Archived posting to the Leica Users Group, 2002/04/27
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Hi Everyone As you may all know, for the past few weeks I've playing around with EOSes in an unsupervised environment. I've got the concept more or less nailed down. The sensor responds to changes in contrast and the lens focuses in or out until that max value has been reached or perhaps trapped predictively and attained. That's simple enough but assuming a static situation at the time when the sensor is activated, how does the camera know which way to turn the lens? At first I assumed that it was turning a tiny increment and measuring contrast change from the initial position and deciding from that whether to keep turrning or stop and turn the other way. But there is no backtracking whatever on the barrel and it always turns in the right direction. Somehow the camera knows whether the object is closer or farther from the initial focus point. The focusing barrel on eos lenses does have a VR slider which can be used I guess to inform the camera on the distance of the focus point in order to allow the thing to land on the right spot without bounding but it still can't tell the camera whether the object is ahead or behind the focus point. The only other idea I can come up with is that a separate sensor working like a split image can figure it out the same way you would based on the position of the split images.*But that sounds wacky and trouble prone. Also, It can't be measuring differences in contrast between closely spaced objects and acting on that because it can AF properly on anbsolutely flat expansive surfaces as long as there are vertical lines. And as far as I know it's not an active system with respect to the outside environment either. The info I've gotten off the net is superficial and I'm still stumped! Does anybody know how the camera figures out which way to turn? Javier * On most split image screens the top image towards the right and the lower image towards the left indicates that the lens should be turned CCW from the shooter's standpoints and that the object is farther away. I think it's the standard. Though some companies did it backwards for reverse helicoid lenses like some of the old Schneiders used on Exaktas requiring a reverse split image. Finally, some had a lens or screen reversed and you have to turn backwards. Double pain in the ass! __________________________________________________ Do You Yahoo!? Yahoo! Health - your guide to health and wellness http://health.yahoo.com - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html