Homework Assignment No. 3: Focal Lengths of Lenses, Pay Attention to Backgrounds.
In my third class, we look at the focal lengths of lenses. Some have fixed focal lengths. Other lenses have variable focal lengths and are called zoom lenses because you can zoom in and zoom out: zoom in to photograph a narrow area, zoom out to photograph a wide area.
The most common zoom lenses have a range of focal lengths from 18mm to 55mm. Other common zoom lenses can zoom out to 200mm and 300mm. An 18mm lens is called a short or wide-angle lens, while a 200mm or 300mm is called a long or telephoto lens.
Short focal lengths usually have high depths of field; that is, close and far-away objects all are more in focus. Long focal lengths lenses have very short depths of focus, also called depths-of-field; that is, the object is in focus but the background is blurred: the further away the background, the more blurred it will be.
Out-of-focus blur usually looks different from motion blur because the former blurs objects in all directions while motion blur only blurs objects in the direction of motion.
Your homework assignment is to take two photos of an object using two very different focal lengths while keeping the object the same size in both photos. Pay attention to how the background changes. Using a short (wide) focal length, more of the background will be included. Using a long (telephoto) focal length, much less of the background will be included. By moving the camera, and yourself — up, down, and side-to-side — pay attention to how the background changes. If you have a short focal length, moving the camera does not change the background much at all. If you have a long focal length, the background changes a lot!
Also pay attention to how the relative size of objects changes as focal lengths change. Short focal-length lenses make objects appear more distant relative to one another. They make space appear bigger. Use short focal-length lenses to make rooms look bigger. Long focal-length lenses make objects appear closer to one another. They compress space. Use long focal-length lenses to make objects look closer to one another.
In general, fixed focal-length lenses are better than zoom lenses. (1) They have fewer pieces of glass and so produce sharper images, especially at the edges of the frame. (2) They let in more light so you can see better when taking photos through the lens. This is not a factor in mirrorless cameras. (3) Fixed focal-length lenses have lower f-number (aperture) settings and so create more focus blur. We look at that in detail in Lesson Four, Aperture Priority Mode.
Samples
Short focal-length lenses capture big scenes and make spaces appear more open. At the same time, everything is easily held in focus.
Short focal-length lenses capture big scenes and make spaces appear more open. At the same time, everything is easily held in focus.
A lens with a short focal length can make a small room appear much larger than it really is.
Long focal-length lenses have less depth of focus. Backgrounds are easily blurred.
Long focal-length lenses have less depth of focus. Backgrounds are easily blurred. With the background blurred, the subject looks more in focus and more detailed.
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