12 Rules of Composition in Photography: Depth of Field
Demonstrating Depth of Field
Understanding Depth of Field
Depth of field refers to the portion of the photograph that is in focus. A shallow depth of field means that from front to back very little of the photograph is in focus. The opposite is a deep depth of field meaning that most or all of photograph is focused and sharp.
Depth of field can emphasize portions of the photograph. Shallow depth of field also creates isolation of your subject and eliminates structure in backgrounds. Good uses of shallow depth of field include shooting any portion of a subject for emphasis while blurring everything else. Depth of field can set the mood of a photograph creating a soft luxurious look while a deep depth of field can give a hard look typical of the photographs that come out of a point and shoot camera. Looking at many portraits you’ll see that they’re shot with shallow depth of field having the eyes of the person in focus and the balance of the photograph starts to soften the face and hair in most cases almost imperceptibly.
Some photographs were meant to look soft all over. This is most likely due to post processing and not using depth of field. There is a post processing technique called the Orton Effect (popularised by Photographer Michael Orton) where an overall glow is given to a photograph.
Depth of field starts with the lens that you purchase. You’ll see numbers like f1.8 or you may see a range like f4.5-f5.6. These numbers may also be expressed as 1:4.5-5.6. The f-stops value is an indicator of how much light passes through the lens. The amount of light that passes through the lens is controlled by an aperture. An aperture is a number of blades that open and close within the lens to create a larger or smaller opening for light. The smaller the aperture opening the deeper the depth of field achieved. So it stands to reason that the lower the f-stop number the shallower depth of field you will have. This also means the lens was built with more expensive clearer glass. The lens with a smaller f-stop like f1.2 may cost well over $1500 and the same lens with a minimum f-stop of 3.5 may cost 200 to 300 dollars. Unfortunately achieving a shallower depth of field comes with a cost.
Depth of Field
Depth of field is set on your DSLR camera by using the f-stop scale on the camera. You should become familiar with your camera controls so you can manipulate the f-stop. On a Canon camera the dial on the top of the camera indicates “AV” to control the depth of field. AV stands for aperture value, another term indicating depth of field.
The Effect of Focal Length
The amount of magnification of a lens is indicated in millimeters and is called the focal length. A 20 MM lens has a short focal length while a 300 MM lens has a long focal length and is called a telephoto lens. Most standard lenses that come with your DSLR camera are around the 50 MM mark. The lens with a shorter focal length generally can keep more of your subject in focus given the same f-stop value as a lens around the 300 MM focal length. See the photographs included with this article to see the difference.
If you’re looking to get the maximum depth of field you need to shoot at a larger f-stop usually F8 or greater and as high as a f32 and some lenses. The problem with many lenses is the characteristic of focusing ability (sharpness) at the high and low ends of the aperture range. Many lenses do not focus quite as well at the outer limits of the focal length range as they do in the middle aperture values. You can do a little test yourself by placing a printed sheet of paper on the wall and shooting at different aperture values. Most lenses will have the greatest image quality between f11 and f16. So it stands to reason that you’ll get your sharpest photograph from a shorter focal length lens using a middle aperture value. This is not always the case but you can use it as a general rule of thumb. When purchasing a lens make sure that you can return the lens within a reasonable period of time so you can try it out and determine if it fits your needs.
Depth of Field - Keep the Eyes Sharp
The Effect of Sensor Size on Depth of Field
Since the advent of digital photography sensors have replaced film. The size of the sensor that records the image has morphed into many sizes. Today most sensors in digital cameras with interchangeable lenses have a “crop sensor” which is approximately 2/3 the size of a full frame sensor. The more expensive digital cameras, generally over $2000 often come with a full frame sensor. The sensor is the same size as a 35 MM film. The larger the sensor the more control you have over depth of field. That being said crop sensors used with different lenses can achieve similar results as a full frame sensor. Longer focal length lenses will help in achieving the shallow depth of field desired by many photographers. Just remember that longer focal length lenses need more room between you and your subject for focus. A 50 MM lens may require only 18 inches in front of the lens before the photograph comes into focus while a 300 MM lens may require 4 to 6 feet beyond the front of the lens before you can start to focus your photographs.
Understanding Depth of Field
Very Shallow Depth of Field
Extension Tubes and Macro Lenses
When using macro lens or an extension you can enlarge your subject can get great detail. You may be shooting the centre of a flower or the head of the pin or a coin but nothing much bigger than this. Macro photography, or shooting the smaller things in life, deals with very small items and the depth of field is extremely shallow. To overcome this many photographers take several shots of the same image with varying areas of focus. The focus point may change by as little as 3 mm in each shot and these photographs are combined through software to create an image that has more focused possible through a single shot regardless of the depth of field that can be achieved.
The concept of depth of field is simple but complex at the same time. To generalize we can say that the smaller the sensor size and the shorter the focal length of the lens the less control we have over depth of field. The aperture value or f-stop of your lens allows you artistic freedom to create a photograph that you want. It allows for isolation of your image through blurring backgrounds while maintaining a sharp subject. To become a competent photographer you must know what your camera and lens combinations can do. This is achieved through much experimentation and maybe some note-taking to fulfill your artistic vision.
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