Impressions from focus tuning

Started Apr 22, 2012 | Discussions thread
OP (unknown member) Veteran Member • Posts: 4,734
target distance differences

I did find differences based on target distance, but not in every lens. LensAlign tells you that, and while they recommend test distances for each lens, they also tell you that since there are differences, your best strategy is to test a lens at the distance where you most frequently use that lens. Other than studio-only lenses, that's almost an impossibility. I think that's why my adjustments overall were small - I tested at a couple different distances, and on the zooms different focal lengths and different distances, and only did an adjustment when I saw something really consistent.

The most striking thing was seeing how much influence on accuracy there is from which direction your focus is coming from, and from how far the lens must move to get to the focus point. If the lens only has to move a little to hit the focus point, I got really high consistency. If it goes from really close to 25 feet, I got low consistency, even with AF-S lenses, although the non-AF-S lenses had the biggest variations. I'd always known that, and when I'm shooting relatively static subjects I've always used two half-presses on the shutter before shooting when I'm wide open. Still, it's a little startling to see.

Judging the need for adjustments is made harder by the fact that at the distances LensAlign recommends for each lens, blowing the image up to over 100% to look for accuracy almost always results in pixelation with the 12mp D700. Moving the target closer may get you better visualization, but unless you shoot that lens that close much of the time, you may adjust your way to worse results.

I understand the prevalence of f2.8 lenses now... there's enough DOF slop there to handle the variations in focus locks.

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