PDAF issue on D800E

Started Nov 17, 2012 | Discussions thread
Robin Casady Forum Pro • Posts: 12,898
Re: I'm wondering...

S1m0n wrote:

Robin Casady wrote:

1. The AF indicator boxes in the viewfinder are not always in perfect alignment with the AF sensors. Could this have had any effect on your distance tests?

Hi Robin.

a1. I guess that is possible,

although I tend to use only the centre AF point. I would expect the same focus results at varying distances keeping same aperture.

The scale of your target changes as the distance changes. So a misalignment, between where you are aiming and where the camera is focusing, could produce different results.

2. Am I correct in understanding that the inconsistencies you describe only show up at apertures f/1.8 and wider?

a2. Correct, Its more visible at the wide end. F:2, and smaller apertures the depth of field is wide enough to get the subject in focus.

3. Nikon's D800 Technical Guide recommends using Live View for critical focus. I would say that pin sharp at f/1.8 and wider could be considered critical focus. It is possible that Nikon found no problems with your cameras because they don't expect PDAF to be that good. Your cameras probably fall within what Nikon considers acceptable parameters.

a3. Fair point. I did read this.

If this is the case Nikon should have been more upfront about the usage of their 1.4 lenses with this camera. I use a tripod a lot of the time, but there are also instances when I would like to use the camera handheld. Using CDAF handheld is not so easy and of course slower. Robin, keep in mind that at some distances the PDAF can also give sharp focus with the widest apature. Its was just not constant at varying distance with my cameras. This again makes me think its a tollerance issue.

Something to try:

  1. Set the lens at infinity and then activate AF. 
  2. Take the shot and record the number and the infinity starting point.
  3. Set the lens focus to much closer than the subject then activate AF. 
  4. Take the shot and record the number and the infinity starting point.

Do this a number of times and see if there is a consistency about nailing focus and missing it.

I can take critical wide aperture portrait shots (at varying distances) in decent light with the D700 and focus will be spot on. I understand there is a resolution difference here, this is negated with the D800 PDAF issue.

If by "negated" you mean that the AF errors bring down the D800 to the sharpness of the D700's resolution, I would not be surprised. If you mean that AF renders the D800 image much worse than a D700 image, I would suggest putting the D700 through as rigorous testing as you have done with the D800.

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