How to focus on more objects at the same time while maintaining shallow DOF?

Question

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.

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?

Answer 1

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.

---

Addendum

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.

Answer 2

There is software that uses a technique called "focus stacking" which may help with the first part of your question (Helicon Focus for example). You take several photos, each with a very shallow DOF and stitch them. This is sort of like a panorama stitch, but in the forward-backward (i.e. focus) direction instead of left-right. The technique is used to get good microscope images of insects, for example, because microscopes have a very shallow DOF.

EDIT: see also this post.

Answer 3

There are two ways to measure camera to subject distance for subjects not in the center of the frame. Which one works best depends on the characteristics of the particular lens being used.

If a lens has a fairly flat field of focus then the distances should be measured from the camera's focal plane using perpendicular lines to each of the subjects. Note that if the subjects are lined up parallel to the camera's sensor plane the focus distance is the same to all three subjects.

If the lens has field curvature then the distances should be measured at various angles from the optical center of the camera's sensor to each of the subjects. Note that of the subjects are lined up parallel to the camera's sensor plane the focus distance is NOT the same to all three subjects. To place all three subjects at the same focus distance one must arrange them in an arc that mimics the lens' field of focus.

Some lenses, such as Canon's EF 85mm f/1.2 L II intended for portraiture usage, are intentionally designed with field curvature to give a certain look to the images created with them. Other lenses, such as the Zeiss Planar series, are very well corrected to have a very flat field of focus. These lenses cited are all fairly expensive and designed to do one thing very well. Most lenses are designed not only with optical considerations but also manufacturing and pricing considerations as well and tend to accept design compromises to allow them to do many things sufficiently well at an acceptable price to the potential purchaser.

In reality most lenses have fields of focus that are slightly (or sometimes not so slightly) irregular in a shape that falls somewhere between the two theoretical extremes illustrated above. Roger Cicala, the founder and chief lens guru at lensrentals.com, has even developed a way to test a lens and graphically represent a lens' focus field at a particular aperture and focus distance. Part 1 and Part 2 of a series about it from his blog.

So what does this mean when you are shooting three dogs and want all three of them to be inside a fairly narrow field of focus?

It means you must either situate the dogs so that each one falls within the shape of your lens' field of focus or you need to increase the depth of field by stopping down until all three dogs are acceptably sharp wherever they are sitting. To do the former you must know the shape of your particular lens' field of focus for that distance and aperture. To do the latter means giving up the absolutely shallowest DoF for which your lens is capable at that focus distance.

One way to get an idea of the shape of your lens' field of focus is to stand on some kind of grid with a highly detailed surface. I like to use an athletic field used to play American football. It's often referred to as "the gridiron" due to the yard lines, hash marks, etc. painted on its surface. For a longer focal length lens I stand at the center of the goal line, center my viewfinder on the X in the middle of the 40 yard line that is used for kicking off the football to begin a half, focus in the X and take a picture. The blades of grass provide enough detail to see how far in front of the 40 yard line the field of focus curves towards me as one moves from the center to the edges of the frame.

Perhaps the field of focus moves towards the camera as we look further left and moves away from the camera as we move further right. That would indicate some lens misalignment known as tilt, in this case the lens would be tilted towards the left of the line perpendicular to the center of the camera's sensor.

Whatever the shape of your lens' field of focus, you must learn what that shape is and use that knowledge to place different objects within that field of focus when you are using very shallow depth of field.