Why do my photos have a black half moon at the bottom?

I have a Nikon D300 and a Tamron 18-200 lens. Sometimes when I use the pop up flash I get these half moon black spots at the bottom of the pictures. It's great lens but I'm still learning and I'm thinking of getting a new flash for the camera as well.

Shadow of you lens when you are too close to your subject. An external flash on a bracket will cure this.

It's because the lens barrel is blocking some of the light from the pop-up flash & causing a shadow.

Better to use off camera flash. You can either use Nikon speedlights (expensive, but you'll have TTL control from the onboard commander flash on your camera) or use manual speedlights & radio triggers (much cheaper - all you have to do is set the power output manually).

Nikon SB600 speedlight = £220
YN560 manual speedlight = £60
RF602 radio triggers = £27.

Yes, it is the lens blocking out part of the flash. Are you using the hood on the lens? If you are, you can try taking that OFF and see it that cures the problem. It is always good to use a hood, but with a pop up flash, it will often times cause light blockage with the pop up flash.

I agree with the last statement, Shadow of you lens when you are too close to your subject. An external flash on a bracket will cure this.