AF fine tune and focus shift for fast lenses

Started Sep 12, 2016 | Discussions thread
primeshooter Veteran Member • Posts: 6,067
Re: AF fine tune and focus shift for fast lenses

Marianne Oelund 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?

Many people do not understand that the additional light brought in by faster lenses is entirely peripheral to the light brought by slower lenses. They just think everything is brighter with a faster lens, but that's not how it works.

The AF system must be selective about the pathways that it accepts light from, that is, one from each side of the exit pupil (and/or one each from the top and bottom). In order for the system to work with slower (f/5.6) lenses, it must be designed to use light paths that are included within the f/5.6 circle. This means that all of the extra light provided by lenses faster than f/5.6 will be blocked from entering the (Nikon) AF module.

Why 5.6! That seems ridiculous? Why doesn't the system in nikon slrs take advantage of at least 2.8, or wider?

Even in the Canon cameras which do provide some f/4 and f/2.8 capability, the f/5.6 points are primary. The camera uses those first, then when focus is nearly achieved, it will switch over to a more sensitive point for the final precise positioning.

Correct me if I'm wrong, but I constantly see people going on about focusing fast manual lenses with nikon DSLRs. They say something like 'well I just MF using the viewfinder at f/1.4'. Some admit to using the range finder dot but many argue that the screen shows 1.4 dof which I am sure it doesn't? It's correct isn't it, that the focus screen shows the light and dof for about 2.8 and nothing less, so with a 1.4 lens, MF must be with the rangefinder dots, right?

You are correct and the original-equipment viewscreens in today's cameras are not adequate for accurate manual focusing at apertures wider than f/2.8. In fact they are often not adequate for f/2.8 or even f/5.6 in some cases, as the manufacturers no longer care about how accurately the screen is aligned for showing true focus.

I was taking some photos in the garden last weekend, with an AI 105/2.5 lens on a D4 body. Even at f/5.6, if I focused the lens for best sharpness on the viewscreen, the image turned out severely back-focused. It was very frustrating, I can assure you! I had to resort to using the AF dot indicators to obtain correct manual focus.

Thank you, confirmed my suspicions!

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