I feel misled: Panasonic 20mm f1.7 II AF is SO SLOW!

Started 3 months ago | Discussions thread
micksh6
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Re: I feel misled: Panasonic 20mm f1.7 II AF is SO SLOW!
In reply to Anders W, 3 months ago

Anders W wrote:

micksh6 wrote:

Why do you feel mislead? Of course it's slow and it has been shown here long time ago. You just needed to look at the right tests:
http://www.dpreview.com/forums/post/52045407
http://www.dpreview.com/forums/post/52049352

A "controlled" test can also show that 20mm focuses much slower than other lenses and hunts more often too. It's not needed to refocus from close-up to infinity in order to see that. These tests show refocusing between about 5 and 7-8 feet.

Those tests are not controlled. They are biased in ways and for reasons spelled out in the thread to which you link. That's why they are not the right tests.

I didn't start this thread stating that 20mm is slow, so you might as well tell OP that he is biased and the lens is actually fast.

Whether you like my tests or not they showed exactly what OP is seing now. Your tests, on the other hand, showed that 20mm is as fast as other lenses.

BTW, a year ago I replied to your post with test results which you linked today: "the claim that 20mm focuses as fast as other fast primes is just plain wrong and it can mislead people". http://www.dpreview.com/forums/post/51100467

Perhaps, this is why OP feels misled now.

I think a curved focus target attributes to slow AF with this lens.

Any lens will have greater difficulty focusing on a curved than a non-curved surface since for the former there is no single point where the entire surface contained within the focus box can be in perfect focus. But on what grounds would the 20 have greater difficulty focusing on a curved surface than other lenses of similar FL?

This is what tests show. Explanation of test results can be a different topic.

Some time ago I suggested that it might be because of noisy focus motor. Noise is caused by vibration. Vibration also leads to blurred images, and it's much harder for camera to find peak contrast on blurred images. Peak contrast of curved surface is already is harder to determine than of flat surface. And if some contrast samples are impaired by blur this can make AF fail, or it can lead to slow AF.

Because of that 20mm lens may require more AF iterations, and it's more likely to start hunting than other lenses in the same conditions.

This isn't happening in good light, but in low light exposure time during autofocus becomes longer and if it's around 1/125-1/250s the quality of AF samples is very likely to be affected by low-frequency vibration (around 200Hz or so).

You know very well about shutter shock. You can think of this as focus motor shock. You can feel vibration when 20mm is focusing, and camera is taking images when doing it.

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