Sunday, March 21, 2021

Sky Bracketing in Deep Focus Photography

As a 'scape photographer' my usual workflow is to ensure I've covered the dynamic range of the scene, exposure bracketing if required, and ensure I've covered the focus definition of my nearest point of interest out to 'infinity'. 

The challenge being, when you need to bracket in both ways, in the same scene.

This is why Lua scripting, via Magic Lantern or CHDK, represents one of the most powerful features of these two pieces of software technology. Allowing you to carry out complicated and/or complex workflows.

Many/most times high dynamic range in a landscape is defined through the two main areas in the scene, ie the land and the sky. To cover the times when you need to bracket both focus for the land and exposure for the sky, I've added in a new feature to my CHDK M3 Bracket script, that I'm calling 'Sky Bracketing'.

By definition, a sky is at infinity and doesn't have high frequency content that needs sharpness, ie we don't need to focus bracket for a sky, once we are shooting past the hyperfocal distance. 

On the other hand, near field objects of interest will/may have high frequency content and thus require focus bracketing; plus, if we ignore the sky, ie let it blow out, on modern cameras one exposure is usually good enough for the land alone.

In my new sky bracketing feature you thus set the exposure for the land and the script will capture the perfect focus bracket set for the land using this exposure. It will then add in one additional sky capture, at your infinity focus point, either 3Ev or 4Ev lower than the land exposure. 

The M3 Brackets menu looks like this:

In the above screen capture we see that I've selected focus bracketing from the current point of focus to infinity (X2INF) and that I've defined infinity focus as 4 times the overlap hyperfocal, based on a defocus blur of 15um. Thus the infinity defocus blur of the last image in the land focus bracket set will be at a blur of around 15/4um.

We also see the new sky option under the 'Exposure bracket?' menu. In this test example, I decided to use a 4Ev offset for the sky, ie relative to the land's exposure.

As I selected Bookends, I switched off the delay. I also left the screen on so I could see the on-screen feedback, eg how many brackets will be needed and the focus position of each capture.

Today's test was outside in my back garden, and there was a little bit of wind, which in this post I haven't bothered addressing in post processing.

My workflow with the sky bracketing feature is simple:

  • Compose
  • Adjust the exposure until the histogram's highlights, ie the sky, are just clipping
  • Re-adjust the exposure by the sky Ev you have chosen
  • Enable CHDK Alt mode and select the script if not already selected
  • Press the shutter and let the script do its work

In this case the script captured the following image set:

Here we see the 5 focus brackets, covering out to 4 x the hyperfocal, and a sixth bracket at 4Ev down from the land exposure, that 'ensures' we don't blow out the sky, also taken at 4 x hyperfocal.

Post processing such an image set is up to you, but the way I did it for this test is to:

  • Adjust and sync the five land captures in Lightroom
  • Do a round trip to Helicon Focus
  • Export the HF processed image and the sky capture as Smart Objects to Photoshop
  • Use luminosity masks to blend in the sky, having adjusted the exposure of the smart objects in ACR as requrired
  • Finish off as needed in PS and/or LR

Here is the finished test image, hardly a winning entry in a competition :-)

In conclusion, the new sky bracketing feature brings another useful capability to the Canon M3, and one that I will add to ML DOFIS as soon as I can.

As usual I welcome any feedback on this post or any of my posts.

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