Back Focusing HD DA 70mm F2.4 Limited

I've never been fully satisfied with the sharpness of my HD DA 70mm Limited that I had bought this summer. When I took a candle lit photo of my grandson last weekend with the lens wide open it was apparent the focusing was definitely out. I've had a play around today checking for back focusing by focusing on the lens cap at 15cm on the inclined ruler and the result is below:




You can see that the lens is focusing at 18cm, 3cm behind where it should be.

I then adjusted the AF Fine Adjustment to +7 and success the focusing is now sharp at 15cm.




This is the first time that I have found a lens to be this far out, particularly with it being a new Limited.

Back/Front focus issues have been with us since AF first appeared in the 1980's. Only with higher MP cameras and the ability to pixel-peep do we notice them as much.

They can be caused by the camera (AF sensor registration distance) or the lens , or a combination of both.

I have done extensive testing on My K1. It suffers with a general BF. I need on average +7 for accurate focus, but on some lenses it requires +5 and on a couple +10 is only just enough. I have also discovered that making the camera focus from infinity or closest-focus distance to the subject can make a difference.

What can also make a difference is the light under which you take your picture. I learned a while ago that trying to calibrate a lens for AF under artificial light can lead to odd results. If you really noticed the issue when photographing under candle light, that may itself be a contributor to the problem.

Front or back focusing is quite common but can be remedied, though it's been my experience that the AF systems on my K200D and K-3 still aren't precise enough to always hit the mark.

Focusing on a ruler is not a good way to do this because the AF "point" is not really a point at all, rather an area. Anything within that area is fair game for the AF system, regardless of where the user wants it to focus.

Here you have a link to a proper test chart with full instructions on how to do it properly: http://fotospec.com/doc/focus.pdf

Print out the chart itself rather than using a computer screen. Focus as close as possible and at maximum aperture.

Have you considered using the debug menu to adjust the fine tuning to include this offset? It doesn't take long at all, I performed it on the cheapo K-1 I picked up that required over +10 adjustment and now it's perfect at 0 for most lenses.

I can support you if you'd like to try it.

That is a very kind offer, but I think I will leave things as they are. The important thing is that AF/FA does not change over time, so at least with my existing lenses they will all continue to work well for as long as the camera does.

The two lenses that probably need +10.5/+11 I can live with that. For a general scene it is always close enough, for a close up I will be using manual focus anyway, so I do not think it is required.

Should anything change I will give you a shout, and thank you again for the offer.