New lens autofocus problem

[_rubinho_]

I've just invested in a Tokina 11-16mm f/2.8 DX-II to go on my Nikon D5100. The first few test shots were disappointingly soft so I tried quite a number at different focal lengths, subject distances and light settings none of which were any good. A bit rubbish for £500 worth of glass with good reviews I thought. As a test I flipped the camera into live view mode and took some more shots and they came out looking as sharp as they should. Very odd I thought so I took some back to back shots using the viewfinder and then live view and for all of them the latter came out much better. I now think it could be some sort of AF problem in the body. Can anybody think what could be the cause of this?

In the viewfinder I was using the centre AF point and single servo mode. I'm puzzled right now!

[_rubinho_]

Ok I have just tried with my 35mm f/1.8 and the results are the same. In focus on live view and out of focus through the viewfinder. I tried another AF spot in the viewfinder and that's no better.

Live view AF mode is single servo and wide area mode. The focus spot is right in the centre.

Viewfinder is single servo and single point mode.

I'll boot up my laptop and get some test shots up.

[_rubinho_]

OK here's what I mean. These are 100% crops of my test pattern (living room wallpaper). First the Tokina.

Viewfinder:


Live view:


Next the Nikon.

Viewfinder:


Live view:


All other camera settings are identical.

[_rubinho_]

Simpo Two said:

I'm not sure what the problem is exactly, but if you want to use the VF then does your camera have a 'focus fine tune' function?

_rubinho_ said:

Live view AF mode is single servo and wide area mode. The focus spot is right in the centre.

What does 'wide area' add to the equation if it's using a single central focus point?

Live view 'normal' mode doesn't seem to make a difference. Seems like somebody has also spotted this on the D5200 http://www.dpreview.com/forums/post/53873061.

There's no AF Fine Tune in the D5100's menus. A video online suggests you can do it manually. http://www.youtube.com/watch?v=Pfxz-Z_uEfU.

Don't want to send it in for adjustment as I'm off on holiday on Monday. Typical!

[_rubinho_]

Just tried the kit lens and it shows the same effect. The wide setting is only available on live view mode.

I put a tape measure down and the camera seems to back focus when using the viewfinder by about 300mm. That is the area around 300mm behind the focus point is in sharp focus.

[_rubinho_]

Good point. The focus distance was around 1m so a 30% error! Back for calibration it is. Will be using live mode on holiday then. Thanks for your input.

[_rubinho_]

Well I've had a crack at fixing it using the linked video as a guide.

It was ambiguous in places but to fix back-focus (like I had) the screw needs to be turned anti-clockwise. To fix front focus the screw needs to be turned clockwise.

I settled on about one flat (1/6th of a turn) anti-clockwise.

It's not perfect but significantly better. It's definitely usable. I've got no excuses when my holiday shots are out of focus now!

[_rubinho_]

To bring this to a conclusion the reason live view and viewfinder autofocus were different is that they use different methods. Live view uses contrast based focussing like a compact camera whereas the viewfinder uses the standard DSLR focus point array.

I'm going to make sure my next body supports AF Fine Tune.

Thanks for all your input.