Shallow Depth of Field techniques [Archive]

Daniel Browning

March 31st, 2009, 04:47 PM

I shoot on the XH-A1 and was wondering if anyone had any techniques to get a shallow (as possible) depth of field out of the fixed lens.

If you really mean "depth of field", then there's literally only two options possible:

Wider f-number (e.g. f/3.4 instead of f/5.6).
Zoom in without moving.
Move in without zooming.

You'll notice that the last two methods both require a tighter composition, such as a headshot instead of three-quarters.

None of the other methods actually affect depth of field. They affect background blur. Of course, most of the time when new photographers think they want "shallow depth of field", what they actually want is "background blur", and are just not aware of the difference.

There are several methods for increasing (or decreasing) background blur. Increasing the distance between the subject and the background is one, for example, that was touched on already in this thread.

Two other methods for increasing (or decreasing) background blur are:

1. Magnification.
2. Depth of field.

The magnification method, or "back-up-zoom-in" technique, has these effects:

Same amount of the subject is in focus (same DOF).
Perspective is altered drastically (and often detrimentally).
Composition of background is altered drastically.
Background blur is larger due to magnification.

The DOF method, e.g. a wider f-number, has these effects:

Less of the subject is in focus (different DOF).
Same perspective.
Same composition of the background.
Background blur is more diffuse due to being more out of focus.

Everything about them is different. Even the one thing they have similar, background blur, is different: blur caused by DOF looks different from (and better than) blur caused by magnification. The lattening/compression of perspective takes away from the perception of distance ("3D" look). Loss of background removes the subject from their environment, whereas DOF allows the background to contribute to the image, even if it's blurred.

Despite the fact that these two methods are different in every way, 99% of people talk as if they do the same thing. It would be better if this myth
were squashed out.

Using magnification (back-up-zoom-in) is a perfectly viable way to achieve greater background blur, and in many circumstances, it's the only method to increase background blur, especially with small-sensor cameras.

Using DOF is also a perfectly viable way to achieve greater background blur, if the camera/lens is capable of it. Sometimes it's an undesirable method, for example if it causes important parts of the subject (e.g. one eye) to
go out of focus. That's one reason why magnification (back-up-zoom-in) is used even with large formats: it magnifies background blur without making parts of the subject more blurry. In other words: DOF stays the same. So even if you have the option to use DOF, another method, like magnification, might be better.

Also, it should be said that there are a variety of ways to make DOF thinner for a given subject:

Wider f-stop.
Same f-stop, but larger sensor and matching lens (same AOV).
Zoom in without moving.
Move in without zooming.

Note that "back-up-zoom-in" is not one of those methods. Also note that the last two methods cause a change in composition (perspective, background, field of view). The first two methods cause the DOF to get thinner without affecting the composition.

Again, there a many reasons why it's important to distinguish the difference between blur-from-DOF and blur-from-magnification.

The biggest, I think, is perspective. Many people do not appreciate the vital importance of perspective in their composition. To downplay the difference between DOF and back-up-zoom-in is to say that perspective is not important. But it is vital. One may compromise perspective to gain background blur, but the image suffers as a result. Perspective is also the reason that "zoom with your feet" is bad advice: the perspective must be chosen that best conveys the purpose of the image, not whatever is forced on you by the focal length.

For example, say the subject is a headshot, and at f/5.6, the entire face is in focus, including both eyes, ear, and nose. But you decide the background blur is not diffuse enough. You open up to f/2.0, and the change in DOF causes the background to become much more blurry and diffuse, just as desired. Unfortunately, it also causes the subject to have thinner DOF: now only one eye is in focus, the other eye is slightly blurry, and the ears and nose are really out of focus.

At that point, you switch back to f/5.6, but increase magnification by backing up and zooming in (for the same field of view). Now much of the background is gone from the image, which could be either good or bad. The background is also much blurrier, because the slight blur was magnified more. And, most importantly, perspective is now much flatter. The subject takes on a slightly more "2D cutout" look instead of 3D, and the background seems much closer than it really is. BUT: the good things are that the face is completely in focus, and the background is blurred nicely. Given how often this technique is used in Hollywood and on TV, you would be in good company to use it. (I like it too, but I enjoy truly thin DOF more.)

Here's a reverse example. Let's say you're shooting a three quarter shot with a RED ONE and a 200mm super telephoto lens at f/2.0. You want the ears and nose in focus, but they're just not inside the DOF. So you do the
opposite of "back-up-zoom-in": you zoom out to a shorter 135mm telephoto lens, still at f/2.0, and move closer to compensate. But the DOF is still the same; the eyes and ears are still out of focus. So you try 85mm f/2.0,
50mm f/2.0, 35mm f/2.0, 28mm f/2.0, and even a 24mm f/2.0. Even with the super wide angle 24mm f/2.0, the ears and nose are still just not in focus. The DOF is the same, all that changed was perspective, background, and background blur. To get them in focus, you have to change something that actually changes DOF, not background blur.

Longer focal lengths increase magnification of the background, so even the tiniest bit of blur becomes larger in relation to the subject. If there is no blur (e.g. at long focus distances), then the magnification method doesn't work. Also, there are many subjects where it's hard to use that method (e.g. cramped rooms, wildlife, the moon, etc.) because it's impossible to backup or you're already using the longest lens you can afford.

DOF and background blur are different. We should know what contributes to each, and why.