Sensor size and Depth of field in film and digital cameras
Depth of field is defined as the range in which the photo is in focus. Achieving a specific depth of field is a useful photography technique used for example to achieve a blurry background effect. Film and digital cameras have different optical attributes as a result of using different sensor sizes. For that reason depth of field is also different in film and digital cameras.
Before you can understand the difference between film and digital cameras when it comes to depth of field, it is important to understand what depth of field is and how it can be set by the photographer. Depth of field (also known as DOF for short) is defined as the range in which the photo is in focus. For example: an infinite depth of field means that the photo will be in focus from a certain distance from the camera and up to infinity. Another example: when taking a blurry photo the goal is to set the depth of field just around the object, which results in the object being in focus while the background behind it is not.
The depth of field is influenced by the optical parameters of the camera and the specific scene. The main parameters that influence the depth of field are:
The distance of the object from the camera influences the possible depth of field settings. For example shooting an object that is very close to the camera will result in a smaller depth of field. On the other an extreme case when the object is in practical infinite distance from the camera will result in the camera focused on infinity and the depth of field being infinite too.
The aperture: The depth of field increases as the f number increases. The wider the aperture (f number decreases) the smaller the depth of field and vice versa: the narrower the aperture (f number increases) the greater the depth of field.
The focal length of the lens: The depth of field grows as the focal length gets shorter. For example if you use normal lenses at somewhere around 50mm your depth of field will be much greater than if you use zoom lenses at 200mm. With a very long focal length the depth of field can be extremely small.
One of the main differences between film and digital cameras is the sensor used to capture the photo. Film cameras use a film sensitive to light that is placed behind the lens. When a photo is taken the shutter opens for a predetermined period of time and light hits the film. The result is a photo “printed” on the film. Digital cameras on the other hand use a fixed electronic sensor (also known as CCD) that is situated behind the lens. The sensor is built from tiny light sensitive sensors each representing a pixel. When the shutter opens light hits the sensor and each pixel gets its “value”. Put together all the pixels comprise one digital photo.
A normal single frame of film size was standardized at 35mm. Digital cameras on the other hand use smaller sensors. One of the reasons why digital sensors are smaller is cost – manufacturing a large electronic sensor is costly (for reasons such as yield and other specific process limitations). The depth of field changes with the sensor size, the smaller the sensor the higher the depth of field. In fact low end pocket cameras use small sensors that together with their other optical limitations result in depth of field that is practically infinite. Because photographers used to measure everything based on 35mm film – many digital cameras still quote “35mm equivalent” measurement. There are a few high end SLR digital cameras that use 35mm sensors thus providing the same optical characteristics as the old film cameras. These are still expensive and considered high end professional cameras.
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