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<blockquote data-quote="J-see" data-source="post: 436460" data-attributes="member: 31330">Every lens has an effective aperture at which auto-focus is done. For an f/2.8 prime, all AF is done at f/2.8. Other lenses like zooms often have a variable effective aperture; the Tam 150-600 is f/5 at the short end and f/6.3 at the long end. If you'd add a TC to it, you extend the lens and thus create a smaller aperture. For an f/4 with a TC2x attached that will be about two stops resulting in f/8. It is not just the light or contrast but also the width of the angle the light enters the lens that affects the performance of focus. By making a lens &quot;longer&quot;, the ° of that angle shrinks (30° becomes 25° as an example). That's why those exotic primes have such massive front elements.New cams can focus even with an effective aperture of f/8 while older cams can no longer focus under such conditions and start hunting.The D7100 has only one focal point that still functions at an effective aperture between f/6 and f/8 which can make it hard to focus the Tamron 150-600. Especially at low light.But there is some trick being used by Tamron since I have been shooting her with my D3300 at 600mm length and that cam should not be able to focus beyond an effective aperture of f/5.6. It clearly did although the results were poor.</blockquote>

[QUOTE="J-see, post: 436460, member: 31330"] Every lens has an effective aperture at which auto-focus is done. For an f/2.8 prime, all AF is done at f/2.8. Other lenses like zooms often have a variable effective aperture; the Tam 150-600 is f/5 at the short end and f/6.3 at the long end. If you'd add a TC to it, you extend the lens and thus create a smaller aperture. For an f/4 with a TC2x attached that will be about two stops resulting in f/8. It is not just the light or contrast but also the width of the angle the light enters the lens that affects the performance of focus. By making a lens "longer", the ° of that angle shrinks (30° becomes 25° as an example). That's why those exotic primes have such massive front elements. New cams can focus even with an effective aperture of f/8 while older cams can no longer focus under such conditions and start hunting. The D7100 has only one focal point that still functions at an effective aperture between f/6 and f/8 which can make it hard to focus the Tamron 150-600. Especially at low light. But there is some trick being used by Tamron since I have been shooting her with my D3300 at 600mm length and that cam should not be able to focus beyond an effective aperture of f/5.6. It clearly did although the results were poor. [/QUOTE]

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