AF fine tune and focus shift for fast lenses

Started Sep 12, 2016 | Discussions thread
Ferguson Senior Member • Posts: 1,826
Re: AF fine tune and focus shift for fast lenses

Lance B wrote:

primeshooter wrote:

Marianne Oelund wrote:

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.

Why do people go on constantly about lenses' that are f/1.4 are able to focus easier as they let in more light? I had heard you post before explaining that it wasn't the case. Is this just misinformation?

I don't think the f1.4 lenses are thought of as focusing faster, at least none of mine focus fast. However, all my f2.8 lenses certainly focus faster than any of my other lenses.

I think that f5.6 is for the focusing system, ie the camera uses the f5.6 focusing circle of f5.6, BUT they can take advantage of the extra light of an f2.8 lens thus making it easier for the camera to focus. I do not think there is any benefit of extra light of lenses faster than f2.8, though. From what I remember, f2.8 is the brightest that is usable.

Marianne will/may have a better explanation or info on this.

I thought that the vast majority of the focus issues with respect to aperture (e.g. better at 2.8 than 5.6) related to ANGLE of the light due to aperture, not the amount of light.

If you look at most decent DSLR's, they focus in darn near dark, so (for example) a restriction on focusing at F8 in bright sun is clearly not caused by inadequate light.  If it was about light, the "which focus spots do F8, F5.6, etc." discussion would relate to EV not aperture.

Now if F2.8 is better than F4, it does not necessarily follow that F2 is better than F2.8, or F1.4 is better than F2.    But if one could plot "speed/quality of focus" against f-stop, I wonder how it looks?   Is there a sweet spot, or is wider always better?   [And here focus speed means detection speed, not how fast it can move the motor, which is a lens mechanics feature.]

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