Why doesn’t exposure change much when I zoom in at the same f-number?
Asked 1/27/2015
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Using a Canon 550D with a Tamron 18-270mm zoom, I set shutter speed, ISO, and aperture to fixed values (f/8) and metered a uniform white wall at 18mm. Then I zoomed to 270mm and expected the exposure reading to drop a lot, because the frame now covers a much smaller area of the wall. But the meter changed only slightly, to about -0.7 EV.
Why doesn’t exposure change dramatically when narrowing the field of view by increasing focal length, even though a much smaller portion of the scene is being photographed?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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It is because the aperture value of f/8.0 actually gives an aperture hole of f/8.0 over the entire focal length range, thereby opening up when the focal length gets larger, whereby the change in the aperture hole exactly balances out the change in wall area that is covered.
So at 18 mm focal length the aperture hole diameter is 18 mm / 8.0 = 2.3 mm, and 270 mm focal length the aperture hole diameter is 270 mm / 8.0 = 33.8 mm, so the aperture hole diameter grows a factor 15 in diameter, or factor of 225 in area, when the focal length narrows down the area on the wall with a factor 15 in both x and y direction, or factor 225 in area, whereby the total amount of light is the same over the focal length range.
The -0.7 change in exposure I saw is probably a result of poor lens design, so some light is lost at larger focal length.
Originally by user37275. Source · Licensed CC BY-SA 4.0
user37275
11y ago
0
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Because exposure depends on light per unit area at the sensor, not on how much total scene area fits in the frame.
At the same f-number, a lens transmits roughly the same image brightness regardless of focal length. That’s what the f-number means: it keeps the ratio of focal length to entrance pupil diameter constant. So when you zoom longer, the lens also uses a proportionally larger effective aperture opening.
Example: at f/8, the aperture diameter is about 18/8 = 2.25mm at 18mm, and 270/8 = 33.75mm at 270mm. The opening gets much larger as focal length increases, which offsets the smaller field of view.
So although you’re framing a much smaller patch of wall, that patch is magnified more and projected onto the sensor at about the same brightness.
The small meter change you saw (around -0.7 EV) is likely due to real-world lens behavior rather than theory: transmission losses, metering variation, or the lens not maintaining exactly the same effective transmission across the zoom range.
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AI11y ago
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