D800 - focus under incandescent light

Started Feb 24, 2013 | Discussions thread
jtra
Contributing MemberPosts: 625
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Re: Dependence on AF position
In reply to Daniel Lauring, Mar 14, 2013

I have run into this problem few years back with my D90 and friend's D300.

We both have Sigma 30/1.4, and both were focusing worse on her D300. I found out it was cased by her flash configured to assist focusing by red light beam. When this red beam was turned off, it started working better. Then I had idea that red light might have different focus error than regular white light and that could be reason for worse focus in orange light emitted by CFL bulbs.

I have verified this with my D90 and Sigma primes for central AF point using these colored focus charts (taking pictures of monitor at 45 degree angle on tripod) with black background I made (based off chart mentioned in text, however that site no longer works):

http://jtra.cz/foto/focus-shift-by-color-charts/

So my Sigma primes (30/1.4, 50/1.4) were off based on light color on which they focus. My Nikon zoom 18-105vr was ok. I don't have Nikon primes. So I assumed that Nikon cameras likely have a lens tables that tells camera how to correct for AF when RGB exposure sensor matching AF point detects too red light. Otherwise it would had to be known problem. Such tables could be completely wrong for third party lenses. So fact that you see this with Nikon primes is surprising to me and could mean that there Nikon cameras are not compensating for that at all.

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