AF fine tune and focus shift for fast lenses

Started Sep 12, 2016 | Discussions thread
Marianne Oelund Veteran Member • Posts: 7,788
Re: AF fine tune and focus shift for fast lenses

Ryan Mack wrote:

My understanding is that Nikon's AF system cannot take advantage of lenses faster than 2.8.

That's the Canon AF system, but only for its better bodies.  Their lower-level bodies, and all Nikon SLR's, will only use the f/5.6 circle for AF.

Does that mean for a 1.4 lens that it is focusing based on where the lens focuses at 2.8?

The AF system uses a parallel optical path, which must be aligned to achieve the same perception of focus as the image sensor.  The answer to your question depends on whether or not the AF system was aligned for the lens as it focuses at f/2.8.  The user can of course modify this with fine-tune.

Specifically, it seems many 58 1.4 lenses need a significant positive fine tune adjustment at 1.4 and that lens is known to have a significant frontwards focus shift at 2.8. Is that causation or are they unconnected?

Lens focus shift isn't seen by the AF system.

Starting with models released in 2014, Nikon added a compensation feature to AF, which basically works by biasing the fine-tune value according to which aperture the lens will be taking the image at.  This requires the camera to have tables of focus-shift values for various lenses.  Cameras made prior to that time, have no such compensation and must be fine-tuned using compromised settings.

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Source credit: Prov 2:6
- Marianne

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