Tuesday, October 29, 2019

Noise Reduction using Median Stack Blending

On October 26 I did another Milky Way shoot at Pigeon Pt Lighthouse. Some of my best material was shot at ISO 6400, which is very noisy on my 5D Mark III.  After exploring alternatives such as the classic Topaz Denoise and their new Denoise AI, I finally resorted to median blending. This worked because I had been taking a more-or-less continuous sequence of photos.  The sequence is marred by various mishaps, so probably not a good basis for a timelapse as I had intended. But it does provide an opportunity to stack multiple frames and use median stack blending to reduce noise.

I knew that this was a thing, but I couldn't figure out how to do it in Photoshop.  I found two good references; there are more I'm sure.

At some point maybe I'll write an expanded post, but for now I just wanted to make sure I had a record of these references.

Milky Way over Pigeon Point Lighthouse, October 26, 2019
Stacked frames at ISO 6400 using median blend
My technique basically followed the work flow in the NatureTTL article, using 5 successive frames.  Stars were manually aligned using Photoshop Warp tool, then combined into Smart Object so that Layer > Smart Objects > Stack Mode > Median could be applied. Note that the layer masks really do need to be deleted, not just disabled.

Lighting on the lighthouse is very inconsistent because of various car headlights, and photographers waving their headlights around. A separate group of frames that had "good" light on the lighthouse was median-stacked as well and blended.

In order to recover the grid in the lighthouse windows I used a single frame from earlier in the evening that was not completely blown out by the light.

For comparison, here is my original effort using a single ISO 3200 frame:

Single frame at ISO 3200
Looking at it now, there are still some aspects of the single frame shot that I prefer.
  • The drama of the deep shadows
  • The multi-frame version now looks somewhat "brown" to me, vs. the pleasant blue of the single frame.
  • The lighthouse lighting on the single frame version is pleasantly shaded. The newer one looks flat.  In fact the lighting overall seems flat.
But the blended one is much more crisp, and there are an astonishing number of stars. So nothing I can't fix.

No comments:

Post a Comment