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Message: <blockquote data-quote="Malcolm Stewart" data-source="post: 3224321" data-attributes="member: 6851">I too have had good long lens results with a 1D3 (and now by its replacement, a 1D4) as well as a 7D.  I'm wondering whether this linked article might throw some light on your problem?<a href="http://www.imaging-resource.com/news/2014/02/27/canon-dual-pixel-cmos-af-autofocus-secrets-of-the-canon-70d-explained" target="_blank">http://www.imaging-resource.com/news/2014/02/27/canon-dual-pixel-cmos-af-autofocus-secrets-of-the-canon-70d-explained</a>Or is the problem that with the higher pixel densities, the 70D is showing up the limits of your lens when used towards its 600mm setting?  On the sensor, 244 pixels/linear mm for the 70D vs 138 pixels/mm for the 1D3.  I assume that you've optimised the AF Micro-Adjust at around the range you're shooting at.</blockquote>