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10-21-2015, 08:26 AM   #1
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AF adjustment prime V zoom

One aspect I prefer about primes is that the AF adjustment is just a single adjustment. Once measured, that's it. With zooms it isn't that simple. Both my zooms have adjustments ranging across 5 or so AF adjustment units. (I appreciate it does vary with focal distance, but this does seem to be a very small discrepancy on the primes I use)

I've just set up a new 50-135 and the 50 needs about +8 and the 135 around +4, and it's not linear. OK it's not always going to be a problem. but at the top end where the DOF can be very shallow, these adjustments at wide apertures can be critical. My other zoom, is the 16-50 and this ranges from +2 to +7 (default to +5).

So my question(s) is: as a mostly prime user, I'm interested to know what others do with their AF adjustments zooms ? Do you take a mean ? Or bias to the end you mostly use ? Don't care - use manual focus or Live View ? Or what ? Or maybe you remember the different settings and adjust in the field (I doubt it) ? Interested to know ...

Another related question. I'm lucky enough to have two K-3s. When I have set up the AF adjustments for my lenses I have found the best settings vary per lens, but are the same for both bodies. I originally assumed the adjustment was there to accommodate camera variations, but it seems that both K-3 are identical and it's the lenses that differ. Maybe the AF adjustments are for each lens/body pair and I've been lucky that both my K-3 are the 'same'. I guess this is my conclusion. Thoughts ...

10-21-2015, 08:38 AM   #2
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QuoteOriginally posted by BarryE Quote
best settings vary per lens, but are the same for both bodies. I originally assumed the adjustment was there to accommodate camera variations,
My assumption is it was both camera and lens. At any rate over several bodies all have had different adjustments, so maybe you got incredibly lucky. I do note that my k-3 and k-3II adjustments are very close and and any difference might be subjective in testing. This was not true for the k-5 series when I used those bodies, each was quite different. Maybe QC on the k-3 has tightened up?

QuoteOriginally posted by BarryE Quote
I'm interested to know what others do with their AF adjustments zooms ? Do you take a mean ? Or bias to the end you mostly use ?
I pick a middle range, or the range I am most likely to use with that lens and adjust for that. For what I shoot that is more than good enough. And if I am shooting very small DOF or very precise work I use a prime anyway. I rarely shoot a zoom at less than f/8 so the AF adjustments are not as critical for me on those lenses. Good enough really is 'good enough' @ f/11
10-21-2015, 08:52 AM   #3
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QuoteOriginally posted by BarryE Quote
So my question(s) is: as a mostly prime user, I'm interested to know what others do with their AF adjustments zooms? Do you take a mean ? Or bias to the end you mostly use ? Don't care - use manual focus or Live View ? Or what ? Or maybe you remember the different settings and adjust in the field (I doubt it) ? Interested to know ...
The two zooms I own don't need adjustments, but if I tested a zoom and found different degrees of AF inaccuracies at different focal lengths, I'd send it to the manufacturer with my camera to have them adjust it. I know that you can send your Sigma lenses to Sigma with your camera and they adjust it for free within a couple of days (at least in Germany).
10-21-2015, 09:37 AM   #4
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QuoteOriginally posted by BarryE Quote
Another related question. I'm lucky enough to have two K-3s. When I have set up the AF adjustments for my lenses I have found the best settings vary per lens, but are the same for both bodies. I originally assumed the adjustment was there to accommodate camera variations, but it seems that both K-3 are identical and it's the lenses that differ. Maybe the AF adjustments are for each lens/body pair and I've been lucky that both my K-3 are the 'same'. I guess this is my conclusion. Thoughts ...
All that the AF 'adjustment' does is to correct for differences in the mount flange thickness - either of the lens or of the body (the global setting). Trying to use it for anything else is futile. Focus correction data relating to focal length or focus distance zone comes from the lens rom (if the lens has one).

10-21-2015, 09:38 AM   #5
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QuoteOriginally posted by BarryE Quote
...So my question(s) is: as a mostly prime user, I'm interested to know what others do with their AF adjustments zooms ? Do you take a mean ? Or bias to the end you mostly use ? Don't care - use manual focus or Live View ? Or what ? Or maybe you remember the different settings and adjust in the field (I doubt it) ?...
For zooms, if you predominantly use one end of the zoom range adjust for that focal length. Otherwise, just average out the 2 ends, or go with the longer end since that's where depth of field is thinnest.

It's unfortunate that camera manufacturers still haven't put automatic AF tuning into firmware. The camera could independently cycle between contrast-detect and phase-detect, tweaking the AF adjustment each time, until the AF is calibrated.
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