Originally posted by StigVidar Sorry, but the EV rating og the AF system is not dependent of the lens. The AF sensor is not capturing light from the whole area of the front element, but only from two small spots. These small spots are not dependent of the lens and a f/5.6 lens is just as capable to focus in dim light as a f/1.4 one. But there are of course mechanical differences on lenses which make AF more suitable on one lens compared to another one.
Sorry thats just wrong Pentax state the spec on their web site and include the conditions required to achieve it.
"Sensitivity range: EV -3 to 20
(ISO 100, 50mm F1.4)"
Af phase detect uses a baseline twin sensors to detect phase variance these are traditional spaced such that F5.6 works.
Unfortunately this means they can only detect DoF Phase changes of F5.6 so use logic to improve accuracy ... i.e swing left till phase varies not position swing right til phase varies note position move lens to center of readings , they use these both because of cost and so any cheap consumer lens can Auto Focus
More upmarket cameras have 'special' f2.8 baseline sensors 1 in the k5ii and 3 in the k3 these allow ev=-3 as per spec and over double the accuracy of f5.6 sensors .... BUT to utilize these sensor you must use f2.8 or better lens.
It makes no difference what portion your using for AF reading as the whole image dims as you close the aperture.
My statements are easy to prove ... Put an f2.8 lens on your camera reduce light until the AF struggles .... Put an f5.6 lens on the camera and it will no longer focus.
This is old news and was known as blackout in MF focus days with split prism panels and the reason it happen in AF is the same as MF.
Nikon F2 "blackout" with slower lenses [Archive] - Rangefinderforum.com
Modern dual baseline sensors is like having twin focus screens 1 allowing f2.8 to work OK and 1 allowing f5.6 to work OK but don't; fool yourself to get the advantages of EV-3 and high AF accuracy you must use fast lens which will activate the f2.8 sensors , Slower lens do not cover the f2.8 baseline and hence useless.
This isn't hard and fast as you can easily get a Pentax camera to focus with f11 max aperture but EV and performance drop accordingly ... There are many AF timing Vs EV available to Google just how slow AF becomes as you approach the Phase limits.
The big difference between Pentax and Canon implementation is Canon artificially kill AF if the sensor baseline is exceeded (hence why teleconverter stop working above f5.6)
Last edited by awaldram; 06-02-2015 at 07:11 AM.