Logan Lee , Feb 07, 2012; 12:11 a.m.
Hi! I was out the other day shooting for anything I found interesting. It was a bright sunny day, I was outdoors. I half-pressed my shutter button to focus on a chair. The AF system indicated that it has locked focus even while the entire viewfinder was still blurry. I released the shutter button and half-pressed it again at the same position as the first try, the same thing happened. So I tried to look for something else to focus and it focused fine. I've only had this lens for less than 2 months and spent a lot of my savings on it. I've read a lot of bad copies going around and I'm very worried that I might be one of the unlucky ones. Is this a case of incompatibility? Is this the failure of the camera AF system or is it a bad copy of the lens? Please help! Thank you!
Lorne Sunley 
, Feb 07, 2012; 12:15 a.m.
It could be dirty contacts on the lens, try cleaning the ones on the lens and the ones on the camera
Wouter Willemse 
, Feb 07, 2012; 03:26 a.m.
It could just be the scene you tried to focus on. Sometimes, the AF just won't lock properly, or won't lock onto what you think it should. It happens. At least, I've seen it happen with all of my AF lenses at least some time.
I wouldn't be worried. If it happens again, try to find something with more contrast to focus on, or focus manually.
Elliot Bernstein 
, Feb 07, 2012; 06:23 a.m.
"I've read a lot of bad copies going around" Where did you read this?
Chances are, the AF system was 'fooled' in some way. It happens.
When I have trouble focusing for whatever reason, I try focusing on a different area of the subject or change the AF mode (for example, change the AF point and or AF mode).
Logan Lee , Feb 07, 2012; 09:18 a.m.
Thank you for your responses.
So, does this mean what happened to me was normal? I'm just starting to learn the ropes of photography so I don't really have that much experiences yet to differentiate what is normal and what isn't. The chair I was trying to achieve focus on had a weaved pattern on it. Maybe the AF did get "fooled".
Wouter Willemse , Feb 08, 2012; 05:14 a.m.
Yes, I think it is normal. AF systems prefer something of high contrast, that works best to lock the focus. Also, the central AF point of your D90 is better than the other ones, so if you're using the off-centre AF points, you may find the camera can have more issues locking on.
Logan Lee , Feb 08, 2012; 07:14 a.m.
Thank you for your responses Wouter Willemse.
I did use all the focus points (including the central AF point) at that time it missed focus twice in a row. I'm worried I might have a bad copy of the lens. But if you experienced guys say that it's normal then I will believe you.
Here are a few sample shots I took before the missed focus happened. Please comment regarding sharpness.
