Lesson 2/Learning Event 2
Figure 2-5.
Different lens to subject distance
4. There are times when you want to maintain the same image size but you
must move closer or father away from your subject. This is done by using a
long focal length if you must move away from your subject or a wide angle
lens when you must move close to the subject.
5. Maintaining the same image size on the focal plans from three different
subject sizes can be obtained from three different focal length lenses at
the same lens to subject distance. For example, in figure 2-6 an image size
of 5mm on the focal plane is produced with each lens by doubling the focal
length as the subject is reduced in half while maintaining the same lens to
subject distance. The figure illustrates an example of:
a. A 25mm lens will produce a 5mm image of a 60 foot tall building at a
given lens to subject distance.
b. A 50mm lens will also produce a 5mm image size of a 30 foot tall
building at the same given lens to subject distance.
c. And the 100mm lens will produce a 5mm image size of a 15 foot tall
building at the same given lens to subject distance.
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