larger DOF on 16-50 on nex6

Started Dec 28, 2012 | Discussions thread
Rehabdoc
Contributing MemberPosts: 911
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Re: Should have not used auto focus. Should have focused manually on the same point.
In reply to dumokun80, Dec 31, 2012

dumokun80 wrote:

thanks for the smart ass comment

so you are basically saying that center focus AF does not guarantee the camera focuses on the center?

it will PROBABLY focus on SOMETHING near the center. The logs in the center area of your picture are at different distances from the camera.

like how much off are we talking about here? like focus in one of the corners or at the edge of the frame?

the issue isn't whether center AF will focus on an object sitting on the corner or an invisible object sitting in a bank vault in Peru. The issue is whether that center AF will focus to the exact same distance. DOF has DEPTH. That's why they call it "depth of field". An "acceptable" AF "lock" onto something in the center can vary, and only   puts its target somewhere in that DOF.

if the center focus can be off, what makes you think that the spot focus is any more accurate?

He's not being a smart ass.

(1) the 18-55 is not significantly sharper in the CENTER than the 16-50 in the middle of its focal length range. It's simply not. Unless you somehow got a "special" copy that's magically unsharp in the center while being extra sharp in corners, your jpeg from the 16-50 has a different focus point than the one from your 18-55. We already know your conclusions don't make sense. We're only carefully going through the list to figure out which of your methodological errors were the worst In this cautionary tale of bad conclusions.

(2) anybody with common sense would think to manually focus a test trying to compare lens sharpness and DOF. It wouldn't even occur to me to go to all the trouble of a tripod, then put my faith in AF. Of course I trust manual focus (especially with magnification view) more than I trust AF. (For example, even when you AF on a face you don't know if it will choose the nose or the eyes. You don't know if it will put the same object more towards the back or the front of the DOF From one shot to another. AF is Not magic. It's faster than MF, but not better, especially on something like this).

(3) in your chosen subject matter, in fact there are multiple potential focus points near the center, all in different distances from the camera. You're shooting freaking stacks of logs that are poking out at you at different lengths. And from your down sampled jpegs, I can't tell which things near the center are truly in focus.

Unless you are vision impaired: of course manually focusing with magnification view is going to be dead on accurate. MF is slower, but you know exactly where your focus is.

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