Some data on Oly "Focus Stacking"

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lester11
lester11 Contributing Member • Posts: 515
Some data on Oly "Focus Stacking"
20

I was curious to get some feel for how the "differential" step between 1 and 10 worked out in practice using the Oly "Focus Stacking" feature on my E-M1. With the Oly 60 mm macro lens mounted, I set up a Lens Align target at 500 mm from the sensor plane, and snapped away with the 8-frame standard focus stacking bracket using apertures of f/2.8, f/4.0, and f/5.6 and differentials of 1, 2, 3, and 4. The setup is shown below.

Setup so as to read off DOF in the image

The two graphs show the resulting data. It was very interesting to see that the actual size of each of the actual focus steps depends on the aperture. For the 60 mm macro lens set-up, only differentials 1, 2, and 3 provided a focus stack of overlapping frame focus ranges. The step size for differential=4 was larger than the depth of focus, and the in-camera stack merge failed (no disrespect to Oly, it would also have been similarly ugly using any other focus stacking software). The actual step size for differential=3 was just within the depth of focus, and so gives a maximum bracket width from the 8 frames.

Actual focus step size (subject at 500 mm) per differential, size of DOF shown for comparison

Width of the focus bracket after merging the frame stack in-camera, single-shot DOF shown for comparison

My take-away is that, if you use the Oly "focus stacking" feature, you should be quite safe with differentials of 1, 2, or 3. You'd look up your DOF (using an app, eg HyperFocal Pro) for your particular lens and subject distance (remember to measure/estimate from the sensor plane, not the front of the lens!), and then you would set your aperture to accommodate the bracket width you wanted, knowing that this is about 3 times DOF for diff=1, 4.5 times DOF for diff=2, and about 7 times DOF for diff=3.

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Lester

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