According to Nikon User :
My Nikon 8000s has an Auto/Manual focusing selector on the body
(actually, two modes of auto), as does my new Nikon D70s. Each of the
lenses for the D70s also have an A/M focusing selector.
I've read the instructions for both the camera and for the lenses, and
I'm still confused. Do I need to set both to A for auto focusing, and
both to M for manual focusing, or what?
*Both* must be set to auto for the auto focusing to work. Think
of them as two light switches in series. If either is off, the light is
off. Only when both are on will the light glow. Or, if electricity is
not a comfortable analogy -- think of a hose with a nozzle on the end.
Both the nozzle and the faucet must be "on" before water will flow.
Yes, the nozzle also has the additional function of changing the spray
pattern, but it is also a "switch".
What happens if the camera is set to M and the lens to A?
You get manual focus only.
What happens if the camera is set to A and the lens to M?
You get manual focus only.
This is easy enough to test. Set both switches to autofocus and
half-press the shutter release while pointing to something near. Then
point at something more distant, and again half-press the button. The
lens's focus will shift to bring whichever into sharp focus (even if
you are firmly griping the focus ring on the newer lens which have the
switch present.)
Then turn off the AF switch on the lens. Repeat the test, and
you will see that one object remains out of focus. Turn the AF switch
on the lens back on, verify that it all works as before, and then turn
off the AF switch on the camera body. Again, it will not change focus,
unless you manually turn the focus ring.
Now -- as to the reason for the double switches.
1) Older Nikon cameras with AF lenses did *not* have focus motors
in the lens. Instead, there was a tiny "screwdriver" which
projects through a hole in the lens mount ring -- at about 7:00
when looking at the body from the front.
This engages a "screw slot" in the back of the lens, and rotates
when the autofocus motor in the camera is attempting to change
the focus.
On this sort of lens, you really need to have you hands off the
focus ring while the camera is doing its thing.
When you switch the body to manual focus, it withdraws the
"screwdriver" so it no longer engages the "screw slot", freeing
the manual focus ring to do its job without having to fight (and
probably damage) the gearing in the lens and that in the body.
2) Newer lenses have an Ultra-Sonic motor in the lens itself. It
adjusts the focus by rapidly vibrating a pawl which engages a
toothed wheel on the focus ring inside the lens. This
disengages the external ring until the focus is complete, at
which point you can adjust the focus with the ring if you so
desire.
The switch on the lens is a bit more convenient to reach than
that on the body, if you want to shut it off in the middle of
shooting. (It falls under the thumb with a relatively small
shift from where it is gripping the zoom or focus rings.) In
contrast, the body-mounted switch (which *must* be present to
allow switching off autofocus on the older mechanical autofocus
lenses) is a bit more awkward to reach while the camera is to
your eye in shooting position.
I have examples of both styles of lenses with my D70 -- the
18-70mm "kit" lens is one of the newer ones (and useless on a
full-frame body)
However, both my "28-105mm f3.5-4.5 D" lens, and my 50mm f1.4
are of the older design, and focused by the "screwdriver". (I
also have a 180mm f2.8 which has been "chipped" to allow the
metering, but has no provisions for autofocus, as well as older
lenses which don't even have the chip, so I must meter with an
external exposure meter.
The newer style lenses work with a somewhat older camera body,
the N90s. I happen to have one which was converted to digital
by Kodak for the AP (so it has a similar crop factor to that of
the D70, and the 18-70mm kit lens works fine on that. (That
camera body has a three-position switch -- which selects "Off",
"Single" mode (focuses once at the half-press of the shutter
release), or "Continuous" mode (follows whatever is in the
center of the focus area as long as the shutter release is held
half-depressed). (To make the choice between the latter two
modes on the D70, you have to go to the menu.) I would like it
to be on the body switch instead, as in the N90s. That would
make it easier to quickly switch between the two modes at need.
Presumably, some day, Nikon will stop producing camera bodies
with the support for the "screwdriver" type of autofocus
mechanism -- perhaps when the come out with a DSLR which costs
less than the D50. But so far, they have not.
I hope that this helps,
DoN.
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