How can I focus on more objects, say three dogs sitting in front of you, while maintaining a nice background blur/shallow DOF? I have a kit lens ranging from 18-55mm with a variable aperture of f/3.5-5.6.
This is difficult to do with any lens, let alone a kit lens with an f/3.5-5.6 maximum aperture on a crop body. I have problems doing this with an f/1.2 lens on a full frame body.
The amount of background blur you get depends on a number of factors, aperture possibly being the least of them.
- subject-to-camera distance (closer you are, the more blur you get)
- subject-to-background separation (the larger it is, the more blur you get)
- focal length (the longer it is, the more blur you get)
- aperture setting (the larger the aperture [smaller the f-number], the more blur you get)
But with multiple subjects, you also have to have enough DoF to cover all three of them, so it depends on how they're lined up. Ideally, you probably want them at the same distance from the camera, and then you might have a shot. If they're spread over 20 feet, it's probably not possible, except with some goofy post-processing.
And how can I zoom in without losing too much background? I tried to zoom out to 18mm and move up closer to the subject to create the impression that the background is farther away but I can't get it blurry then. If I zoom in and back up to keep the subject in the frame, the background becomes blurrier but also less of the background is visible in the frame. What am I missing here? Is the DOF shallower or deeper when zoomed out to 18mm and being closer to the subject?
It depends. But probably deeper.
This is the other issue. All those factors I listed above interact with each other. If you use a longer focal length, you're probably going to be farther away. If you use a shorter focal length, you're increasing the optical DoF. There are certain limits past which you cannot go. If I'm shooting with my 8mm fisheye lens, everything from 3-5 feet to infinity will be in focus at f/4-f/8. It just has enormous depth of field and that's that. My 400mm f/5.6L USM can blur the background for a bird 20 feet away at f/8-f/11.
What you want to do may or may not be possible with your gear. You could also consider using the technique known as the Brenizer method, or bokeh-pano stitching. You use a fast short telephoto lens, like an 85/1.8 or 135/2, to shoot the overall image in pieces, and then stitch them together as a panorama. This was mostly done with full-frame to mimic medium format, but can also be used with APS-C to mimic full-frame.
If you really want to see how all the factors (focal length, format, subject distance, focus distance, and aperture) all interact, I'd suggest playing with a DOF calculator. Keep in mind, however, that everybody's ideas of "acceptable sharpness" have changed a bit with digital and smaller sensel sizes, so don't take the numbers as gospel. Just use it as a rough guide to see how things interact.
And then simply trying out the principles to see results should help you get a better grasp of what will work in which situations.
The other thing you might want to consider, if all you want to do is have your subjects be in contrast to the background, and "pop out" is to also consider off-camera lighting, and changing the lighting ratio between subjects and background. In that type of setup, you need far less blur to emphasize the subject.