right out and slap you in the face on these. In my next post I'll show what the Kolor Autopano Giga software can do with HDR options and these layers.
Woke up this morning to an amazing sunrise, last time this happened we had an amazing sunset the same evening..... let's see where this goes!