Trying to understand micro focus adjustment at 400mm

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Gus Smedstad Forum Member • Posts: 61
Trying to understand micro focus adjustment at 400mm

I own a 70-200 f/2.8 II and 2x TC. I've got a 100-400 f/5.6 II on order, which I plan to use both bare and with a 1.4x TC.

I thought maybe I'd dip my toe into microadjustments, so I found a suitable template on the web and constructed a focus target / tilted ruler setup. I didn't see the point in shelling out $80 for a LensAlign target.

I conducted my initial test at at 400mm wide open (f/5.6) at about 40 feet (30x focal length). I then ran into something that should have been obvious to me when I started: at this distance and this aperture, the entire ruler was in focus.

Focus test at 400mm

Now, in hindsight this setup has some other issues - there are other objects the camera could potentially use for focus, even though I'm using spot focus on the center of the target. Like the rock I was using as a weight, and the leaves in the background. I've added some small taped-on weights that should fix the former, and I can choose another spot to fix the latter.

Regardless, the depth of field issue remains. All the examples I've seen on the web show a narrow depth of field where distant and close parts of the ruler are clearly out of focus. That's not going to work here.

I could, of course, move a lot closer in order to reduce depth of field, but that doesn't seem like a good idea for a number of reasons. I'd end up optimizing the lens for very close targets, and I gather microfocus changes with distance.

Is it just impractical to muddle with microfocus at 400mm unless there's a really gross error? Should I be using a much, much larger target? Is there something else I'm not understanding?

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