Why does my 70–300mm lens lose maximum aperture as I zoom in?
Asked 4/16/2012
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I'm new to shooting sports and using a Canon EOS 60D with the EF 70-300mm f/4-5.6 IS USM. In Manual mode, I noticed I can't keep the aperture at f/4 as I zoom in. At longer focal lengths, the camera won't let me set it that wide and the maximum aperture becomes smaller. Why does this happen on a zoom lens?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
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This is normal for a variable-aperture zoom lens. Your lens is labeled 70-300mm f/4-5.6, which means its widest available aperture changes across the zoom range: about f/4 at 70mm, and only f/5.6 by the time you reach 300mm.
Aperture is related to the size of the opening compared with the focal length. As focal length increases, keeping the same f-number would require a physically larger opening. On many zoom lenses, the lens design does not provide a large enough effective opening at the long end, so the maximum aperture becomes smaller.
So the camera is not malfunctioning, and Manual mode does not override the lens’s physical limits. You can always set a smaller aperture like f/8, but you cannot set wider than the lens allows at that zoom position.
If you want the same maximum aperture throughout the zoom range, you need a constant-aperture zoom, such as an f/2.8 or f/4 zoom.
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