Fast Glass: Lenses that offer faster apertures are valuable tools for depth of field control. Apertures of f/1.4 and f/1.8 are often associated with fast single-focal length lenses: 24mm, 50mm or 85mm. Zoom lenses with f/2.8 apertures that remain constant at any zoom length are incredibly popular too. Common are: 17-50mm, 28-75mm, 70-200mm and more.

ND Filters: In order to get the most out of fast-glass lenses outdoors photographers need to use Neutral Density filters. To use the widest apertures outdoors the amount of light entering the lens must be reduced. Digital cameras can only reduce ISO sensitivity so far (usually ISO100), the photographer then must use an ND filter in order to use f/2.8 or even f/3.5. Most photographers will have both ND4 and ND8 filters to fit their fast-glass lenses.

Nit-Picking Tech Note: ‘Depth of field’ and ‘depth of focus’ are not the same thing. While depth of field describes a range of focused space in an image, ‘depth of focus’ describes the range of distance inside the camera between the lens focus node and the film / sensor plane that provides acceptable focus. Depth of focus is also called focus tolerance.

One Big Argument for Full-Frame Sensors: Perhaps the most compelling argument for owning a camera with a full 35mm sized image sensor is found in depth of field. An APS-sized sensors doesn’t really magnify the length of a mounted camera lens, it crops the image instead. If we look at the image below we see that a 50mm lens provides the exact same image circle to either an APS or Full-Frame sensor camera.

Click to see full-size

To achieve the same composition on an APS-sensor camera as on a Full-Frame camera the photographer must either back up to increase the distance between camera and subject or keep the same working distance and choose a shorter lens length. Either choice increases the depth of field in the final image. This is true at any lens length or working distance when we compare the two sensor formats. For any specific composition the Full Frame sensor will always provide the option of shallower depth of field.

Bonus Info – Hyperfocal and Depth of Field: Hyperfocal length is the closest distance at which a lens can be focused while still keeping objects at infinity acceptably sharp. (Also called the maximum depth of field of a lens.) This point will vary with the lens aperture; as the aperture gets smaller the hyperfocal point gets nearer to the camera increasing the maximum depth of field. The image below is of a wide angle lens, the callout box shows the focus range indicator.

We see the lens is focused to infinity and we also see that there are aperture numbers arranged on either side of the focus mark. This is where hyperfocal length can come in handy. What the aperture scale is indicating is the depth of field on the focus ring for the combination of aperture and focus setting.

On the focus ring we see that the distance scale in yellow numbers indicate ‘Feet’. Let’s assume that we have the camera set to use f/16. We read the depth of filed scale in this way: the focus indicator is pegged at infinity and on the aperture scale below the window we see that on the right side of the focus mark aperture f/16 lines up close to the yellow “4” on the focus ring. Reading this scale tells us that focused at infinity with an aperture of f/16 everything from four feet to infinity will be in focus.

Now here is the hidden trick used by those in the know. Since the aperture scale below the focus ring is indicating the lens’ hyperfocal length we can use this information to get even more of our scene in focus. Rather than focusing at infinity we move the focus ring on the lens so that infinity lines up with our chosen aperture on the left side of the focus indicator, in this case f/16. In doing so we have changed the depth of field in our final image from “4 feet to infinity” down to “2 feet to infinity”. This is a huge advantage in landscape photography, by using this technique the photographer is able to capture images with enormous depth of field.

There aren’t many lenses left that still sport a depth of field scale, those that do tend to be prime lenses and not zooms. The scale is accurate for either full-frame or APS-sensor digital cameras. As lens length gets shorter this technique will have less effect.

1/3 and 2/3 – The depth of field of an image isn’t equally distributed around the subject. Actually the range of acceptable focus is unbalanced with 1/3 of the range in front of the focus point and 2/3 behind it. This is important to know especially when shooting up close and at wide apertures. For example: a close portrait image at a 105mm lens length and a f/2.8 aperture shot from four to six feet away. If the photographer focuses on the bridge of the subject’s nose the ear will likely be out of focus. Focusing instead on the cheek or eye farthest from the camera will often be more successful in achieving an overall focused image.

Because of this imbalance in the range of focus, group shots will often look sharper overall if the photographer chooses the second row of the group as a focus target.

Understanding depth of field and taking control of its effect on an image is an important skill in photography. Depth of field describes the area of acceptable focus in an image. Even though a camera is focused on a specific point in a scene there is a range of space in front of and behind that point that appears to be in focus as well. This range of focused space can be controlled by the photographer and used for creative effect.

A classic example of controlling depth of field can be seen in portrait photography. When a photographer takes a portrait the goal of the image is to focus attention on the subject. By controlling the depth of field the person’s face is sharply focused while the background is a soft blur.

While there are several variables that help create shallow depth of field in a portrait, in combination the outcome is an image that both isolates the subject from the background and creates a feeling of dimension in a two dimensional picture. Photographers can to a great degree control what is and what isn’t in focus in the final image.

The variables that photographers use as tools in creating and controlling depth of field are: lens selection, aperture and magnification. Shorter length lenses set for smaller apertures at a greater distance will have increased depth of field. Longer lenses at wider apertures at closer distances will have decreased depth of field.

The Tools

Turning first to lens aperture the rule of thumb is that larger (wider, faster) apertures create shallower depth of field. Here is a scene to consider: the photographer is shooting a subject that is six feet in front of a bush or tree. If the camera is set to use an aperture of f/8.0 both the subject and the background will be in focus. If instead the photographer sets an aperture of f/3.5 the subject remains in focus but the background is thrown into a blur.

Lens selection impacts depth of field too. The longer the lens focal length is the more shallow the depth of field will be. A 28-75mm zoom lens set to 28mm will offer great depth of field while the same lens set to 75mm will offer much less. This has to do with several technical issues such as hyper-focal length and image compression that we won’t get into here. However we can easily prove this effect by recreating our test scene and then shooting two pictures without moving the camera; one image at our shortest focal length and another at the lens’ longest length.

Last we have magnification. Photographers are used to thinking of magnification in terms of lens length but here we mean how close or far the camera is from the subject. An image taken with the camera closer to the subject will have less depth of field than an image taken when the camera is moved farther away. (Using the same lens length at both distances.)

We can get a sense of how these three tools are applied in general terms by looking at three types of photography:

• The landscape photographer will lean toward short length lenses while using small apertures. In general their subject is at a considerable distance too. The result is an image with tremendous depth of field. Everything from their toes to the horizon seems to be in focus.
• On the other hand the portrait photographer looks to isolate their subject from the environment. By using a moderate telephoto lens (75mm to 105mm) set to a wider aperture and shooting from a distance of six to ten feet the resulting image will have a sharp subject and a blurred background. The degree of blur is controlled by the aperture most often.
• The macro photographer by necessity is shooting with the camera positioned very close to the subject (high magnification). In addition most macro photography is done with a longer lens length. The longer lens permits lighting the subject without casting the camera’s shadow on it. However a camera placed extremely close to the subject and with a long lens mounted is a recipe for unacceptably shallow depth of field – perhaps measured in millimeters. For this reason the macro photographer will set the lens aperture to the smallest extremes in order to maximize the depth of field, favoring f/22 and smaller if available.

Of the three tools available for depth of field control the lens aperture setting is the most measurable and repeatable. For this reason the majority of intermediate to advanced photographers will have their cameras set to Aperture Priority mode (Av) if it isn’t in full Manual mode. Program mode will occasionally hit a setting that creates pleasing depth of field but it’s more often just a happy accident when it does.

Exercising control over depth of field is as much a part of good photography as exposure or composition. Using it to best advantage is a skill worth developing. I suggest practicing using the tools discussed in this article and spend some time reviewing the images you capture in large size on your computer monitor and set the view to show the camera settings with the image.

Be somewhat systematic in learning to use depth of field by taking the same image several times varying one tool only: lens length, aperture or magnification. Keep in mind the relationship between the three tools:

Wider lens + smaller aperture + greater camera to subject distance = greater depth of field

Longer lens + wider aperture + closer camera to subject distance = shallower depth of field

For additional information on Depth of Field please follow this link.