I was monkey-ing around with depth of field the other night.
FIGURE 1. In this image, the aperture value = 29, and the shutter speed , = 30 seconds under these dim lighting conditions.
FIGURE 2. Here, the aperture value = 4.5, and the shutter speed = 1.3 seconds.
There seems to be three main factors that affect the depth of field: The focal length of the lens, the aperture, and the distance of the subject in relation to the background and the lens.
Here I tried to limit the variables to just the aperture. For these images the global values are: An 18-55mm zoom lens (in this case the focal length was set at 35mm). I also had the camera set to aperture priority.
The sock monkey was sitting about 3 feet from the front of the lens, and approximately 4 feet beyond the monkey sits the monitor. The depth of field in Fig. 2 is quite narrow. The material in the chair immediately behind the monkey is already starting to blur (compare to Fig. 1)
The monitor sets up a backlighting situation which I tried to balance with my hallway light. I used my spot meter to expose for the monkey’s brown skin tone. It seems that metering with the camera’s spot meter is somewhat hit or miss.
To really increase the amount of fuzziness in the background, I had to adjust the Zoom lens so that its focal length was at the maximum 55mm. In this image, the aperture value is 5.6, the widest value possible for this lens at 55mm. I haven’t really changed my distance from the monkey, though the longer focal length has the effect of bringing one closer to one’s subject matter. Somehow, it seems that the magnification of the subject has a correlation to the compression of the depth of field, but I still can’t visualize the physics.
Even though I'm becoming aware of the variables involved with setting up depth of field shots, I still can't control it. In this picture of a shell, the sand grains show that my point of focus was a little bit too much towards the front of the shell. This is a case where the subject isn't really engaging in a lot of activity, and yet I still wasn't able to get the whole shell in focus (Sadly this is the best of three attempts). In the olden days, lenses used to have distance/aperature markings on the focus ring that worked to show what the depth of field was for a given distance and aperature setting. These markings don't appear on the Canon EFS 18-35mm kit lens that comes with the Rebel.
While the instruction manual details a depth of field preview button, the only thing that appears to happen to me when I use it is that in some cases, the image in the view finder becomes darker, which, instead of helping me determine the range of acceptable focus, just makes it harder to see. I know there must be some kind of a correlation there, but so far, it hasn't been immediately obvious.