At 09:16 PM 7/12/03, Boris asked:
I am curious about something...
When I look at the rearview mirror, my eyes focus close. If I want to
focus at the image in the mirror, my eyes relax and focus for a longer
distance as if I have turned my head around and focused on the subjects behind.
I wander why is that.
Not quite, but close. When you focus on the image in the mirror, the focus
distance is the sum of eye->mirror distance plus mirror->reflected-object
distance. When you turn around, it's simply eye->object distance, which is
likely not much different from looking it it in the rearview mirror.
Does it mean that if there is a window and I want to focus on the subjects
that the window reflects, the lens has to be set up at a distance equal of
the distance between the window and the subject and not the distance
between the lens and the window?
It must be set up for the sum of the two:
camera->window distance PLUS window->subject distance
The focus distance is the entire light path from subject/object to lens.
Confirm this with the following experiment. Stand about 10 feet in front
of a mirror with camera in hand and focus on the mirror (a mirror edge will
work). Look at the focus distance on the lens. Should be about 10 feet,
distance from camera to mirror. Now focus on yourself, or the camera in
your hands. Should be about 20 feet, the distance from you to the mirror
and plus distance from mirror to camera. Allow for a little error in the
lens markings; they're not always accurate to eight significant digits.
-- John
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