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Reader Question: Focus Shift with Nikon Mirrorless

See my Nikon DSLR wish list at B&H Photo.

See other Nikon mirrorless posts.

Terrence M writes:

Assuming the upcoming Nikon mirrorless camera works properly with the F-mount adapter would you still have focus shift, for example with the 14-24mm f/2.8G, 85mm f/1.4G and the 50mm f/1.2 AIS?

DIGLLOYD: this is a more complicated question thatn it sounds. There are several cases.

Case #1: camera autofocus

Presumably a Nikon mirrorles lens adapter for F-mount lenses will pass through the lens information so that the camera can deal with each Nikkor lens appropriately (non-Nikon lenses who knows).

This is the ideal case—the camera can focus with the lens wide-open, but compensate for the focus shift it knows will occur from any given aperture. The photographer can thus forget about the issue entirely. Presumably Nikon will do little and probably nothing for non-Nikon lenses, but at present it is unclear if Nikon will support any non-Nikon lenses.

Case #2: manual focus lenses with aperture control

This case would be a PC-E lens or non-Nikon lens with aperture control (which as yet does not exist). In this case, the camera could take focus shift into account in the “green dot” focus assists mode. But since this mode is wildly inaccurate and since Live View is so much better, it is nearly pointless.

Case #3: manual focus with lenses having no aperture control

Use Live View and understand the lens behavior—it’s an operator issue. Anything else will be error prone, and there is nothing the camera can do here, since it cannot know the focusing or shooting aperture.

Case #3 Live View manual focus

Cameras like the Nikon D850 default to focusing at the shooting apeture but also allow popping the lens to full aperture via the DoF preview button. The whole operation is WYSIWIG (what you see is what you get)and therefore a photographer who understands the issue can focus manually at an appropriate aperture for the shooting aperture. It’s not clear to me that the camera can do something that won’t trip me up, or anyone who understands what s/he is doing. Hence it is best left alone—no compensation by the camera.

Case #4 Live View auto focus

Here it gets tricky. Should the camera risk focusing error by focusing at the shooting aperture, or focus wide open, the compensate? My assumption, validated by long experience, is that focus stopped down introduces very large errors (Sony A7R II/III delivers very large errors). Hence compensation by the camera is likely to be a disaster.

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