Wednesday, March 24, 2021

DOFIS: Sky Bracketing

In the last post I introduced a sky bracketing feature to the CHDK focus bracketing script that runs on the Canon M3. This new feature takes a perfect focus bracket set, from near to infinity, and then grabs a sky exposure, ie at infinity.

In this post I'm introducing the feature in to my Magic Lantern focus bracketing script: DOFIS; and in doing so removing the current auto exposure mode that operates with auto focus bracketing.

The logic behind sky bracketing is that many/most high dynamic range 'scapes' are differentiated by two main areas in the scene, ie the land/buildings and the sky. 

To cover the times when you need to bracket both exposure and focus for the land, and exposure for the sky, I created the idea of auto 'Sky Bracketing'.

By definition, a sky is at infinity and doesn't tend to have high frequency content that needs tack sharpness, ie we don't need to focus bracket for a sky. The sky focus is a constant once we are, say, beyond the hyperfocal.

On the other hand, near field objects of interest in the land, eg at less than the hyperfocal, will/may 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; noting that in ML we could also enable Dual-ISO.

The new DOFIS menu looks like this:

In the above example, I've set Auto Bracketing to Full mode and that it will be triggered by the RATE key on my 5D3, plus I've set diffraction aware to off. As I'm auto focus bracketing I've also set Traffic Lights to off, as I don't need them. IR frequency is off, as my 5D3 is a normal visible band sensor. The new Sky Bracket? menu item has been set to 4Ev; which means that, after setting an ETTR for the sky, the script will lift the focus bracketing by 4Ev, before taking an infinity sky bracket at the base (ETTR) exposure.

In the above example I also set a 2 second delay, ie after pressing the RATE button. Note  DOFIS has successfully picked up that I'm using a registered lens, ie OK is being displayed, so that DoFs will be calculated taking into account the location of the front principal and the pupil magnification. 

Finally, I've also added in a Bookend feature when auto bracketing, ie rather than taking a picture of your hand ;-)

The DOFIS sky bracket feature is only available when full auto focus bracketing is selected, when DOFIS will take the requested focus brackets up to the hyperfocal, an additional infinity focus bracket at 4*H, giving an infinity defocus blur of the overlap CoC/4, and then the sky bracket at 4*H.

As for a test subject for this post, I’m afraid it’s still my garden for me at the moment.

I composed and focused on a near field subject of interest, and ETTRed, thus setting the exposure for the sky, and, in this example, I requested a 3Ev sky bracket, as the sky was, unfortunately, pretty flat.

After pressing my auto focusing trigger key, the RATE button on my 5D3, DOFIS captured the following bracket set:

Here we see the two bookend frames, that help differentiate this bracket set. We clearly see five focus brackets, taken at +3Ev relative to the ETTRed sky, with the last two at around H and 4H. Finally we see the sky bracket, taken at the ETTR exposure for the sky and at 4H, ie infinity.

Once ingested into Lightroom, my post processing  workflow for a sky and focus bracket set usually as follows:

  • Select one of the focus brackets and make base adjustments, eg chromatic aberration, highlight recovery, shadow uplift, exposure balancing, but no lens correction until after a focus merge, if at all;
  • Do a round trip to to your focus bracketing software, eg I used Helicon Focus to create a focus blended TIFF;
  • Back in LR exposure balance the HF TIFF with the Sky bracket RAW, to facilitate merging the two images in Photoshop;
  • Export these two images to Photoshop as layers and align
  • Blend in the sky image using whatever means you prefer.  On this occasion I didn't need a luminosity mask, as simply masking in the bracketed sky exposure was all that was needed.
  • Note that in addition to blending in the sky, you can also blend in other parts of the sky image, eg blown out specular highlights etc.
  • Finally, adjust colour and tone and dodge/burn etc for taste.

 The final test image being:

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

Postscript: here is another test shot taken with the sky bracketing feature, this one with some structure in the sky:

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