How does a 1.4x teleconverter change the aperture of a variable-aperture zoom?
Asked 7/17/2016
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If I use a Canon 1.4x Extender on an EF 100-400mm f/4.5-5.6L, how does the maximum aperture change across the zoom range? For example, does the f/5.6 maximum aperture at the long end become f/8, and what happens at the wide end?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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Yes. 5.6 becomes 8.0
If you add a 1.4x extender (teleconverter) you will now have a 140-560mm f/6.3-f/8 lens.
Keep in mind that most cameras will only autofocus to a limit of f/5.6 and f/8 will be too dark for the AF to work.
Newer high end cameras like the 7D Mk II and 5D Mk III will attempt to AF with f/8 lenses. Even if AF works (as in "the camera will try to AF") at f/8, it will be slower, less accurate, will need brighter light and/or a higher contrast target upon which to focus, and generally be restricted to fewer usable AF points. The 7DII and 5DIII will only attempt to AF at f/8 with the center focus point selected. The new 1D X Mark II is the only Canon camera that can use every AF focus point at f/8.
For more on the practical ramifications of using teleconverters with slower lenses, please see Will the Canon 5D MK II with 100-400 1:4.5-5.6 work properly with Kenko 1.4 converter?
Originally by user39427. Source · Licensed CC BY-SA 4.0
user39427
10y ago
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Generated from our catalog & community — verify before relying on it.
Yes. A 1.4x teleconverter costs about one stop of light across the whole zoom range, so both ends of a variable-aperture zoom get one stop slower.
For a 100-400mm f/4.5-5.6 lens, adding a 1.4x extender makes it effectively a 140-560mm f/6.3-8 lens:
- 100mm f/4.5 becomes 140mm f/6.3
- 400mm f/5.6 becomes 560mm f/8
So your example is correct: the long end goes from f/5.6 to f/8.
A practical concern is autofocus. Many cameras only autofocus reliably up to f/5.6, and f/8 may limit AF performance or disable it entirely depending on the body. Some newer/higher-end Canon cameras can autofocus at f/8, often with fewer AF points and slower, less reliable performance, especially in dim light or with low-contrast subjects.
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