-2EV shadow noise ~ a look

A friend asked me if I'd tried under-exposing an image, push-process and then stack images to reduce shadow noise?  I have, but I'd not really shared anything on this blog.  Until now.

As always, please keep in mind that what I do is anything that someone with more than a passing interest might be able to duplicate.  I try to keep things as simple as absolutely possible.

There are testers who measure things from a more serious engineering, test and measurement perspective.  Turn to them if you really want to dig into these kinds of topics.  There's a lot to be thought about and pondered that can related directly to carefully managed image making.

Setup -

• Sony A6300
• Sigma 30mm f/2.8 EX DN
• Manfrotto tripod
• 1 normally exposed image to set as a comparison
• -2EV images to stack using 
• Camera 2 second delay timer
• Camera then quickly fired off 5 images
• Processed RAW in RawTherapee
• Capture Sharpened (most series)
• One pass at no Capture Sharpen, but with Noise Reduction (one 3200ISO series)
   
• Image stacked in the Gimp 16bit color space
  

Comparison -

[As always, click and view at 100% to see whatever there is to see]

 

 

Comments -

It can be seen that I shot three different ISOs, 100, 400, and 3200.  I deliberately shot this way as ISO400 is Base ISO 2 on the Sony A6300, and ISO3200 is the last ISO before Sony starts some rather heavy signal conditioning. A person can review Jim Kasson's measurements and comments starting here.

Working with the -2EV under-exposed images, I show a single shot properly exposed, single -2EV Push-Processed image, 3 -2EV image stack, and 5 -2EV image stack results.  At ISO100 I see the shadows clean up nicely, even using Capture Sharpen and push processing (using curves in this case) to ISO400.

Images shot at ISO400 aren't quite as clean.  Noise is clearly visible in all images.  The stacked images seem to try and clean up the -2EV Push-Processed shadows.  But I'm not helping myself by having Capture Sharpen turned on.  Through, frankly, I'd convinced myself that there might be less noise at Base ISO 2 (400 on this camera) than I actually see.

Working on the ISO3200 files I see that Capture Sharpen is definitely not the appropriate image processing choice to make here.  I learned something in having proceeded this way.  By comparison, I can see where Capture Sharpen turned off and Noise Reduction turned on is a much better way to go.

The single properly exposed ISO3200 image with Noise Reduction doesn't look too terrible.  I went purposefully light on the NR to see if random noise in the stacked image would better average out.  In some ways it does exactly this.  The ISO12800 Push-Processed 5 image stack is starting to clean up and is almost usable.

With this small APS-C sensor I'm not surprised with the strong analog gain at ISO3200.  Anything over ISO3200 becomes very salt and peppery.  If you know what I mean.

The underlying lesson for me is that, yes, I can Push-Process images, stack them, and reduce deep shadow noise.  But I have to be careful and intelligently select operators that do what I want. And one good way to achieve that is through careful study, image inspection and comparison.  Which is what I re-started here.