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----- Original Message -----
Sent: Monday, December 16, 2002 4:08
PM
Subject: Focus bracketing (was: Re: fa
85mm 1.4 vs 77mm limited)
Hello Dario,
my experience is that focus bracketing is
absolutely essential. 4 times I have done comparative lens resolution tests
(Only my comparison of 77 and 85s is in the WWW so far, see http://www.arnoldstark.de/aufloesung85er.htm).
My first test (without focus bracketing) would have led me to believe that the
FA50/f2.8 Macro (just replaced by the F50/f2.8) was inferior to 6 other
Pentax normal lenses. A newer test with focus bracketing proved that the
opposite was true, and that it is only particularly difficult to focus the
FA50/f2.8 (and most AF lenses due to their small turning angle of the focusing
ring) perfectly.
The problem with not focus bracketing is that the
result of such test will tell you the perforamnce of the whole system: Focus
system + lens + film. If you do focus bracketing, you eliminate the dependance
on the quality of the focusing system, thus one gets the performance of lens +
film. In my experience with several cameras, a lot of focusing aids/systems
are not perfectly aligned.
If one does focus bracketing, one gets one
picture with optimum performance, and the pictures around it prove that this
really was the optimum performance. You have a proof, not just a probability.
For my tests with focus bracketing, I choose about 6 (10 would even be better)
focusing positions at and very very near the position with a not-split image
in the viewfinder. Neighbouring positions typically give 10% difference in
resolving power at the image center. Sometimes, due to lack of image flatness,
the resoltion goes up in the corners while it goes down in the centre.
However, the centre resolution is what determines the "best"
focusing.
I believe that a lot of contradictory lens performance
experiences can be easily explained by imperfect
focusing.
Arnold
Dario Bonazza 2 schrieb:
just one question about your testing procedure: Do you do focus
bracketing? I found careful focus bracketing to be quite important in my
own tests.
Most previous tests were done by Carlo Lastrucci, not by me (with the exception of the 24-90mm, published in Spotmatic No. 30, October 2001, that we made together). Then I commented Carlo's pictures on Spotmatic magazine. No, we usually don't do focus bracketing, since most people in most pictures don't. I'd appreciate your further comments on this.
Carlo's tests were done focusing with MZ-5 autofocus, with the exception of the latest comparison (35/2 FA vs. 31/1.8 Ltd, published in Spotmatic No.34), where focusing was manually adjusted by looking at the split-image in MX viewfinder.
My test about the 24-90 was done with MZ-S, autofocusing on subject (either infinity or mid-distance) always going farther away from a closer subject (hence reaching focus from close distance). I was thinking of doing the same this time. Suggestions are also accepted.
Bye,
Dario Bonazza
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http://www.dariobonazza.com
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