My two EOS mount Tokinas have push/pull focusing rings for switching
between auto and manual focus. The 80-200/2.8 is an older design than
the 28-80/2.8 and is somewhat cumbersome in comparison.
On the 80-200 the push/pull motion doesn't turn off the focus motor but
simply disengages or reengages a clutch between focus motor and manual
focus ring. In the autofocus position the manual focus ring is free
turning with no connection to the focusing drive. In the manual
position the focusing drive clutch is engaged and both auto and manual
focus are active at the same time... which also means that the manual
focusing ring turns during autofocus operation. But the lens also
includes the more typical Canon MF/AF on/off switch. Turning that
switch to MF operation will disengage the autofocus motor and drive
train. In that position the MF ring only turns manually and it also
turns more easily with the drive train disengaged. Also, the push/pull
operation of the focusing ring can only be engaged at one point in its
rotation. The push or pull must be accompanied by a rotation of the
ring until it finds a detent.
The 28-80 on the other hand is a much later design and a simpler and
more elegant animal. There is no MF/AF switch at all. The only control
is the push/pull action of the focusing ring... MF in one position, AF
in the other. When in AF the focusing ring does not turn and there is
no detent position required to effect the switch... just push or pull
from any position.
The saving grace for any of this is that I have never felt the need to
use manual focus on either of these lenses. On the other hand, neither
focuses particularly close where the focusing target might get critical
within a very limited DOF. The 80-200 only goes down to 1.8 meter (1:8
at 200) and the 28-80 to 0.5 meter (1:5 at 80)
Some later model Canon lenses have full time manual focus but I don't
own any Canon glass and don't know how it works. I assume that the
focus ring must turn during autofocus like the 80-200 Tokina. Some long
tele lenses also have a focus distance limiter switch. That's to keep
the lens from hunting for focus within a range where you know the
subject isn't going to be.
Chuck Norcutt
Moose wrote:
>>
> Odd how little design decisions can make a big difference. Canyon put
> the switch on the lens, so it can never go "software". More important,
> it gives some design flexibility for lenses. Although most opt for a
> switch of the standard design in the standard place, a few do something
> different. The Tamron 90/2.8 macro, and I believe some others, uses
> push-pull of the focusing ring. It is just natural and perfect with
> macro. If the AF nails focus, shoot, otherwise, grab the focusing ring,
> pull back a click, focus and shoot.
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