AF Microadjustment versus temperature

I've accumulated some data that shows that AF microadjustment is a function of temperature. The attached plot shows the microadjustment value decreasing with increasing temperature. I've also seen a similar downward trend with my 300 f/2.8 + 2x TC III. Air temperatures were 5 to 7 degrees lower for the low temperatures and 2 to 3 degrees higher for the internal temps near 30 C.

I'm using these methods in the field to do my microadjustments: http://www.clarkvision.com/articles/microadjustment/

Roger

Hi Roger, there is no image showing for me.

Quote:

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Originally Posted by Jerry van Dijk

Hi Roger, there is no image showing for me.

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Hmm, not sure why you wouldn't see it. It is a gif image. I've attached a jpeg. I also added it to my article on microadjustment in the field at:
http://www.clarkvision.com/articles/microadjustment/

Roger

I've had some offline questions about the data and the trend. Here are some Q&Aa:

In reviewing the date there is something which struck me as odd and I wanted to run it by you for your consideration.

Basically, there are three groups of data points:
Group I: CIT between 6 - 10C
Group II: CIT between 12 - 20C
Group III: CIT between 28 - 30C

What's remarkable to me is that within each group the MFA is very consistent despite the fact that the temperature spread within Group II for instance is quite large. In fact, the temperature spread within Group II is larger/same than between the averages of Group I and II.

Of the 3 groups, really only the middle group has more spread
than the trend would indicate. This is more a function of my
resistance to changing the microadjustment value unless I saw a
clear indication that I should in the field. In reviewing images,
it looks like the higher tempperatures in group II might have
benefited with a slightly smaller microadjustment value, consistent
with the trend.

I presume that the data points for each group were obtained within one experimental series and hence the data in Group I-III represent three different outings in the field. Given the consistency of the MFA within each group I am wondering if the differences between the groups may reflect something other than differences in the CIT but changes in the experimental settings, e.g. subject distance. There may of course also be other factors at play.

Actually, each point mostly represents a single outing (only a couple of
points were from the same day). The trend covers outings from march
through May, to the same group of newborn owls. Distance to the owls
was virtually the same for all the points. The last two (warmest points)
are the exception. The baby owls flew the nest, so those points are for
a different target but the same distance.

Further, both camera/lens setups you observed a "MFA temperature dependence" with employ the 2x TC III. Do you not observe such dependencies with the bare lenses?

I don't yet have enough data to confirm other situations, but I
will work toward getting such data. If the variation originates
in the camera, as I suspect, then all lenses would show it.

Have you investigated a distance dependence of the MFA? At dpreview there are some folks claiming that MFA changes with distance but it's just anecdotal and I haven't seen any data.

I have not investigated other scenarios yet. A distance dependence
might occur with lenses having spherical aberration that varies
with focal distance (many lenses will show such aberrations as the
subject moves closer). The question is will it be enough
to show a microadjustment change? Hopefully not.

I am getting excellent focus on the Moon and sun with my microadjustment
values derived on owls, so that implies I'm not seeing a distance
dependence.

Roger