Reflection 6
In this week, we talked about focal length and review the knowledge point of depth of field.
About focal length, Lens focal length tells us the angle of view—how much of the scene will be captured—and the magnification—how large individual elements will be. The longer the focal length, the narrower the angle of view and the higher the magnification. The shorter the focal length, the wider the angle of view and the lower the magnification.
The focal length of an optical system is a measure of how strongly the system converges (focuses) or diverges (defocuses) light.
In a single lens objective, the focal length is the distance to the film plane, but since most objectives are a rather complicated set of lenses it’s not that simple.
That’s rather technical, but the most interesting characteristic of focal length is that it’s one of the decisive factors in how the angle of view of your lens is. The longer the focal length is, the narrower the field of view. The other decisive factor in the angle of view is the size of the film or sensor.
Lenses with a fixed focal length are also called primes, and lenses with a variable focal length are called zoom lenses.
The physical properties of a lens at a given focal length also affect the depth of field. A shorter focal length lens (say 27mm) focused at 5 meters, set at f/4 has a deeper DOF (perhaps from 3 meters in front and 20 meters behind) than a longer focal length (say 300mm), also set at f/4 focused at 5 meters.
There are both practical and creative consideration when making these decisions, so understanding focal length might be the most important visual tool for a director to comprehend.
A longer focal length, such as 135mm or 85mm in the diagram below, results in a narrower angle of view and greater magnification. Meanwhile, a shorter focal length, such as 24mm or 14mm, translates to a wider angle of view and less magnification.
In this first example, below, the camera is positioned the same distance from the subject and different lenses are used:
In the next focal length example, the camera is positioned at varying distances with different lenses, as indicated in each photo:
In summary, the lens mm number refers to the focal length or magnification. This can also be casually referred to as the lens size. Different manufacturers make lenses with different characteristics. So when I film something, I really need to think about what lens I want to use in the shot.