Control Depth of Field
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Photographs are two-dimensional representations of our three-dimensional world. Depth of field allows us to reach into the depths of our image and highlight a certain part of it. Basically the depth of field is how wide - or more practically how deep - the area in focus is. The truth of the matter is that there is only a single point of true focus for a lens. However because visual imperfections in an image must to be within a certain size limit for the human eye to perceive them it becomes possible to take a picture where objects within a certain distance in front and behind the point of focus will also appear clear and crisp. This range is the depth of field.
Focal LengthThe first concept that needs to be taken into account is focal length. In short focal length is the distance from the center of the lens (or lens assembly) to the point where the light will be focused. Focal length is a characteristic of lenses not of cameras themselves and each lens will have a different focal length which corresponds to a different purpose for that lens. Shorter focal length lenses (ex. 28mm) will have a wide field of view which they capture and are most suited for tasks such as macro photography. On the other hand are lenses with very long focal lengths (ex. 400mm+). These lenses have a narrower field of view but have amazing zoom capabilities.Lenses can change the distance to the point that is in focus is by adjusting the focal length (zooming). A normal focal length for most DSLR cameras is around 50mm. In most cases when the focal length of a lens is given this is actually the maximum focal length possible through the zoom function of that lens. An interesting side note about focal length is that different frequencies of electromagnetic radiation (i.e. different colors) will have slightly different focal lengths for the same lens. This has to do with the way different frequencies of light pass through glass. The artifacts that result from this are known as chromatic aberrations and can appear as a halo of color or rainbow fuzz around certain parts of the image.
F-numberOnce you have an understanding of focal length the next concept which needs to be understood is the f-number or f-ratio. The f-number determines the aperture settings for a given camera. This number is actually the ratio of the focal length divided by the diameter of the aperture. Because this number is a ratio based on characteristics of a lens f-numbers should be equivalent between different lenses. Higher f-numbers correspond to a larger aperture. This allows more light to fall on the sensor potentially resulting in the necessity for a lower ISO setting. Another result of changing the aperture setting is the depth of field changes. Higher aperture settings will result in a wider depth of field. Inversely, working with a lower f-number will result in a narrower depth of field.In an effort to express this concept more clearly I performed a test sample. I used a macro lens so the changes in depth of field would be extremely clear. Using a scrap of 2x4 I marked a foot long segment of wood with twelve marks each an inch apart. At each point I placed a single nail. Using a low f-number (f/3.3) I focused as best I could on the middle of the nails and proceeded to snap a shot at each f-number supported by my lens all the way up to f/36. It was very clear to see the depth of field expand as the aperture steadily grew. See a sample below.
FocusSo as you can see the climbing aperture size results in the area of the image that is in focus to be "deeper" or cover a larger 3- dimensional area. The reason for requires a closer look at the concept of focus itself.Light which lands on a camera sensor has come from many different directions and many different distances before being focused through the lens onto the film or sensor. As mentioned above the light is focused onto a single point (ideally right on the sensor!) and this point corresponds to a plane of depth a certain distance from the camera. When light is out of focus it tends to be affected by the lens in particular ways. With a circular aperture the effect tends to be towards circular distortions with points of light blurring out into a circular shape. Now the truth of the matter is that the human eye can pick up only certain levels of distortion. At small enough distortion levels an image will still look sharp. Controlling the area which will appear sharp (i.e. focused) to the human eye is the goal of controlling the depth of field. If you can't determine what the depth of field for a particular focal length and f-number is there are many online depth of field calculators such as this one or this one. |
