manual focus in dSLR

[Guest]

Archived from groups: rec.photo.digital (More info?)

I notice by chance that even when the Pentax DS' auto focus switch is
off, it still does autofocusing, and when I turn the focus correctly,
the red focusing dot flashes. I tried it with the Canon 20D, it doesn't
do it automatically, but when I push the shutter button half way, the
red focusing dot would flash too if the auto focus agrees with my manual
focus. I would like to know the threholds for the red dot to flash.

 

[Guest]

Archived from groups: rec.photo.digital (More info?)

"l e o" <someone@somewhere.net> wrote in message
news:joDUe.35606$1g2.3561@fe05.lga...
>I notice by chance that even when the Pentax DS' auto focus switch is off,
>it still does autofocusing, and when I turn the focus correctly, the red
>focusing dot flashes. I tried it with the Canon 20D, it doesn't do it
>automatically, but when I push the shutter button half way, the red
>focusing dot would flash too if the auto focus agrees with my manual focus.
>I would like to know the threholds for the red dot to flash.

On the 20D the tolerance is a fraction of the depth of focus. An article
can be found: http/doug.kerr.home.att.net/pumpkin/AF_accuracy.pdf

 

[Jim]

Archived from groups: rec.photo.digital (More info?)

"l e o" <someone@somewhere.net> wrote in message
news:joDUe.35606$1g2.3561@fe05.lga...
>I notice by chance that even when the Pentax DS' auto focus switch is off,
>it still does autofocusing, and when I turn the focus correctly, the red
>focusing dot flashes. I tried it with the Canon 20D, it doesn't do it
>automatically, but when I push the shutter button half way, the red
>focusing dot would flash too if the auto focus agrees with my manual focus.
>I would like to know the threholds for the red dot to flash.
Isn't that just the electronic rangefinder hard at work?
Jim