Manual focus issue with a7r3 and 100-400

I was shooting some images of the moon with the 100-400 and the 1.4 converter and tried to manual focus set at infinity.The results were out of focus images. Therefore, I switched to auto focus, center wt, and had perfect sharpness. Do I assume that a preset of infinity is not going to work? If so, why not? Correct me if I am wrong, but the moon should be at infinity?

Not knowing the exact procedure you used I can only speculate... If the camera goes to sleep or you cycle it on and off between manually focusing and taking the shot you will get an out of focus shot.  That's the bane of focus by wire lenses.  To avoid this, simply move the switch on the lens from AF to MF.

Infinity is a locked point with a dslr...usually a bit past infinity with AF lenses (to allow for heat expansion) but focus by wires can be all over the place. Get yourself a nice manual focus lens and have a real infinity point.

Mike in O wrote:Infinity is a locked point with a dslr...usually a bit past infinity with AF lenses (to allow for heat expansion) but focus by wires can be all over the place.  Get yourself a nice manual focus lens and have a real infinity point.

OK, this makes sense and if I back-off the point all should be good. I will explore this and try to get a feel for how the lens will react. Thanks E.J. and Mike for the explanation.

You may have misunderstood what Mike and I were trying to say.

You can't simply "back-off" and be OK. A focus by wire lens has no mechanical linkage between the focus ring and the focusing lens elements. It's an electronic coupling. If you manually focus the lens and it is sharp, as long as you take the shot before the camera goes to sleep or it gets shut off for some reason, the shot will be sharp assuming you focused correctly and didn't bump anything after you focused. But if for any reason the lens loses power, when it regains power you will need to refocus.

What Mike was suggesting is to not use a focus by wire lens for this type of photography and use one with a mechanical focus linkage. in that case, nothing other than you can cause the lens to change focus since it is not an electrically driven focusing element.