Aperture
For the first and second photos I used a small f-stop to create less focus throughout the photo. In the first image I used f4 so the everything besides the alien would be blurred but he would be in focus. Also with a large aperture, f5.6, I was able to capture the red food coloring in focus while blurring the rest of the food coloring in the background. The last photo I used a small aperture, f22, to allow all parts of the image to remain in focus. By setting the camera to a large aperture, blurring the subjects not in focus created a large depth of field, which I did manually by adjusting the knob at the end of the lens. For instance, when looking at the second image the blur in the background shows some distant between the foreground and the background. Personally I think that the first photo is better because the blur along the sides leading up to the alien give this object much more importance. You would want to use a shallow depth of field when there is something specific you wish to put in focus so that the viewer is drawn to this object over the blur around it. A large depth of field would be better when your are taking a photo of a large subject so that everything can be in focus or when you do not wish to highlight anything specific by using blur.
Depth
For the first photo, i attempted to use leading lines to allow the viewers eyes to follow the stick up the bark of the log. The other sticks at the bottom of the photo also have the effect of leading towards the bigger stick and eventually towards the bark. Most of this photo is positive space but I think one interesting part of the negative space created at the top of the leading stick shows that the log is hollow on the inside, something you might not have expected. The second image frames the object that the photo is focusing on. The small berry in the middle of the photo has been framed by the design of the window and immediately captures the viewers eye towards this aspect of the photo. The last photo has a Christmas tree in the background, the main focus of the photo, but the strong foreground object steals some of the significance away. The way this object is placed it becomes difficult to determine what exactly the object is, possibly because it appears to have the same height as the Christmas tree.