Focus tuning

I’ve long been an advocate of focus fine tune aids for camera/lens systems, in particular, the docks offered by Sigma and Tamron for their lenses. The system allows for focus fine tuning at multiple distances, and in the case of zoom lenses, at multiple focal lengths. This is better than what Nikon offers in its cameras, and which is the only solution for Nikon lenses. My problem is having a reliable focus target for testing. Several systems are on the market, but they are in my opinion, overpriced. You need a reasonably sized focus target with good contrast, and front to back scale so that you can determine front focus, back focus, or correct focus.

I felt that I had the right tools to make my own target, and made it in about 5 minutes, not including the cleanup of the workshop before starting.

I created my black and white target using a Silhouette vinyl cutter, and its associated software. This is something my wife uses on a daily basis, so it was handy to be able to avail of it. I designed a very simple target with good contrast to be able to best use my camera’s focusing system. The frame was a piece of white painted pine that I had lying around, and I added a couple of hinges that I also found in the workshop, some rubber feet which were door bumpers left over from the kitchen reno. I added a piece of dowel sticking out the side of the wood, at the height of the centre point of my target. I added a push pin at the bottom of the wood, and with the dowel, I can rest some kind of ruler so that I can easily determine whether a lens is front or back focusing. I’m not sure what kind of scale I need on my ruler at the moment, so I’m starting out with an actual ruler. That should help me figure out what scale is appropriate.

As long as I line up a measurement with the plane of the front of the wood, I should be good to figure out front and back focus pretty well. I’ve tried it with my 50mm f/1.4 and 35mm f/1.4 so far. It seems to work well. One thing I do notice is how its hard to determine precise front/back focus from a distance. As soon as my camera is 15 ft or so, the level of detail drops to a point where I can’t determine the exact point of focus. The contrast of the black target can help here, since it maximizes the likelyhood of fringing, which appears when things aren’t quite lined up.

This looks good. Focus on the 16″ mark, which is lined up with the front plane of the target.

The auto-fine tune of my D850 should speed up the process of fine tuning my lenses. I’ll hopefully give it a go this week.