Improving focussing with a manual lens

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My trusty Nikon 85mm f/1.8 is in for repair at the moment after taking a fall from our dining room table onto a hard floor. Whilst it’s being brought back to life I’m using a Samyang 85mm f/1.4 manual focus lens that I picked up off here second hand a couple of years ago.

It’s an excellent lens really; it’s very sharp, contrasty, great feeling focus, but it’s manual focus and getting the focus just right at f/1.4 is mostly impossible. I get it right about 25% of the time on my Nikon D800, even when the focussing assister shows that it’s in-focus. I can tell it’s not going to be spot on as it says the subject is focus for a few degrees of movement on the focussing ring.

I was wondering if there’s any way of improving my hit-rate whilst manual focussing at large apertures. I think I’ll treat myself to a magnifying eye piece, but it only increases the magnification by about 30% or so. Is there anything else I can use; are there any other tricks I can employ to improve this?
 
Stop down to f2.8 since modern SLR viewfinders can't actually show the depth of field of anything faster than 2.8. The magnifying eyepiece still won't help if you're shooting at f1.4, but it helps a little at 2.8 (I use a 28/2.8 AIS and a 105/2.5 AIS on my D800).

For accurate focusing at 1.4 you'll have to use live view since that will actually show you the DOF you're working with.
 
If you're looking longer term ... you could consider fitting a spit prism type focusing screen.
 
They are nice lenses. I usually use mine at 1.7 or 2 (it's a little too soft at 1.4 & CA is very noticeable) and try to choose a mid-point on the focus ring when what you describe happens. Since mine is almost always used in low light handhelp I can't do as sk66 suggested & use live view, though that would be a good solution if you're on a tripod.
 
I won't use anything else these days for landscape. Zoomed in Live View and MF wins for me every time. I've been let down too many times with AF missing focus. You get away with it shooting wide but not with 70-200 or similar.
 
Split prisms screens knock out a lot of light through the viewfinder... that's why they went away from ground glass screens.

That was one of the biggest changes for me when I went from manual to AF cameras in the 80's - suddenly there was this marvellous bright, clear viewfinder instead of a rather dim and gritty image with stuff in the way in the middle of the view. Made a huge difference.
 
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