Why does my macro lens show a smaller maximum aperture at close focus?
Asked 7/8/2018
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I’m using a Sigma 180mm f/2.8 macro lens on a Nikon D850. At infinity the camera reports f/2.8, but as I focus closer the reported maximum aperture gradually changes to smaller values, reaching about f/5 near minimum focus.
Can a macro lens be forced to stay at its full wide aperture when focused close, or is this normal behavior? If not, what optical effect is causing the camera to report a smaller maximum aperture at higher magnification?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
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No, not possible. That is simply what all macro lenses do. They can only focus closer by moving the lens farther forward (so to speak, it's probably done internally today). Because, it is necessary to increase the focal length to increase the magnification. And f/stop is defined as focal length / aperture diameter, so the f/number increases somewhat. The chart you show is very normal for that macro lens.
Marked focal length is only valid at infinity, but focusing closer changes it a bit. Holding f/stop constant to avoid that change is one reason regular lenses are not allowed to increase magnification to more than about 1:10.
Originally by user38978. Source · Licensed CC BY-SA 4.0
user38978
8y ago
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Generated from our catalog & community — verify before relying on it.
No—this is normal for macro lenses, and you generally can’t override it.
The lens is not physically stopping down as you focus closer. At the widest setting, the aperture is still fully open. What changes is the effective aperture reported by the camera.
At close focus, a macro lens must increase the lens-to-sensor image distance to achieve higher magnification. As magnification rises, the effective f-number rises too, even though the physical opening has not changed. In macro work this is commonly described by:
effective aperture = set aperture × (1 + magnification)
So the camera reports a smaller maximum aperture because the lens transmits light as if it were at a higher f-number at close distances.
This is expected behavior, not a fault. It’s part of how macro lenses focus closely. In practical terms, you lose light at higher magnification, which affects exposure and depth of field. The chart you listed is normal for a macro lens of this type.
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