aperture of focus?

[CardBoardBoxProcessor]

Hello, having consistency issues with AF on my Sigma 20mm f/1.4. It has been suggested to me that perhaps because 1.4 is so soft anyway the camera has issues focusing wide open. is there anyway to have it set the aperture to something lime f/2.8 while it is attempting to focus?
using a D810.

[Akira]

The only conceivable way is to set the aperture to f2.8 and enter the live view mode.  Some newer Nikon bodies allow you to change the aperture while you are in the live view mode.

There is no way to use the AF function or the the focus aid in OVF once you push the stopping down button.

Apparently the focus issue is caused by the residual spherical aberration:  The focal plane changes when the lens is stopped down from f1.4 to f2.8.

[Macro_Cosmos]

To prove this theory, you would have to stop it down and try, using LV.

I think it's just Sigma's famous jerky Af and horrible sample variance. Is the lens adjustable on their dock thingy?

[Les Olson]

A 20mm lens has a hyperfocal distance of less than 10m at f/1.4 and less than 5m at f/2.8: unless you are focusing on something very near minor inaccuracy of focusing should not have any easily detectable effect.  So, if you are getting obviously out of focus images there must be a major problem. 

Is the problem mostly in focus images and occasional out of focus images, or consistently out of focus images?   All AF systems have occasional bloopers, where the lens elements just do not go where the camera intended them to, which can be reduced by giving the system more time to work: pressing the shutter slowly or repeated shutter button half-press/AF-on button.