The user Laurent also has the Eos 5D, he wrote in another thread that I just came by :
Pentax K10D beats Nikon D700 in low light AF [Page 2]: Pentax SLR Talk Forum: Digital Photography Review
On my own account, I had been having fun shooting with manual focus lenses on my K10, (and slower zooms in good light). So when I finally got my first AF fast prime for my K10, I was looking forward to see how it would fare compared to my film Pentax SLR.
I did some shooting in very limited lighting, shooting Iso 1250, f/1.9, and shutter times of ¼ sec, ½ sec, 0.4 sec.
I found that it did surprisingly well. It locked onto areas where I had trouble distinguishing the contrast myself. The setting was at night, with one light source set to another side of the room.
I really like that the Pentax AF system will not give up, but will keep trying till it finds the setting. Whereas other systems tend to try, but then give up if EV too low.
I think the trouble is that people come from P&S with unlimited DOF, and then get a 50/1.4 lens with extremely narrow DOF, and then expect it to lock on instantaneously on random moving targets like children moving about.
If I’m covering an event, I’m also very happy that not using P&S with flashes going off all around. Or a Rebel cam, with the bright AF white assist light, disturbing speakers, etc.
People can be nervous enough, making a speech at a wedding, or at an important reception; without the photographer putting more pressure on them. I’ve often encountered how people thought that I had hardly taken any pictures, and then be surprised to see that I had covered most. I prefer that the photographer be more invisible and just reporting what is going on, without playing a major part oneself.
Being inspired by the real Pro’s, I work to optimise equipment performance. I don’t put everything in auto, but apply my settings, maybe sometimes using spot AF, going with the appropriate WB, Iso setting etc.
Edit :
besides, regarding the Eos 5D, people buy it for FF, not blazing fast AF. With narrow DOF, people gotta be careful anyway, selected what they want in focus.
A friend of mine was using his 5D in tracking mode, and found it difficult. But most of the time he is happy, going manual with his Leica glass