Sunday, July 2, 2017

I wouldn't start from here if I were you

I think all of us know the old joke that is mentioned in the title of this post. Well doing focus stacking also brings that joke to mind. Do I start at infinity and work towards the foreground, our start in the near field and bracket towards infinity?

If you are using my auto focus bracketing script you only have one option: focus on the nearest thing of interest and the script will automatically fill in the required focus brackets to cover you to infinity.

If you wish to have more control over focus bracketing, then focus bar will allow you to first of all start in the far or near field, or anywhere. From this first image, you can then bracket towards or away from this first point, and decide on the amount of focus overlap you want. I personally look at 10-20% of the DoF, but often am limited to what I can do as Canon lens control is not always working in your favour.

For landscape focus bracketing I personally would suggest the following workflow using the focus bar; because, if you start at infinity, you don't always know when your near field point of interest is in focus:
  • Set your non-focus camera settings, eg shutter, iso and aperture;
  • Finalise composing the scene;
  • Focus on the closest point of interest you wish to see in focus;
  • Take an image with your (closed) hand in front of the camera, ie creates a starting 'bookmark' image;
  • Take the first image (using a remote shutter trigger to minimise camera-tripod movement);
  • Rotate the lens focus ring towards infinity and stop when the magenta overlap meets your needs, eg about 10-20% overlap;
  • Take the next focus bracket and repeat until your end point has been met, eg infinity;
  • If infinity is your goal, keep focus bracketing until the focus bar switches into 'blur reporting mode', ie when the far DoF is beyond infinity, but the focus point is less than infinity. In blur reporting mode, adjust the final focus bracket until your infinity blur criterion is met, eg between 15-30 microns on a full frame camera;
  • Take a final image with your (open) hand in front of the camera, ie creates an end 'bookmark' image.
As for post processing: I personally ingest the bracket sequence into Lightroom, minimally adjust one image as required (don't do any lens correction at this stage), and sync the others to this one. Then do a round trip to your focus stacking software; and finish the final image off in LR and Photoshop as required.

As a test example, here is a final focus stack of five Dual-ISO images, taken with my EOSM at 11mm and at a non-optimum aperture of F/4.5. I decided the corner of the chair was my nearest point of interest.

BTW if you are interested in the focus distances, and forgot to write them down, you can look into the EXIF data of your .cr2, ie under Maker Notes. I use RawDigger if I wish to look at an image's details, including the EXIF. For example, the first image's EXIF reports the following:
Focus Distance Upper
0.36 m
Focus Distance Lower
0.33 m

Looking on line, it's not fully clear what Canon means by upper and lower focus distances. But at least the info is indicative of where you focused. However, on the limited images I have looked at, the upper value appears to be close/correspond to the ML reported focus distance.

Bottom line: the focus bar is a very flexible tool for those seeking to get optimum focus, either in a single image or in a focus stack. The UI is clear and you remain in charge throughout the capture.

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