Hi--

I am using Canon SLR EOS 300 with 75-300 mm zoom lens.
I have noticed a difference between this lens and my
binoculars(a very simple one). I am myopic(near
sighted) and so cannot view far objects with sharp
focus. I wear glasses to correct my vision.
I notice that I look through my binoculars with
glasses and focus at a pirticular object to get a
sharp image. Then when I remove my glasses, the image
gets blurred, and I have to readjust the focus,
following which the sharpness is achieved again.
But in the case of the SLR cmera it is not so. When I
focus on an object while wearing glasses, and I remove
the glasses, the object gets blurred. But the
difference is that if I now try to readjust the focus,
the image gets blurred in both directions, and never
goes towards sharpness(which actually gets sharpened
in case of binoculars).
Can you please help me understand this difference, as
I presume that both optical devices have similar
lenses.

Sumeet Puri


Both devices do have similarity in that they are lenses with
focal-length, and therefore focus. What is different is the
target upon which they focus. In the case of the binoculars,
it is an "aerial" image that your eye focuses on (the focus plane
is in the air), and changing your eye focus by removing glasses
changes the location of best focus in the air, correctable by
refocusing the binoculars. With the camera, there are two fixed
focus systems, one through the lens to the "ground glass" focus
screen (correct focus to this is absolute, and independent of the
other focus system); the other is your eye focus on the other side
of the fixed-position focus (or "viewing") screen, which is also
absolute (you must be able to focus sharply on this screen with
your eye to see it properly, and to see proper focus of the lens
on the other side, and the eyepiece lens is what allows you
to do this). If you were to remove the focus screen upon
which the lens image is focused, you would have an aerial image
to focus on which could change position, and focus would be similar
to the binocular situation, but you would be unable to properly
focus the taking lens on the camera to get a sharp image on
film. The viewing screen fixed position is the same distance from
the lens center as the film is, so focusing on one with the taking
lens will also cause the focus to be correct on the other...