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Thursday, April 18, 2013

Photography: Chapter 3

Motion & depth of field

Let’s start with aperture first. So what is aperture?

An aperture is a hole or an opening through which light travels to the photographic film or sensor

Before discussing “depth of field” and how aperture determines it, let’s understand what is collimated light. Collimated light is light whose rays are parallel, and therefore will spread slowly as it propagates. In other words it does not disperse with distance (ideally), or will disperse minimally (in reality)

The aperture determines how collimated the admitted rays are, which is of great importance for the appearance at the image plane. If an aperture is narrow, then highly collimated rays are admitted, resulting in a sharp focus at the image plane. If an aperture is wide, then un-collimated rays are admitted, resulting in a sharp focus only for rays with a certain focal length. This means that a wide aperture results in an image that is sharp around what the lens is focusing on and blurred otherwise

Depth of field (DOF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image. The area within the depth of field appears sharp, while the areas in front of and beyond the depth of field appear blurry (as depicted below)



So the crux of the above discussion is that smaller the opening, greater the sharpness of objects from near to far (as depicted below, this one was clicked with aperture value of f/16)


A large opening means more blurring of objects that are not at the same distance as the subject you are focusing on (as depicted below, this one was clicked with aperture value of f/5.6)


Shutter Speed

Shutter speed controls the length of time the light strikes the sensor; consequently, it also controls the motion blurriness of the image

Excessively fast shutter speeds can cause a moving subject to appear unnaturally frozen. This is because, the less time light hit the sensor, the less time subject has to move around and become blurry. Therefore, fast shutter speed gives more control to freeze the motion of a fast-moving subject (as depicted below, this one was clicked with exposure time of 1/500 sec)


Slower shutter speeds are often selected to suggest movement in a still photograph of a moving subject i.e. intentionally blur subjects (as depicted below, this one was clicked with exposure time of 2 sec)



References
1. Wikipedia
2. Canon EOS 60D from Snapshots to Great Shots by Nicole S. Young




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