Lens keeps changing focus distance upon repeated AF?

Energize · 7 Feb 2013 at 10:21

I've noticed that my lens will slightly change the focus distance upon repeated half-presses of the shutter, I focus perfectly in the liveview and then switch to af through the ovf and sometimes the distance scale remains static and sometimes it changes slightly, which at f/1.4 results in a soft image. What is causing this?

Energize · 7 Feb 2013 at 12:27

a1ex2001 said:
take two(or more) pictures it sounds like inconsistent focus rather than a back front focusing issue which would make it harder to fix with micro adjust if you have it.

Still worth doing some repeated stable testing to see if the issue is random or a more consistent issue you could tune out.

The lens back focuses as well, I have it set to -15 to get the correct focus most of the time.

Energize · 7 Feb 2013 at 21:13

An Exception said:
What lens is it?

Nikon 50mm f/1.4D.

D.P. said:
Do some repeated shots. Use a solid tripod, remote release and mirror lock up. Take a large set of photos against a static high contrast subject. Take a photo, then reset the focus to either extreme but take not on whether you are pulling near or far focus. Say take 15 photos when you manually pull the focus to the MFD, then another 15 when you push the focus passed infinity.

What do you mean by mfd?

Energize · 7 Feb 2013 at 23:03

An Exception said:
Umm, at 1.4 the lens is soft anyway?
Tbh, there isn't much more anyone can help you with unless you post some samples and preferably do a real test as already been advised, unless for some reason test charts is uncool or something...

I'm doing that now, hence asking what MFD stood for.

I don't see what the resolution of the lens at f/1.4 has to do with the autofocus.

Energize · 8 Feb 2013 at 02:37

f/1.4 gives the shallowest depth of field so shows up focusing errors the most.

Energize · 8 Feb 2013 at 22:55

It doesn't look out of focus to me at f/1.4, there is certainly a big difference between the lens being correctly focused by MF and incorrectly focused.

See the difference I'm talking about here looking at my wallpaper texture? And that's a 1:4 crop.

http://i46.tinypic.com/14up37.jpg MF
http://i48.tinypic.com/os8x2s.jpg AF sample 1
http://i47.tinypic.com/10hr195.jpg AF sample 2

Energize · 10 Feb 2013 at 17:07

Upon further examination it seems that this is not a lens issue, the body appears to back focuse all lenses by 15 points and has variable AF.