observations of AF micro-adjustment in low light

Last night I decided to have a look at the AF micro-adjustment on my 400DO/1DIV combo. This was all done in the house under relatively low incandescent light. I set a target up about 20+ feet away and made some comparisons with different adjustments. I was amazed to find that under these conditions the rig was quite severely front focusing and required a +15-20 adjustment to make the images sharp. This was consistent with the bare lens and the 1.4 and 2 x TCs, and repeatable.

This morning under much better light I repeated the experiment and the +15-20 micro adjustment requirement has vanished and I'm back to close to where I was, about +5). Could low, warm light cause this effect? Anything else?

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Originally Posted by John Chardine

Last night I decided to have a look at the AF micro-adjustment on my 400DO/1DIV combo. This was all done in the house under relatively low incandescent light. I set a target up about 20+ feet away and made some comparisons with different adjustments. I was amazed to find that under these conditions the rig was quite severely front focusing and required a +15-20 adjustment to make the images sharp. This was consistent with the bare lens and the 1.4 and 2 x TCs, and repeatable.

This morning under much better light I repeated the experiment and the +15-20 micro adjustment requirement has vanished and I'm back to close to where I was, about +5). Could low, warm light cause this effect? Anything else?

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Hi John,

What was the temperature different? You dod not say the temperature of the second measurement. Light level should play no roll, but I have seen shifts with temperature. Also, was the temperature stable, meaning had the system been in a different environment and then was warming or cooling when you made the measurement?

Roger

The diffractive optics in that lens means contrast in lower than usual, so in the case of low/flat light contrast difference may not be enough for the AF sensor so it causes a false front lock. Temperature is a factor but in my experience MA does not show significant change in a temperature range of low 20F to high 80F (MK4 + 300 f/2.8 and 500 f/4 lenses w and w/o TCs).

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Originally Posted by arash_hazeghi

Temperature is a factor but in my experience MA does not show significant change in a temperature range of low 20F to high 80F (MK4 + 300 f/2.8 and 500 f/4 lenses w and w/o TCs).

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Arash,
Interesting. That is exactly the opposite with my experience with the same gear (1D4 + 300 f/2.8 and 500 f/4 and TCs). I'll start recording conditions when I do MA. (assuming I have time when on location).

Roger

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Originally Posted by Roger Clark

Arash,
Interesting. That is exactly the opposite with my experience with the same gear (1D4 + 300 f/2.8 and 500 f/4 and TCs). I'll start recording conditions when I do MA. (assuming I have time when on location).

Roger

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Thanks Roger, please let us if you find a difference.

BTW, I was never happy with the performance of the DO with the TC.

Thanks guys. Interesting thoughts. The adjustment in the evening was done indoors so temp was roughly 20°C. The temp of the light was warm (incandescent). The redo the next morning was done in the same location and indoor temp under brighter light from the window and a halogen ceiling light. If I get a chance I'll try it again tonight to see if I can duplicate the front focusing.

My 400DO is a really good performer and I'm very happy with the results bare and with the 1.4 tc. The files do not look like what I produced with my 500/4 out of the camera but I can get very close. After processing you would never know where the image came from.