Is there something defective with my lense?

Oi vey. What a mess.

Here's a bit of hopefully helpful advice as you go back out and try to correct your error. DoF is a product of three factors: The effective focal length of your lens (focal length of the lens in relation to the sensor/film size), f-stop, and subject distance. As a general rule, longer focal lengths, wider f-stops (smaller numbers), and closer focusing distances will all decrease your DoF, and vice versa. There are some handy DoF calculators online. If you have an iPhone, there's a really handy app I love (after trying a few DoF calculators) called PhotoBuddy, that will help you calculate DoF (as well as exposure, flash exposure, bracketing—handy dandy for HDR—, angle of view and bellows compensation calculators, a grey wedge and a bulb timer).

If you have an iPhone or iPod Touch, PhotoBuddy's a must-have.

Here's a quick bit of information to help put things in perspective. For the very first shot you linked, "A", let's just for a moment ponder what the DoF is likely to be. I'm going to, for simplicity's sake, I'll assume you were about 100cm from your subject, give or take. How I wish that focus distance was in EXIF data. I'm guesstimating here. That's about 3ft. I could be horribly wrong. But it should be good enough to illustrate my point.

At 100cm, an 40mm on the D90 (so effectively 61mm on a 35FF camera), and f/2.8, the total DoF is 67.3mm. That's all of 6.73cm. Not very much at all. If you were closer, say, 80cm, it would be down to 42.6mm, and the closer you get, the smaller and smaller DoF gets. (This can become a pain in macro photography, where focusing distances can be very, very close. If you used a 100mm macro lens on your camera, and focused on something 20cm away at f/2.8, DoF would be in the range of 0.2mm, which is er...rather tiny, razor thin and absurd.) Hopefully you get the idea after that rambling.
 
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A on Flickr - Photo Sharing!

A: focal length: 40 f-number: 2.8 exposure time: 1/80
B: focal length: 55 f-number: 3.2 exposure time: 1/200
C: focal length: 48 f-number: 3.2 exposure time: 1/200
D: focal length: 48 f-number: 3.2 exposure time: 1/200
E: focal length: 42 f-number: 3.2 exposure time: 1/200
F: focal length: 52 f-number: 3.2 exposure time: 1/200

again i know they're more of snapshots than anything else =\ i wanted to do it at an actual location but the kids didn't want to and yet still wanted photos. two of the guys go to the same church as me so when they asked i couldn't really say no. the crappy composition isn't my concern, it's that after having read so much about this lense, i would expect it to be much sharper than my product. imo, my kit lense produced sharper image than this....(this is saying what i'm seeing, not what is suppose to be. just putting it out there because i know someone is bound to see that i said i feel like my kit lense is sharper and immediately start bashing without even the slightest intent to concentrate on the problem presented in this thread -_- )

Honestly, I can't see what you're complaing about. Looking at the full res images of where you linked, those look pretty freakin amazing to say the least. Great exposure (on most) and nice contrast.

I don't see what the problem is

~Michael~
 

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