From: phg+@CS.CMU.EDU (Paul Gleichauf)
Subject: Re: Photograping Whales [8008s autofocus problem]
Organization: Carnegie Mellon University
Date: Tue, 1 Sep 1992 13:42:47 GMT

In article <PHILG.92Aug31190654@zug.ai.mit.edu>, philg@zurich.ai.mit.edu (Philip Greenspun) writes:
|> 
|> In article <6031@transfer.stratus.com> savoia@sw.stratus.com (Rick Savoia) writes:
|> 
|>    This past weekend, I went on a whale watch and brought
|>    along my 8008s. ... I
|>    was using a 35-105 AF Nikkor lens.  The problem I had
|>    was when the whale breached, the camera would rack 
|>    back and forth trying to find the focus.
|> 
|> Your problems include the following:
|> 
|> 1) Nikon autofocus is basically decorative
|> 
|> 2) Neither whales nor water have much contrast to speak of
|> 
|> 3) autofocus systems like light and a 35-105 zoom (f5.6?) won't give
|> them much
|> 
|> Could you have won with a Minolta 7xi and an f/2.8 lens?  Probably.
|> 
|> Philip Greenspun
|> MIT Department of EE & CS
|> 
|> P.S.  Yes, Nikon is the only 35mm camera I regularly use (6006, 8008,
|> and F4 under duress).

	I really do dislike flaming people, but there is a real question 
of competence that has to be raised when answers like this are posted.  AF
systems are servo controls, they are not really intelligent, at best they 
have a rudimentary pattern match facility.  If you understand their 
limitations they can be used effectively, even on breaching whales. 
	The best solution in general is to use manual focus when it is simplest.
If you find that you need to use AF in this situation note the cues that you 
use for focusing and try to accomodate to the camera AF mechanism.  Breaching
whales usually present a dark vertical line against either the bright horizon or 
against a usually lighter sea.  This line should fall across the horizontal sensor
array used in the pre-N90 Nikon AF sensor.  If there are cues on the ocean 
surface, prefocus there and disengage the AF.  
	Continuous mode focusing is more likely to produce hunting unless you can 
track the subject as it moves (a breaching whale dives first so this is not the
case here). Instead use servo and leave the shutter release half way down 
to lock focus (or a focus lock button where available).  Some Nikon
lenses have a range limiter to control the focus range with the placement 
of mechanical stops.  Use them if available and remember that you can exploit 
the same technique with other lenses that leave the focus ring spinning around the lens barrel in AF focus mode. Do not torque the ring to override the AF 
mode, rather just stop its travel at some point with your hand, it will reverse 
its focus direction when resistance is sensed.  The camera does not know how long
the focus travel is for every lens that is attached to it, it senses the end
of focus travel and changes direction.  If you try to manually refocus the 
lens against the AF action you run the risk of stripping the focus cam coupler beteen the camera body and the lens.  It can be done with care, but I use it 
as a last resort. The exception to this pattern is the newest long telephoto 
design which has a special manual override mode to AF.

							Paul