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From: jans@mako.UUCP (Jan Steinman)
Newsgroups: net.rec.photo
Subject: Re: Multiple exposures ??
Message-ID: <439@mako.UUCP>
Date: Mon, 10-Dec-84 13:04:46 EST
Article-I.D.: mako.439
Posted: Mon Dec 10 13:04:46 1984
Date-Received: Wed, 12-Dec-84 05:16:37 EST
References: <41600001@hpfcms.UUCP> <1539@drutx.UUCP> <4257@cbscc.UUCP>
Reply-To: jans@mako.UUCP (Jan Steinman)
Organization: Tektronix, Wilsonville OR
Lines: 56
Summary: 

In article <4257@cbscc.UUCP> rsg@cbscc.UUCP (Bob Garmise) writes:
>  1) backwind the film until it's tightly wound, then...
>  2) press/flip the release button/knob
>  3) thumb wind the film as if you were advancing to a new frame...

A cleaner solution exists for some cameras, notably Olympus OM1, OM2 and some
Cannons.  Many cameras have a ratchet mechanism for the rewind button which
causes it to pop up after re-winding a bit.  This keeps you from leaving the
camera in rewind mode after loading a new roll.  (If your camera instructions
tell you something like "hold in the rewind button while turning the crank",
you probably have such a mechanism.

Follow steps 1) and 2), above, then rewind the film a tiny bit and release 
the rewind button.  Continue rewinding until the rewind button pops up -- the
film should not rewind further if there is such a ratchet mechanism.  The
mechanism is usually keyed to fractions of a frame (I think the Olympii take
four rewind-button-pop-up cycles per frame, but it's so easy to find out, I
haven't memorized it.)  To find out how many times to repeat the procedure in
order to rewind exactly one frame, follow step 1), above, note carefully where
the rewind knob/crank is positioned (a grease pencil is great for this), shoot
the first exposure and wind the film slowly, noting how far the rewind knob/
crank has moved, and perform enough of the push-rewind-button-rewind-tiny-bit-
release-rewind-button-rewind procedures to get the rewind crank/knob back to
it's original position.

It sounds complicated, is difficult to describe, and is somewhat more awkward
than the simpler method cited, but it results in much greater accuracy and 
removes the requirement to waste a frame on either side of the multi-exposure
shot.  If step 1) is performed prior to the FIRST shot, pin-registration
accuracy can be achieved.  (Auto-processing equipment also tend to get
confused by the first method if registration is off slightly.  Before
discovering the method presented, I got back a roll of slides with every 
frame after the multi cut neatly in half, due to sloppy the registration of
the first method!)

As far as I know, this method has not been presented in instruction manuals
or magazines, although it works with every camera I've tried it with.

>Now for my question...let's say you're taking a picture of a lake and it's a
>sunny day. On top of that you wish to multiply expose a car to make it look
>like the car is hovering over the lake. For the sake of argument let's also
>say the proper settings for each individual picture would be:
>   1) Lake: f8 at 1/250 sec
>   2) Car: f4 at 1/60 sec
>To make a multiple exposure, what settings do I use?

The key is to think in terms of exposure value.  Each single f-stop change
doubles or halves the amount of light reaching the focal plane.  Reduce each
exposure by one stop for a double exposure, two stops for a quad, etc:
	Lake:	f11 @ 1/250	OR	f8 @ 1/500
	Car:	f5.6 @ 1/60	OR	f4 @ 1/120
This rule of thumb is sometimes tempered by reciprocity -- if in doubt,
bracket using 2/3 stop and 1 1/3 stop decrease per exposure.
-- 
:::::: Jan Steinman		Box 1000, MS 61-161	(w)503/685-2843 ::::::
:::::: tektronix!tekecs!jans	Wilsonville, OR 97070	(h)503/657-7703 ::::::