Help with strange D800 Live View problem
I recently got a D800 and just got the Tamron 70-200 zoom (sorry, just couldn't pull off the Nikon version on top of the D800). Having read about the inconsistent focus issues some have had with the Tamron, and wanting to tune my AF for this lens regardless, I did some testing. I set up a cereal box next to a tape measure and put the D800 on a tripod pointing to the cereal box. I set the aperture wide open and zoomed to 200mm. I shot test exposures 3 ways; regualr AF (center point), Live View AF (also center point) and manual focus in Live View using the magnification function to get precise focus.
I have uploaded 100% crops of the key focus point from representative images for each of the different focus methods. Note that despite its reputation, the lens focused consistently every time. From my perspective, the regular PDAF shot looks pretty much spot on. There may be a tiny bit of front focus, but it sure looks like the 2 FT mark is at or near the center of focus. It is a bit blurry in general as I was too lazy to use mirror lock up and the shutter speed was about 1/40th. However, it is clearly focused in the right spot. To back this up, I repeated this set up with the shutter speed at 1/200th and used the built-in flash to eliminate some of the mothion blur. Even though the ambient light was fairly dim by this time, the lens looks like it hit focus just right. The manual focus also looks spot on (and has less mothion blur).
The odd thing is that the Live View AF shots consistently back focused. Is this just due to the low light, the odd nature of the target, or is there some other issue?
See attached photos (PDAF = normal autofocus). Note, focus point in all cases was centered around the lower corner of the cereal box but also covered the part of the tape measure closest to that lower corner of the box.
Thanks for any thoughts or ideas.
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