# 70-400/4.5-5.6 ... Why not 4.5-6.3?



## JumboShrimp (Feb 4, 2014)

I'm only asking because folks at Sigma and Tamron seem to have no issues with producing lenses with f/6.3 at their longest zoom that seem to work just fine on the Canon bodies. Is there a strong technical reason why Canon is stuck with f/5.6 ILO f/6.3? Obviously, f/6.3 could provide a lighter and cheaper product with more zoom design range/flexibility.


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## neuroanatomist (Feb 4, 2014)

AF requires f/5.6. Sigma/Tamron lenses lie to the body, telling it they're open to f/5.6 when the max aperture is narrower. I suppose Canon feels they'd lose face by lying to themselves, so to speak.


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## 9VIII (Feb 5, 2014)

My understanding is that it's also a regular quality assurance thing. While AF may work beyond f5.6, it may not work optimally, thus Canon would rather tell people what to expect up front rather than have customers complaining that their camera has bad autofocus under less than ideal conditions.

That's actually one thing I've been wondering about lately, when I'm shooting in bright sunlight, why can't I use autofocus at f11 or f16? If I'm getting enough light for a proper exposure you'd think AF should still be fine.


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## neuroanatomist (Feb 5, 2014)

9VIII said:


> That's actually one thing I've been wondering about lately, when I'm shooting in bright sunlight, why can't I use autofocus at f11 or f16? If I'm getting enough light for a proper exposure you'd think AF should still be fine.



It's not the amount of light, it's the diameter of the iris diaphragm. When we refer to an f/5.6 or f/2.8 AF point, it's not about the light sensitivity (intensity of illumination) but rather that the aperture determines the baseline for the rangefinder system that is required for that line to function. Personally, I think better terminology might be to use threshold instead of sensitivity, so an f/2.8-threshold line would require an f/2.8 lens to function, and if you mounted an f/4 lens, that sensor line would not operate. An f/5.6-threshold sensor would work with any lens having a max aperture of f/5.6 or wider.


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## RGF (Feb 5, 2014)

At one point Canon a 500 F4.5 but then dropped it favor of the 500 F4 IS (original version). Not sure but think there was some problems with AF and 500 F4.5 (but that years ago).


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## Blakegard (Feb 14, 2014)

To me having a 70-300 with the aperture being f/4-5.6 is already frustrating enough, and at the long end in harsh conditions AF does take a major hit. I think a 70-400 f/4 would be much better for the lineup, but that's just my shooting preference.


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## neuroanatomist (Feb 14, 2014)

Blakegard said:


> I think a 70-400 f/4 would be much better for the lineup, but that's just my shooting preference.



There's a 200-400 f/4 now, and it costs over $11,000…but you think a 70-400 f/4 would be better for the lineup? ???


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