Monday, March 29, 2021

More on Deep Focus Bracketing with the Canon M3

In a previous post I discussed the addition of sky or infinity bracketing in the Canon M3, ie at the end of a deep focus bracket, based on the logic that in a landscape the sky will be the brightest object and that this doesn't need focus bracketing, ie it will be at 'infinity'.

In this post I'll discuss the latest M3 Bracketing script's updates and provide an example of using the script.

The latest version of M3 Brackets may be downloaded from the link on the right. This version has the following new features:

  • Sky or Infinity bracketing may be undertaken at either 3, 4 or 5Ev, relative to the land or foreground, ie this feature may also be useful for real estate photography where one needs to exposure for inside as well as outside.
  • You can now switch on a simple logging feature, eg to record the focus distances of your focus bracket set. Logging is either on or off. Logging is recorded/appended to a single file 0001.txt in the CHDK log folder. You can reset the log file, ie delete the contents, from the M3 Brackets log menu. Once you have reset the log file you will need to (re)select your logging option before using the script.
  • M3 Brackets, once initiated via a CHDK full shutter press, will pause and tell you how many focus brackets will be taken. At this point you can terminate the script with a full shutter press, or continue with the script with a half shutter press.
  • Finally, a word on M3 and CHDK. To eliminate the CHDK screen flashing, simply cycle through the Canon INFO button until things stabilise, eg histogram not showing.
The following is a test/demo of the script. The scene was taken indoors and didn't require a sky (exposure) bracket. The camera settings were: focal length 14mm, ISO 400, f/8, at 0.6s.

Logging was switched on and the following log created:

Mon Mar 29 11:34:48 2021
Start @ 150mm
2/11 @ 157mm
3/11 @ 167mm
4/11 @ 179mm
5/11 @ 196mm
6/11 @ 219mm
7/11 @ 253mm
8/11 @ 306mm
9/11 @ 404mm
10/11 @ 640mm
@H = 164cm
@4H = 658cm

Here we see the log record starts with date-time of the start of the capture, so you can find your image sequence in post, and focus positions (requested) for the focus bracket locations. In the above we see that 12 images were captured, included one at the requested infinity focus of 4xH, giving an infinity defocus blur of a quarter of the overlap blur that was used for focus bracketing.

The M3 Bracketing script uses a version of the DOFIS split/thick lens model, albeit ignoring the pupil magnification, which is compensated for by being aware of this and choosing a suitable overlap defocus blur, 15um in this case.
After ingesting into Lightroom the image capture stack looks like this:

Here we see the script's bookend feature, which creates a dark frame at the start and end of the capture.

After basic exposure tweaking of one of the images in LR, ie ready for focus merging, I synced those settings across the focus stack. I then did three round trips. Two to Helicon Focus to explore two different blending strategies; and a third to Photoshop. The results, after common post processing for a bit of 'pop', being:

The first image above is processed using Helicon Focus's method B, the second is using HF's method C, and the last image using Photoshop's focus blending feature.

None of the above were focus tweaked/masked and thus, if you zoom in, you can see 'artifacts' in the focus blending, usually where there are focus 'gaps' in the scene, ie at edges. Having said that, I'm still impressed that my secondhand M3, running my in-camera CHDK M3 Bracketing script, is able to support my deep focus photography using an APS-C size sensor.

As usual I welcome any feedback on this post or any other post on my blog.

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