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12-21-2011, 02:31 PM   #1
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Autofocus sensor sensitivity

I hope you can help explain something I've been wondering about. Autofocus sensors have a certain sensitivity, i.e. f/2.8 or f/5.6. I have read that this means that f/5.6 sensors need less light to work on than f/2.8 sensor do, so they work well with f/5.6 lenses (that only transmit a quarter of the light that an f/2.8 lens does). When using a lens with a maximum aperture that is slower than f/5.6, autofocus would stop working reliably with such sensors.

The thing I don't understand is this. It would seem to me that the amount of light that passes through a lens depends on the amount of ambient light as well as the maximum aperture. So how can autofocus sensors be specified in an F-stop number, leaving the ambient light levels out of the equation? Surely an f/5.6 lens used outside on a bright day transmits more light than even an f/1.4 lens does at night?

Does that make sense?

12-21-2011, 03:32 PM   #2
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I believe its more about the sensors ability to cope with the lesser DoF of the 2.8 or larger aperature lenses, to be able to react quicker and find focus in shallow feilds.
12-24-2011, 03:59 AM   #3
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QuoteOriginally posted by cmohr Quote
I believe its more about the sensors ability to cope with the lesser DoF of the 2.8 or larger aperature lenses, to be able to react quicker and find focus in shallow feilds.
I believe I read somewhere that it's both. E.g. f/2.8 sensors are great for precision and all that, but don't work so well when focusing with slower lenses.
12-24-2011, 05:02 AM   #4
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This should help to shed light on PDAF: http://dougkerr.net/pumpkin/articles/Split_Prism.pdf.

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