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Telephoto Lenses with focal lengths longer than 60mm.

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Old 13th June 2019
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Tordan58 Tordan58 is offline
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4/3 telephoto lenses AF calibration on m4/3 bodies

Hi,

I resumed my "AF calibration project" and put additional lenses to the test. (You can read about the method in this post: http://e-group.uk.net/forum/showthread.php?t=50698

I calibrated the zoom lens at the long end 200mm) only, will calibrate at the short end, time and enthusiasm pending.

For each lens/camera combination the results are based on 5 series of 10 shots at different calibration values, in order to reduce the impact of variations/imperfections in the AF system. I also assessed each frame twice to mitigate incorrect assessments from my part.

Calibration distance was done at 25 times the focal length, in line with the recommendations I could find on Internet.

The final results are found below.



Some thoughts...
  • The E-M1 Mark1 and Mark2 perform differently. In most cases the Mark1 has better accurracy than the Mark 2 although there are exceptions when used with the 50-200 SWD. (I was expecting the Mark2 to be better and this surprised me)
  • All lens/camera combination suffer from back-focusing to lesser or larger degree.
  • The most accurate lens prior calibration is the Canon EF 400/5.6.
  • The two Zuiko lenses both have about the same amount of backfocus issues without TC.
  • Using teleconverters usually seems to amplify the backfocus issues, with one exception (Zuiko 50-200 with EC14 on Mark 2). It would make sense thinking that adding extra glass in the path would worsen the optical performance and the AF accuracy however I have no insight into this. With the EC20 on the 300/F2.8 it becomes very pronounced, the AF calibration can barely compensate for this, I had to push it to the limit (-20) to offset the issue. I did not calibrate the the 50-200 with the EC20 as I do not intend to use that combo.
  • Statistical standard deviation is in the range of 0.3 cm for the most consistent lenses/cameras and up to 1.6 cm for the least consistent. Does not sound that much but looking at the samples the min-max variations were as much as 7 cm for a series of 10 shots when using TC on the Zuiko 300/2.8. Half of that is 3.5 cm, peak deviation from average value, which is alarmingly close to what the depth of fields covers. Takeaway: take several shots, allowing the AF to to re-acquire focus when using TC.

Any comments or questions are welcome.

Cheers
Tord
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