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09-22-2014, 06:59 AM   #31
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QuoteOriginally posted by James78 Quote
Doesn't it mean body needs pro calibration?
Could be. I'd always found that a distant (infinity) target with a good vertical contrast (tree, building etc. against the sky) in good light works best and that anything else is a waste of time.

09-22-2014, 09:18 AM   #32
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QuoteOriginally posted by James78 Quote
p.s. all posts with embedded images updated to link to images now.
Sigh of relief...


Steve

---------- Post added 09-22-14 at 10:14 AM ----------

QuoteOriginally posted by kh1234567890 Quote
Could be. I'd always found that a distant (infinity) target with a good vertical contrast (tree, building etc. against the sky) in good light works best and that anything else is a waste of time.
I remember you offering this advice in the past and thought you might be on to something, so I gave it a try but had mixed results. Unfortunately precise calibration of infinity focus may be difficult or impractical depending on the lens. For example, on my Sigma 17-70 the difference between 30 meters and 1 km is less than one degree of arc on the focus ring and is evaluated by the AF system as being the same distance. I would suggest that the same thing might be accomplished by establishing a distance value of "distant"(3).* With many lenses this may not be that far away and might be easier to manage.

kh1234567890, do you have any recommendations?

For the OP, there are multiple considerations here:
  • The body may well benefit from a professional calibration
  • Whether the Rokinon is capable of returning sharp results (say using optical focus rather than focus confirm) has not been established
  • Focus sensitivity of the PDAF system (regardless of brand) is limited with fast lenses and is not adequate for fine focus at taking apertures wider than f/2.8


Steve

* There are anecdotal reports that the AF behavior is managed, in part, by subject distance. A distance value is shared by the lens at various points in its focus range and that value is included in the exif as SubjectDistanceRange. Defined values are:
  • 0 = unknown
  • 1 = macro
  • 2 = close
  • 3 = distant
For a given distance, the value may be different depending on the lens mounted. What is "macro"(1) for one lens might be "close"(2) for another. The exif specification does not provide guidance here. The default is "unknown"(0) for manual focus lenses.

It is unknown how a service facility calibrates AF. Most online instructions suggest a subject distance in the "close"(2) range. The Sigma USB Dock accessory (used for calibration of some model Sigma lenses) allows for in-lens calibration at "macro"(1), "close"(2), and "distant"(3) at several focal lengths (choices depend on lens). Apparently the matter of AF calibration may not be as easy as applying a simple bias.

Last edited by stevebrot; 09-22-2014 at 10:20 AM.
09-22-2014, 02:25 PM   #33
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I'm waiting for Pentax support to quote for body focus calibration. Since this was purchased from ebay Japan is it something that I should expect seller to take care of? I mean should I have known all this I won't have purchased with that price if ever. What is etiquette in these things?
09-22-2014, 03:17 PM   #34
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QuoteOriginally posted by stevebrot Quote
I would suggest that the same thing might be accomplished by establishing a distance value of "distant"(3).* With many lenses this may not be that far away and might be easier to manage.
That would be eminently sensible. The whole phase detect system AF is a kludge and is beginning to show its limitations. It's always been a bit unreliable with ultra-wides (eg the DA15 etc.). The OP original question digressed onto fast 50's though, which is what I was really commenting on.

We simply do not know enough about what what the lens communicates to the camera and how that information is used. I'm surprised that whenever someone gets the chance to talk to Pentax, the question of 'correct' AF calibration distance is never asked - the interviewer always seems to be more interested in FF rumours and fantasies.

09-22-2014, 05:07 PM   #35
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QuoteOriginally posted by kh1234567890 Quote
I'm surprised that whenever someone gets the chance to talk to Pentax, the question of 'correct' AF calibration distance is never asked
Perhaps if several of us were to suggest an FAQ article to Ricoh, they might share a recommended procedure. That would make things so much easier here. We could simply post a sticky pointing to the article


Steve
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