Understanding Depth of Field
Depth of Field is term used to describe how much of a photo is in focus. When a picture is taken the camera will focus on a specific point and the amount of area that is acceptable in focus both in front of and behind this main focal point is what is referred to as the depth of field.
There are several factors that influence the depth of field in any photograph. Photographs where a larger area of the photo are in focus are said to have a deep or large depth of field. This is often seen in landscape photos where both the foreground objects and the background are relatively in focus and sharp.
A shallow or small depth of field means that the area in focus is relatively small and that the distance of an object being either in focus or out of focus can be a matter of a few feet or even a few inches depending on the camera and lens. A shallow depth of field is often seen in portraits and macro photography. It helps isolate the subject from the background and results in a nice blurred background (bokeh) that enhances those types of photos.
Factors that determine depth of field
There are four basic factors that determine the depth of field of a photo.
- The aperture or f-stop of the lens
- The distance the subject is from the camera
- The focal length of the lens
- The size of the image sensor
The relationship of aperture and depth of field
Aperture is a term used to refer to how large the lens opens up to allow light to enter your camera and is one of the primary ways of controlling the depth of field of your photo. Without getting into all of the science behind it, the simple thing to remember is that a larger aperture will result in a smaller depth of field and a smaller aperture will result in a larger depth of field. One thing to keep in mind for those who might be new to photography is that a smaller f-stop number such as f2.8 means the lens opening is wider and will result in a shallow depth of field. On the other hand a larger f-stop number such as f11 means the lens opening is smaller and you will get a larger depth of field.
The relationship of distance between the camera and the subject and depth of field
The closer the subject is to the camera the shallower the depth of field will be. Therefore if you want a small depth of field you should move closer to your subject and if you want an increased depth of field you should move farther from the subject.
The relationship of focal length and depth of field
Focal length is a term used to refer to the angle of view and the magnification factor that a lens has. The focal length of a lens is expressed in millimeters (mm) with smaller numbers like 18mm indicating a wider angle of view lens with less magnification and larger numbers such as 500mm indicating a telephoto lens with a smaller angle of view and more multiplication. When it comes to controlling your depth of field the important thing to remember is that using a lens with a longer focal length will result in a shallower depth of field and using s shorter focal length lens will result in a greater depth of field.
While the science behind this aspect can be rather complex there are some great tools online that allow you to calculate your depth of field for any combination of these factors.
The relationship of image sensor size and depth of field
Cameras with larger image sensors will have a shallower depth of field than those with smaller sensors for any given aperture setting. The reason for this is the change in equivalent focal length between a full frame sensor camera and a camera with a smaller image sensor. The same relationship between image sensor size and focal length that affect what is known as the “crop factor” of a camera and lens combination also affects the depth of field. For example a camera with a full frame sensor will require a smaller aperture setting to achieve the same depth of field while maintaining the same equivalent focal length as a camera that has a crop factor of 1.5X. There is a helpful calculator on this page that shows this relationship very well.
How to control your depth of field
Your ability to control your depth of field is somewhat determined by the lighting conditions you are shooting in and the type of camera you are using. For those with point and shoot cameras that do not have advanced modes such as aperture or manual modes using your “scene modes” will give you some control over the depth of field. For example selecting the “portrait mode” should result in a smaller depth of field while selecting the “landscape” mode should result in a larger depth of field.
For those with DSLR’s and other camera types with advanced modes the easy way to control the depth of field is to use the aperture mode and adjust your f-stop to get the depth of field that you want. This allows you to control the lens opening (aperture) while the camera automatically adjusts the shutter speed and ISO.
Understanding the relationship between aperture, shutter speed and ISO will help you control your depth of field while avoiding other issues that could negatively affect your overall image. Using the right combination of these settings will allow you to get the best depth of field for your image given the lighting conditions that you are shooting under.
Why should I be concerned with depth of field?
Understanding depth of field and how to control it is a key to getting sharp, in focus images as well as creating the look and mood you want your photo to convey. Shallow depth of field can be used artistically to create great images and learning how to have razor sharp photos is key when shooting many subjects like wildlife and sports. Understanding how depth of field will affect your image is important to mastering the art and science of photography.
When should I use a shallow depth of field?
You should use a shallow depth of field whenever you want the subject to stand out from the background. A shallow depth of field is great for portraits, macro photography, sports action photography and some wildlife photography because it helps the subject stand out from its surroundings and draws the viewer’s eye to the main subject.
When should I use a deeper depth of field?
It is common to use a greater depth of field when doing landscape photography. This allows both the foreground and distance objects to be in focus. Also when doing group photography you will want a somewhat deeper depth of field than you would when doing individual portraits. That allows all your group to be within the depth of field range and in focus.
How can you know what your depth of field will be?
The simplest but often overlooked way of knowing what your depth of field will be is to use the Depth of Field or Preview Button that is common on many advanced cameras. This button quickly allows you to preview what the depth of field will be by stopping down the lens to the aperture setting you have selected. If the image appears somewhat darker when doing this you should not worry as the camera will compensate by adjusting the shutter speed as needed to get the correct exposure.
There are also several online tools as well as apps for smartphones that can calculate the depth of field based for you. Below are some links to some of the most popular ones.
Is it possible to have a depth of field that results in everything being in focus?
There is a point known as the “hyperfocal distance” that will result in maximum sharpness from half of this distance all the way to infinity. Knowing this distance is very useful for landscape photography as it allows you to produce a sharp in focus print.
In order to calculate this ideal or hyperfocal distance you can use a mathematical formula or there are some charts and programs available that do the calculations for you. However for practical day to day shooting there are some quick alternatives that can help find the right focal point to achieve this result.
The first method is somewhat of a trial and error basis but can get you satisfactory results if done properly. First begin by focusing on the most distant object in your scene. Then manually adjust the focus so objects closer to you are in focus without making the distant background appear to be soft or out of focus.
Additionally for those scenes that do not extend all the way to the horizon there is a rule of thumb one can use to get the greatest depth of field. That rule of thumb is to focus about 1/3 of the way into your scene while using an f-stop of f11. However with so many variables for best results it is better to use a depth of field calculator and make sure that the closest and farthest parts of the scene are within the depth of field range.
Increasing the depth of field through focus stacking
Focus stacking is a digital darkroom technique that combines multiple images that were taken at different focal distances into a single image. This results in an image with a greater depth of field than any single image. Using Adobe Photoshop or several other digital editing programs one can use this technique to produce a finished image that shows remarkable depth of field.
To increase your depth of field you can:
- Mover farther away from the subject
- Reduce your aperture….set your lens to a larger f-stop number.
- Decrease the focal length of your lens…zoom out if using a telephoto lens.
To decrease your depth of field you can:
- Move closer to your subject
- Increase your aperture…set your lens to a smaller f-stop number.
- Increase the focal length of your lens….zoom in if using a telephoto lens.
The links below are for examples of the type of tools available. Practical Photography Tips has not tested nor do we endorse these products.