Sunday, July 5, 2020

Quick update on Deep Focus Photography

As I'm still shielding, because of C19, and am also recovering from a broken leg and surgey, I thought I would take a quick relook at my auto deep photography experiments.

Currently I have auto deep focus capture running on the following cameras:
  • Canon G1X - via a CHDK script
  • Canon G5X - via a CHDK script
  • Canon G7X - via a CHDK script
  • Canon EOSM3 - via a CHDK script
  • OMD EM5II - via the in-camera functionality
I'm going to ignore the OMD in this post,  as it relies on a propriety solution, and is not optimised for landscape deep focus capture.

The G5X and G7X have the smallest sensors, both using a "1 inch" format sensor, with the G1X upping the size to "1.5 inch" format.

This leaves my EOSM3 camera as the only APS-C format that runs one of my CHDK auto focus bracketing scripts. The latest version of the script can be downloaded from the link on the right: M3 Brackets. 

Once you have CHDK running on the M3, the script is simple to use, with the following UI:
  • Select the focus bracketing logic, ie OFF allows you to 'just' exposure bracket; X2INF focus brackets from the current position to infinity; and Min2INF covers to entire focus of the lens, irrespective of where you are focused;
  • Select what ETTR-based exposure bracketing logic you wish to use at each focus bracket. The options are: none or +4Ev or +3Ev or +2Ev or +2/+4Ev or ISO1600. Alternatively, for the M3 version, use Auto and set the base exposure for the shadows. The script will then take as many +2Ev exposure brackets as required;
  • Select your infinity focus, ie the last shot in the bracket sequence, at the overlap defocus blur divided by either 2, 3 or 4;
  • Select the overlap defocus blur in microns, between 5 and 20, with 15 being a reasonable number to use on an APS-C sensor;
  • Select if you wish to use a start delay time in seconds;
  • Select whether you delineate the bracket sequence with dark frame 'bookends';
  • Select whether the LCD is off during capture;
  • Select a so-called nudge distance to ensure the script doesn't freeze at the macro end. The default is 10mm;
  • Select a lens thickness to be used, in the split thin lens model, if you haven't explicitly coded a lens into the script. The script has the 11-22mm lens encoded, but you can add other lenses. Unless you know what you are doing, leave the lens thickness at zero and use a thin lens model;
  • Select 'Get Lens Name' to allow you to programme your own lens thickness.
As to some test results, I set the 11-22mm to 11mm, F/7.1 and ISO100, at an exposure of 0.5s. I used the Min2INF option and the script took 13 images, which I post processed in Lightroom, with a round trip to Helicon Focus.

Using Jeffrey Friedl's Lightroom “Metadata Viewer” Plugin, it was easy to extract the following (lower) focus distances from the EXIF data:
  • 0.15m
  • 0.16m
  • 0.17m
  • 0.18m
  • 0.19m
  • 0.21m
  • 0.24m
  • 0.25m
  • 0.33m
  • 0.45m
  • 0.71m
  • 1.54m
  • 3.84m
The resultant, deep focus, 'post processed' image looks like this:

Hopefully this short post has illustrated the power of CHDK to automatically create deep focus images on an APS-C cropped sensor, ie the EOSM3.

As usual I welcome feedback on this post.

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