Focus Fine Tune

Started May 13, 2014 | Discussions thread
EinsteinsGhost Forum Pro • Posts: 11,977
Re: Focus Fine Tune

stevo23 wrote:

I don't find it to be all that difficult or complex as you describe. I've managed to find critical focus fine tuning for a given lens in an hour or so. It's a lot of shooting and re-shooting to get it right, but if it takes a whole day, I think something else is wrong. Seriously, it's not and can't be a perfect science because lenses don't behave linearly.

I also think flat targets are useless. 3 dimensional subjects are best as that's what we shoot. Trying to determine focus from a flat, black and white target won't tell you what the lens will do in real life. You need all the colors and all three dimensions as that is what your lens will be seeing when you shoot a photo.

And therein lies the problem with AFFT. Manufacturers use flat targets for their calibrations, but utilize color in their focusing algorithms. 3-D targets are what we focus on, but what the camera actually does is come up with what it thinks is the focal point based on striking an average across the focus zone used. And that might not be what we thought we were focusing on. But starting with the flat target gets you quite close, and then you can tweak from there.

Admittedly, I was calibrating 4 lenses: 3 zooms, 1 prime, and FoCal is a statistical analyzer, and I was being anal retentive because I wanted to see the AF tuning spreads as well as get an AFFT coefficient. You can use DotTune or any of the other camera-based methods, and if you're using a Canon instead of the Nikon I am using, you have the wonderful advantage of fully automating the calibration process. Nikon doesn't give 3rd parties access to its camera control firmware like Canon does. But even with all that, I spent a couple of hours per lens. So not that far off from your hour or so.

Its not that complicated. And it doesn't have to be a 1.8 (fast) lens either. Case in point: Minolta 24-50/4. This mid 80s lens required MFA when I noticed backfocus on my Sony NEX. It didn't take long to figure out that at 24-28mm, the lens wasn't focusing where I wanted it to. A simple adjustment and now a non issue, at either end or in between. The camera/adapter remembers the lens, so put it on and go shooting with precise AF.

Not having access to MFA would have rendered the lens pretty useless except with MF. The first thing with any newly received lens I do is a basic test of focus (If it is AF, or infinity focus if it is manual lens with adapter).

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