When you say "appears"...do you mean you're shooting in Manual? I'm pretty sure you're not, but thought I'd ask just in case.
The very first thing I'd do is: check to see if you have your "focus indicator" turned on...? This is a beep that tells you "yep, I'm focused and locked in" - so that you don't have to judge if you're focused in on something, the camera tells you when it's focused. NOW, as to whether the camera has focused on the POINT you wanted it to, is another subject (ie, did the lens focus on the eyes, or on the tail...and that's when the human factor has to be involved).
2nd thing: what is closest focus distance of that 35mm? (just so it's in the back of your head)
That makes me think that 1) you may have been even just a CM too close for the lens in some instances (therefore, the shutter could not release) and/or 2) the lens focused, the shutter engaged, but then she moved just a BIT - and the pic was taken....if you had your aperture wide open, say at 1.4/1.8 or something - all she has to do is barely tilt her head a fraction, and she could be out of the field that
was "in focus" when the picture snapped.
Maybe an analogy using space? (depth of field=DOF)
Like, this is your DOF at 1.4 [ ]
And, this is your DOF at 22 [_____________________________________]
So, at f1.4, the subject has this much depth of space [ ] in focus during the shutter release. Very shallow depth of field....therefore, one major area of focus, rest of piccie blurred out (bokeh).
But, at f22, the subject has this much depth of space [_____________________________________] in focus during shutter release. Very wide depth of field....therefore, almost everything in focus, very little blur (think: landscape pictures).
Argh....don't know if that made sense....
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