Why does a variable-aperture zoom have a larger effective aperture at 200mm than at 18mm?
Asked 7/7/2011
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For a lens labeled 18–200mm f/3.5–5.6, using D = f/N gives about 5mm at 18mm f/3.5 and about 35mm at 200mm f/5.6. Does that mean the aperture gets physically larger as you zoom in? If the iris opening itself stays the same, how can the effective aperture change? I’m asking about the visible opening / entrance pupil as seen from the front of the lens.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
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The physical apterture opening is the same size. It's fully open in both cases.
The reason that the f numbers are different, is that they don't measure the physical size of the aperture, they measure how much light the aperture lets through. If you look into the lens from the front, you see that the aperture appears to become larger when you zoom in. The light that falls into the lens is also affected the same way, so that more light can pass through the same physical opening.
Originally by user149. Source · Licensed CC BY-SA 4.0
user149
15y ago
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Generated from our catalog & community — verify before relying on it.
Yes—the value from f/N refers to the entrance pupil, not necessarily the physical iris opening itself. The entrance pupil is the apparent size of the aperture as seen through the front optics.
On a zoom lens, the optics in front of the diaphragm magnify that opening differently at different focal lengths, so the apparent aperture can change as you zoom even if the iris mechanism itself does not change much. Looking into the front of the lens while zooming, you can often see this.
So for an 18–200mm f/3.5–5.6 lens, it is normal that the entrance pupil is about 5mm at 18mm and much larger at 200mm. That does not mean the lens is “faster” at 200mm—the f-number already accounts for focal length divided by effective aperture.
Your measurement method is broadly valid if you are measuring the apparent opening from the front while focused appropriately: that is a practical estimate of the entrance pupil, which is the quantity used in the f-number.
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