Question on "f/8 AF"

[billtils]

I was aware of the various methods used to autofocus, but got really confused by a segment on this in an online tutorial on the D810. Specifically, the tutor (who is pretty good) moved on from discussing horizontal vertical and cross phase contrast to the different versions used, namely f/5.6 using all 51 points with the centre 15 being cross and the others horizontal, f/5.6-f/8 having 9 centre cross and 8 horizontal, and f/8 having 3 cross type in the centre and 8 horizontal.

Does this mean that any time there is a lens of max aperture 5.6 or wider it will always use the centre 15 cross plus 36 horizontal no matter what the actual aperture is set at? I suspect that this is the case since the AF operates with the lens wide open and not at the selected shooting aperture.

And if not - what does it mean? I just want to be sure .

[george013]

I was aware of the various methods used to autofocus, but got really confused by a segment on this in an online tutorial on the D810. Specifically, the tutor (who is pretty good) moved on from discussing horizontal vertical and cross phase contrast to the different versions used, namely f/5.6 using all 51 points with the centre 15 being cross and the others horizontal, f/5.6-f/8 having 9 centre cross and 8 horizontal, and f/8 having 3 cross type in the centre and 8 horizontal.

Does this mean that any time there is a lens of max aperture 5.6 or wider it will always use the centre 15 cross plus 36 horizontal no matter what the actual aperture is set at? I suspect that this is the case since the AF operates with the lens wide open and not at the selected shooting aperture.

And if not - what does it mean? I just want to be sure .

You're referring to an online tutorial. Maybe you could give a link so we know what you mean.

George

[DanK]

I don't shoot Nikon, but if this is similar to Canon, then yes.

What is at issue is the actual aperture when the camera is focusing, so if you use a lens that stays wide open until you take the photo, this is determined by the maximum aperture of the lens. As the aperture gets narrower, some AF points may lose either vertical or horizontal functions or stop functioning altogether. Here is the diagram for my 5DIII:



I believe the issue is the physical separation of the sensors, not the amount of light.

[billtils]

Maybe you could give a link so we know what you mean.

George

Fair question George, just one problem, it is a paid class and not much help to you unless you want to pay the registration fee. It's also copyrighted so can't copy and paste the relevant material.

[george013]

Fair question George, just one problem, it is a paid class and not much help to you unless you want to pay the registration fee. It's also copyrighted so can't copy and paste the relevant material.

I did ask for I didn't understand the problem. I just didn't know that the outer cross focusing points are loosing function as a cross focusing point. Never knew they could change behavior.
On my D700 and the 15-30 Tamron 2.8 I've difficulties focusing on the sides with vertical lines. It could be the horizontal lines too, I forgot.
Now I'm curious for the why, an answer to your question.

George

[loosecanon]

Most Canon instruction books have a section where it lists ef lenses and which ones can use which focus points. Older designs such as the 50mm macro and 180mm macro cannot use all the focus points towards the screen edges. In practice I find most of my photography uses the central point, or not far off the central area. Occasional landscape shots may need wider points, but then these I would often be looking to set manually, looking at depth of field etc.
I think too much is played on the number and position of focus points, rather then accuracy and spacing of those towards the centre. Just look at the comments made about the 6D using the excellent 80D focus unit.

[billtils]

I think too much is played on the number and position of focus points, rather then accuracy and spacing of those towards the centre. Just look at the comments made about the 6D using the excellent 80D focus unit.

I'm not so sure about that. If my understanding about what the f5.6/f5.6-8/f8 AF settings are doing is correct, then (for example) my f/4 300mm lens will always autofocus with the 15 centre points being cross type no matter the actual aperture used which means that when single point focus is selected the point will be cross type no matter which of the 15 I lock in to (and similarly if I use the 5 point Group selection). However if my understanding is a misunderstanding then the AF system will be using 1 or 3 horizontal lines respectively.