Why doesn’t camera-to-subject distance affect exposure meter readings?
Asked 1/5/2018
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In the exposure triangle we use aperture, shutter speed, and ISO, and flash/light meters give settings based on the light at the subject. But light intensity falls off with distance, so why doesn’t an incident meter reading change when the camera moves farther from the subject? For example, photographers often meter at the subject’s face and then shoot from different positions. Why is that still valid? Shouldn’t exposure depend on the distance from the camera to the subject?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
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You do get less light as you move further from a object. However, that less light is focused on a smaller area. It so happens that the two effects cancel out, and the focused image of a object will be the same brightness as distance is changed, assuming the f-stop is held the same.
For example, moving twice as far away means the lens intercepts ¼ the light from the same object. However, the size of the focused image shrinks by two in linear dimension, meaning a reduction of area by 4. So ¼ the light is focused on ¼ the area, resulting in the same brightness image.
Originally by user7603. Source · Licensed CC BY-SA 4.0
user7603
8y ago
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Because exposure depends on illuminance at the subject and image brightness at the sensor, not directly on camera-to-subject distance.
As the camera moves farther away, the subject sends less total light into the lens, but the subject’s image also becomes smaller by the same proportion. These effects cancel out: less light is spread over less sensor area, so brightness per unit area on the sensor stays the same at a given f-number.
That’s why an incident meter can be read at the subject and still give a valid exposure even if the camera position changes. The key distance for inverse-square falloff is light-to-subject distance, not subject-to-camera distance in normal photography.
So if the lighting on the subject stays the same, and you keep the same aperture, shutter speed, and ISO, exposure does not change just because the camera is farther away.
What does change exposure is moving the light source relative to the subject, or changing framing by using a different focal length/aperture combination.
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