Manual focus back focusing

[Bill_Green]

This is driving me nuts. I have one lens that I can't nail focus for all the tea in China and most shots are either front or back focused depending on the lens. I know the lens is super sharp, I just can't seem to get the subject in focus. If it is sharp in the viewfinder, it is off in the file. Conversely, there are some shots that I knew I missed the focus, but the file is critically sharp. I notice this mostly with a 180/2.8, but also (though to a lesser degree) with a 90/2. Is this a screen issue, a viewfinder issue or just user error. I'm running a M screen on a A900.

Perhaps my eyes just aren't that good and I need to give up and switch to autofocus.

Any help is appreciated.

 

[das_schlechte_gewissen]

I think that there is an effect with a few lenses that can be called aperture issue. If you stop down the lens the focus distance shifts in one direction or the other.

You can test this by pressing the button for stopped down preview at the body below the lens.

 

[Jan Brittenson]

Check if you get good manual focus at infinity. If not, the capture plane is out of alignment. If infinity is good, then the viewfinder is out of alignment with the capture plane. It's easier to identify mount register problems with wider lenses.

 

[edwardkaraa]

Check if you get good manual focus at infinity. If not, the capture plane is out of alignment. If infinity is good, then the viewfinder is out of alignment with the capture plane. It's easier to identify mount register problems with wider lenses.

This is quite difficult actually to identify. It could be that the focusing screen is not sitting exactly in its place, as I have had this problem once with my A900, and before that with the 1Ds2. Just removing/reinstalling the screen might fix this.

If it's a VF problem, shimming the screen will probably take care of it.

However Bill seems to have this problem with only one lens, so mirror box/VF loose tolerances are probably not the cause. Some lenses are very difficult to focus manually especially longer lenses, as slight movements of the focusing ring or the camera will have huge consequences because of the shallow DOF. That is why Zeiss won't make MF lenses longer than 200mm as they stated at last Photokina.

 

[Jan Brittenson]

This is quite difficult actually to identify. It could be that the focusing screen is not sitting exactly in its place, as I have had this problem once with my A900, and before that with the 1Ds2. Just removing/reinstalling the screen might fix this.

If you turn the lens to infinity and don't get infinity in focus, then this has nothing to do with the viewfinder... It doesn't even have to have a screen installed because you don't even have to look through it.