Learning Event 2:
DETERMINE THE EFFECT ON IMAGE SIZE FOR A GIVEN LENS
1. The focal length of a lens determines the image size on the focal plane.
For example, three different focal length lenses are focused at the same
distance. This will produce different image sizes on the focal plane. If
the 50mm, normal lens, produces a 10mm image size, then the 25mm, wide
angle, will produce an image size of 5mm, and the 100mm, telephoto, will
produce and image size of 20mm (fig 2-4).
Figure 2-4.
Different image sizes
2. With the lens to subject distance the same, the longer the focal length
the larger the image size on the focal plane. A shorter focal length will
produce a smaller image size on the focal plane.
3. By changing focal length and subject distance you can maintain the same
image size.
For example, assuming that you are using a normal 50mm lens,
the same image size is obtained by doubling the focal length to 100mm as the
lens to subject distance is doubled (fig 2-5).
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