Focus Repeatability

Started Dec 5, 2005 | Discussions thread
Wally OZ
Veteran MemberPosts: 5,194
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Re: Focus Repeatability
In reply to Takeda, Dec 6, 2005

Takeda wrote:

Wally OZ wrote:

RLD wrote:

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Here's a couple thoughts...
1) When you defocused did you try both directions, and did it matter?
2) Flourescent lights should be flickering at 120 Hz. In other
words, get dimmer as the AC power goes through 0, and brighter on
the + and - half cycles of the 60 Hz sinewave. How does this effect
the autofocus sensing mechanism? I would think that going from
bright to dim to bright every 8ms could interact with the autofocus
control loop. Is this a known phenomenon?

I have noticed that when shooting a burst of shots under
fluorescent lighting, the exposure and color varies between shots,
so it does have an effect, nothing else was altered.

3) Try incandescent lighting. The filament will respond too slow to
flicker at 120 Hz. If incandescent lighting gives consistent focus
it would support the idea of foourescent flicker as the cause of
your focus problem.
I'll be interested to see how this discussion plays out.
RLD

Yes, I tried de-focusing in both directions. There maybe something
with the pulsating light from the fluorescents. Even though I have
the exposure "fixed", there does seem to be a slight difference to
the exposure across the 10 pictures.

That sort of points the finger at the light source a bit, it is possible that the AF is getting confused under fluorescent lighting.

I am convinced that if you want to test focus you have to do it in daylight else you will go nuts trying to figure out the inconsistent artificial light results.

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