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500mm vs 600mm?

Started Nov 13, 2017 | Questions thread
WilbaW
WilbaW Forum Pro • Posts: 11,643
Re: 500mm vs 600mm?

BirdShooter7 wrote:

WilbaW wrote:

Tannin wrote:

And there is nothing, repeat nothing, so useful as having f/4 at your disposal. Be it for depth of field control, for faster shutter speeds, for faster, better auto-focus...

It's a common misconception that an f/4 lens will PD AF better than an all-other-things-being-equal f/5.6 lens. There's no advantage since they both pass the same intensity of light to the AF sensors. With PD AF it's a can the sensors see through the window thing, rather than a how bright is the room thing.

ok so then if what you say is correct how come there is a minimum illumination required to achieve focus?

That's exactly the misconception right there. 

We agree that an f/5.6 lens will work with f/5.6 AF sensors, whereas an f/8 lens (or lens and TC combination) won't, right?

If I put a 1-step ND filter on the f/5.6 lens so that the same illumination reaches the image sensor as through the f/8 lens, will it still AF in good light? Or will the f/5.6 lens with ND filter fail to work in the same way as the f/8 lens does?

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