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If suppose I want to take a photograph of a poster on the wall of the room, so I can't back away any farther than 5.0m to take the shot(max dist of the room). The poster is 0.80m wide and 1.2m tall, and I want the image to fit in the 24 × 36 mm frame of the film in your camera.

What is the longest focal length lens that will work?

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The ratio of subject width to distance corresponds to the ratio of sensor (or film) width to focal length.

Thus, for portrait orientation, you may estimate the focal length as follows:

horizontal: 24 mm · 5.0 m / 0.8 m = 150 mm

vertical: 36 mm · 5.0 m / 1.2 m = 150 mm

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