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04-06-2014, 12:39 AM   #1
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Lens Calibration

What is generally considered the best setting and distances for calibrating lenses?


The lenses in question are:


DA* 300 F4
Sigma 70-200 F2.8 EX DG OS HSM
DA 55-300 F4-5.8 ED
DA 16-45 F4
DA 12-24 F4 ED AL
D-FA 100 F2.8 Macro


Presumably with the primes wide open at their shortest distance? The Zooms at what?


All of the lenses are excellent but I'd just like to check

04-06-2014, 12:43 AM   #2
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The degree of FF/BF may vary depending on the focal length on zooms, so I would test them at both ends and somewhere in between as well, but only if you're experiencing AF issues.

The general guide is here:
Fixing Front and Back Focus - Introduction - PentaxForums.com

And yes, wide-open is best as that gives you the narrowest DOF.

Adam
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04-06-2014, 06:32 AM   #3
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Distance should be no less than 50 times the lens focal length. For example, a 50mm lens should be checked at a distance of not less than 2.5 meters. See this site for more info: AF microadjustment for the 1Ds mark III, 1D Mk3, 5D Mk2, 7D, 1D X
04-06-2014, 08:53 AM   #4
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Best distance is the one you generally shoot at. I don't know about telephotos, but my experience with a 55mm and a 28mm lens is that it can look perfect at 'lens chart' distance, and be quite back focussed at landscape distance.

04-07-2014, 05:06 AM   #5
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QuoteOriginally posted by Adam Quote
The degree of FF/BF may vary depending on the focal length on zooms, so I would test them at both ends and somewhere in between as well, but only if you're experiencing AF issues.

The general guide is here:
Fixing Front and Back Focus - Introduction - PentaxForums.com

And yes, wide-open is best as that gives you the narrowest DOF.



QuoteOriginally posted by MPrince Quote
Distance should be no less than 50 times the lens focal length. For example, a 50mm lens should be checked at a distance of not less than 2.5 meters. See this site for more info: AF microadjustment for the 1Ds mark III, 1D Mk3, 5D Mk2, 7D, 1D X




Thanks very much. I'll try them out
04-07-2014, 09:13 AM   #6
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QuoteOriginally posted by johnyates Quote
Best distance is the one you generally shoot at. I don't know about telephotos, but my experience with a 55mm and a 28mm lens is that it can look perfect at 'lens chart' distance, and be quite back focussed at landscape distance.
I my experience setting up AF calibration at infinity works best with any lens - all those gizmos are a waste of time.
04-07-2014, 03:09 PM   #7
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QuoteOriginally posted by johnyates Quote
Best distance is the one you generally shoot at.
+1

It is typically not possible to find an AF micro-adjustment that works for all focus distances, and in the case of a zoom, for all focal lengths.

That's why Sigma's lens dock allows you to calibrate different distances and focal lengths individually. If one AF adjustment value were fine for all combinations of focus distances and focal lengths, Sigma wouldn't make you fill in a 2D matrix for all combinations.

Unfortunately, for some lenses, the optimal adjustment also depends on the f-ratio used (due to a phenomenon called "focus shift").

So while wide-open, and the highest focal length would make it easiest to see the focus error, optimising the AF adjustment value for this case may mean that you may not get the best results for your most frequently used f-ratio and focal length. A similar argument can be made for the distance. What good is it, if you optimise a portrait lens for infinity?

You may want to look at the AF adjustment hints I compiled.
04-08-2014, 12:59 AM   #8
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QuoteOriginally posted by Class A Quote
So while wide-open, and the highest focal length would make it easiest to see the focus error, optimising the AF adjustment value for this case may mean that you may not get the best results for your most frequently used f-ratio and focal length. A similar argument can be made for the distance. What good is it, if you optimise a portrait lens for infinity?
The lens will communicate to the camera the lens ID, the set focal length (if it is a zoom) and the focus distance zone (read from the slider contacts inside the lens barrel). The camera already knows the f stop. The camera processor should then apply the relevant corrections - firmware look-up table stuff.

All that the focus 'calibration' is meant for is to allow for offsetting of physical tolerances of the mount flange, either of the camera (global setting) or of the lens itself.


Last edited by kh1234567890; 04-08-2014 at 01:26 AM.
04-08-2014, 08:57 AM   #9
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QuoteOriginally posted by kh1234567890 Quote
The camera processor should then apply the relevant corrections - firmware look-up table stuff.
That's not quite right since the respective correction values are not pulled from the (camera-)firmware but from the lens EEPROM.

Everything would be great if the values in the lens EEPROM (which differ from copy to copy) were 100% appropriate. In this case one adjustment per lens would be sufficient.

Unfortunately, that's not the case. Evidence:
  1. There are lenses that you optimise for close focusing that then suck for infinity or vice versa.
  2. Sigma's dock allows you to modify a whole matrix of values (not just one value per lens).
04-08-2014, 02:01 PM   #10
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QuoteOriginally posted by Class A Quote
That's not quite right since the respective correction values are not pulled from the (camera-)firmware but from the lens EEPROM.

Everything would be great if the values in the lens EEPROM (which differ from copy to copy) were 100% appropriate. In this case one adjustment per lens would be sufficient.
I think that you are presuming a level of sophistication which Pentax lenses do not have. It is all 1980's technology. No individually programmed EEPROMs.
QuoteOriginally posted by Class A Quote
Unfortunately, that's not the case. Evidence:
  1. There are lenses that you optimise for close focusing that then suck for infinity or vice versa.
  2. Sigma's dock allows you to modify a whole matrix of values (not just one value per lens).
Evidence for Pentax lenses (beside anecdotal or 'it says so on t'Internet') ?

Last edited by kh1234567890; 04-08-2014 at 02:28 PM.
04-08-2014, 06:47 PM   #11
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QuoteOriginally posted by kh1234567890 Quote
I think that you are presuming a level of sophistication which Pentax lenses do not have. It is all 1980's technology. No individually programmed EEPROMs.
The patent you referenced not only describes a lens with a ROM but also with a CPU. Quote:
"...the lens CPU 66 has a function which receives the quantity (amount) of defocusing detected by the camera body, references data stored in the ROM 67, computes the amount of drive force, detects the amount of drive force by the AF pulser 68, and drives the AF motor 60; "
The EEPROM of the DA* 50-135/2.8 not only contains information about it being an SDM lens (which people modified to turn it into a screw drive lens), but also multiple AF adjustment values for various focal lengths and distances. The precise locations for these values are known.

QuoteOriginally posted by kh1234567890 Quote
Evidence for Pentax lenses (beside anecdotal or 'it says so on t'Internet') ?
The phenomenon is real and Roger Cicala from LensRentals.com reported about it. People have reported about their respective problems with Pentax lenses and at least one has fixed his issues by patching the EEPROM of his DA* 50-135/2.8.

EDIT: Here's evidence pertaining to a Pentax lens (DA 21) that is beyond anecdotal. In a poll regarding the focus shift between f/5.6 und f/6.3 of the Pentax DA 21, ~75% of all DA 21 owners observed the problem.

Note that numerous users reported that the observed focus shift disappeared once they replaced the lens EEPROM data with data from another DA 21 copy (that did not exhibit the focus shift).

In the same thread I referenced above, someone reports that their focus shift depends on the focus distance. You'll find more of such reports, if you look for them.

Last edited by Class A; 04-09-2014 at 06:53 AM.
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