Calibrating Focus Mask tool

Kit Laughlin

• March 31, 2011 21:51

On the Luminous Landscape C1Pro5 tutorial, Walter Borchenko states that the Focus Mask tool's default thresholds are for Phase backs, with no AA filter. He states that some DSLRs will need to have the Focus Mask thresholds adjusted to reflect the difference in focal plane of cameras with AA filters.

I have noticed that the Sony A850 images which can look perfectly sharp on a 30" monitor may or may not display the mask when enabled. Now that I have a 'faux-tethered' way of shooting the A850, I would like to know at a glance which images are properly focussed from where I am shooting—and the Focus Mask will be perfect for this if I can reply on it accurately showing the plane of acceptable sharpness.

Can anyone post a method for adjusting the thresholds? TIA, KL

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  Paul Steunebrink

  • April 01, 2011 06:58

  Look for some images which are properly focussed, with different apertures preferably. Open them in CO6, activate Focus Mask. Next, open the Preferences, Focus tab, and drag the Threshold slider until you are satisfied with the result.
  
  You can also adjust opacity of the mask and its color in the same screen.

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  Kit Laughlin

  • April 02, 2011 09:42

  Paul; thanks. What, exactly, is the threshold an indication of—how does the program see "sharp"?
  
  Let me reframe the question: if we see any of the green Focus Mask, we can assume that this area is acceptably sharp, based on an internal criterion of the programs', and adjusting Threshold simply delivers less of that range as we adjust it up, or a broader range if we pull it down?
  
  I am trying to get a handle on what the program is understanding in an image by changing the its (the tool's) thesholds.
  
  TIA, KL

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  Paul Steunebrink

  • April 02, 2011 13:46

  [quote="Kit Laughlin" wrote:
  ...
  Let me reframe the question: if we see any of the green Focus Mask, we can assume that this area is acceptably sharp, based on an internal criterion of the programs', and adjusting Threshold simply delivers less of that range as we adjust it up, or a broader range if we pull it down?

  
  Yes, at least that is how I think it works! 😉

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  Gareth

  • April 04, 2011 08:08

  AFAIK the focus mask in simple terms looks for contrast in the image.
  
  The more focused the light is the more edge contrast is will have. Hence when your using a DSLR with an AA filter, the preferences have to be adjusted as the light hitting the sensor is slightly fuzzed to avoid moire.
  
  Excuse my rather crude analysis of a rather more complicated process! 😕

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