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Rebel SL1 and Tamron 100-400mm F4.5.5-6.3 autofocus issue

Started Aug 25, 2019 | Questions thread
Doug Pardee
Doug Pardee Veteran Member • Posts: 9,920
Re: Rebel SL1 and Tamron 100-400mm F4.5.5-6.3 autofocus issue

I can't speak to the combination in question, but I can address this:

AlaskaAce wrote:

The camera spec sheet says auto focus up to 5.6 only.

If the aperture is too small, autofocus will simply fail. It's not a matter of how good the AF is, but rather whether AF functions at all.

With an f/6.3 lens, a typical f/5.6 AF point is losing about one stop of light compared to an f/5.6 lens, so it won't won't work as well in dim light. The center AF point is rated down to -0.5 Ev on the SL1, but at f/6.3 it'll be more like +0.5 or maybe even +1 Ev. But the AF should work fine in good light.

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