----- Original Message ----- From: "David Mitchell" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Wednesday, March 08, 2006 1:14 AM Subject: Re: EOS AF and max ap
> High-end Canon bodies have center-point AF which work at f/8 > instead of the usual f/5.6, at the expense of being bit less > accurate. What is it that stops the AF working past a certain aperture? It doesn't seem like it should be light levels because there is more to that than just aperture: A very bright room at f/6.3 would still be easy enough to focus in, whereas a camera can really struggle to focus in dim light at f/1.8. What gives? David *********************************************************** Geometry. The AF sensors need light rays to appear to be arriving from separate directions. With a wide aperture, this is easy. With a pinhole it isn't. Most AF systems work down to an aperture of f5.6, with some specialised sensors being able to cope with f8. (These are maximum apertures i.e. those at which the AFing is done.) Some of Canon's bodies give better (more accurate) performance when the lenses are brighter than or equal to f2.8. Remember the old split-image focus devices blacking out when used with an f5.6 lens and a 2x converter ? Same principle. (AFAIK All SLR AF systems need contrast in the subject for the AF to work. Hence the use of AF assist patterns when trying to focus on a plain surface.) Malcolm Milton Keynes, UK http://www.megalith.freeserve.co.uk/oddimage.htm * **** ******* *********************************************************** * For list instructions, including unsubscribe, see: * http://www.a1.nl/phomepag/markerink/eos_list.htm ***********************************************************
