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I actually did this procedure this past weekend. Without boring anyone with the details I fine tuned each lens using all the proper procedures (tripod, remote trigger, good light, accurate measurement of distances etc). Several of my lenses front focused even with the fine tune at its max value. I measured each lens using the dot tune method, recorded it precisely and then re tested each lens after moving the secondary mirror adjustment screw until I had the majority centered around the 0 value. I assume from this that the PDAF mirror was off as when I measured all the lenses after each adjustment the moved as a unit. The slight variation of fine tune once the lenses where grouped around the 0 fine tune was due to lens variation. At the end of the process now everything is sharp and in focus. This includes live view(Contrast phase AF)=Phase Detection AF= viewfinder. I finally feel that I will be able to get more consistent shots from my set up. The sample shots I’ve taken since then so far bear this out. Just wanted to make sure that I didn’t mess anything up in the process (unintended consequences). Thanks
there is a possible answer on your very good results: your brand new camera has been adjusted in the factory to it’s best values. At that time the focus was probably perfect. After a certain time, the camera parts get a wear (for ex the touching point between second mirror and it’s excenter), which is normal – but the AF will be worst and worst.
Now You adjusted back exactly the amount of wear from the second mirrors mechanism – so, again all lenses are perfect.
I have a question about autofocus. When I came across your blog, I figured you are the first person who I came across who could answer a question that has been bothering me for a while. On a dslr that doesn’t have an in camera focus adjustment (microadjustments for Canon, Fine Tune for Nikon), one can use a small hex screw to change the angle that the secondary mirror lines up to the PDAF sensor, in an effort to pair the two autofocus methods accuracy (for Nikon’s at least). If one does this, does it alter any other aspects of the autofocus system? If not ,then could one do that adjustment to fix the autofocus issue and not send it in for camera service? Thoughts and insights would be greatly appreciated.
hi,
if you think on the way of one single light beam, which goes from the lens reflected on the secondary mirror to one special AF sensor, its true, that if you change the mirrors inclination, then the length of the beam will be changed – so the AF ystem will find the distance longer or shorter.
BUT. The picture which you see in the viewfinder should be realistic – which can be fulfilled only for a single mirror position. from this point of view, you can adjust only a little bit (but normally this will be enough.
The bigger problem is, that the beams which have to go closer to the mirrors axle will have another modifications, than that ones which are on the other side.
Conlclusion: you can use thist method . but a real precize AF you will have on only 1 pont (for exampl the middle one)
Try it!