D800: Getting AF to work on your fast primes, how to
It seems that a number of people are having trouble particularly with their fast primes. This is a 'how to' or a 'how I got mine to work'.
We are not going to discuss left/right AF points, quality of targets etc. or anything else - just work on simply getting the center AF point to focus accurately.
Firstly, you will need to get a lens alignment tool. They are far more accurate, easier to use, and you will eventually have to run alignment on all of your lenses to get optimal AF from your D800. Out of 6 lenses that I tried, 4 needed adjustment, 2 didn't - the two that didn't were f4. They were all new out of the box so its pretty safe to say that you should check and adjust any lens certainly f2.8 or faster. You've now spent $3000+ on the camera and a similar amount or much more on glass - $80 or thereabouts to get it all working is well worth it. I use the LensAlign Mk II.
This is about lenses faster than f2.8 (not f2.8) and special considerations that need to be made when dealing with those.
The main problem with fast lenses is focus shift, almost all fast lenses exhibit this and the shift almost always causes the focus point to shift backwards (away from camera) as the aperture is stopped down.
Here is an example of this issue with the AF-S 50mm 1.4G, please download the full-sized original so that you can view it at 100% vertically and scroll left/right to observe the effect (you can select the link below and choose original).
If you had fine-tuned your lens with the target in the middle of the area in focus, you might not be able to achieve focus at some apertures. Nikon probably compensate for this which is why most fast primes exhibit a tendency towards backfocus wide open out of the box.
Please note that the camera and target are aligned (you can see the smaller hole in the back of the target through the larger hole in the center of the target)
You can see how much the focus shifts between f1.4 and f2.2 - going from having most of the area in focus in front of the target at f1.4 to very little by f2.2. As the aperture is stopped down further you eventually start to see the area in focus increase and start to expand towards the camera.
This is because we have two effects going on:
'focus shift' as you stop down the lens moving the focus point further and further behind the target.
and an increasing depth of field.
At larger apertures the effect of focus shift is greater than the countering effect of increasing depth of field resulting in this pattern. The relative effects of focus shift are large at larger apertures but decrease and the effects of increases in depth of field become dominant at smaller apertures.
PDAF works by focusing with your lens wide open, then it stops down before shooting - hence the problem.
(Please use delayed shutter, 'A' mode, auto ISO, JPEG normal, tripod of course, and RESET focus between each shot - you can put your hand in front of the lens and half press the shutter that will cause the AF system to hunt between each shot).
To solve this we have to do this step by step (try to leave your camera and target as they are until finished and just take out the CF card to save the trouble of realigning):
(i) Properly and accurately fine-tune the lens wide open to get the target into the center of the area in focus.
(ii) Once fine-tuned, take a series of shots from wide open through to f5.6 (by which the effects of increasing DOF far outweigh the effects of focus shift)
(iii) Identify the range that is in focus at each of those apertures we shot in (ii) above
(iv) Find the range (lets identify it as x-y) that is in focus in ALL of the ranges in step (iii) - i.e. the intersection of all ranges
(v) Work out a new midpoint for the range x-y, and again fine-tune the lens wide open. This time placing the target at the midpoint of the range x-y.
(vi) Check that you have been successful so that the target is in focus at every aperture and that you are still able to focus at infinity.
None of this is destructive and you can always go back ans turn off tuning for that lens, or reset to 0.
However this procedure worked for me and I like the fact that my lens will now focus properly. For people with the 50mm 1.4G, could try the example above and see if it does the trick for you.
You might have to play around + / - 1 step in fine-tuning to get the best result.
Why do we have to fine tune our lenses when they may have worked perfectly before?
there are variances from body to body and lens to lens
what may be considered to be in focus on another camera may not on the D800 as the greater resolution will reveal even minute focusing errors.
It is entirely possible that there is a lens where there is no range in step (iv) where focus is achieved at all apertures
You should note down where your in focus range is for different apertures that you shoot at to have an idea of what will be in focus when shooting and where to focus. You might even want to make small temporary fine-tuning adjustments if you're going to do a lot of shots wide open for example.
For distance to target please use 25 x focal length
Good luck and I sincerely hope that we can all get this issue sorted soon.
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