The idea of fill-flash is to balance a dark foreground with a bright
background. To accomplish this, one sets the f-stop per the manual guide
number of the flash to obtain proper exposure of the foreground, ignoring
for the moment the background. The shutter is then adjusted to give a proper
exposure of the background based on the f-stop chosen. This will give a 1-1
fill ratio. One can then adjust the exposure compensation dial (or the
shutter, since this is what the exposure compensation dial does anyway) to
give less exposure to the background. Adjusting the shutter will have no
effect on the foreground since this is exposed solely by the flash which is
faster than the shutter in any case. The "normal" fill ratio is considered
either 1-2 or 1-3, although this is an individual preference. These
adjustments are easiest to make with a leaf type shutter since the shutter
will sync with the electronic flash at all speeds. A disadvantage of the
focal plane shutter is you are limited to a top speed that will sync with
the flash. Olympus suffers greatly in this respect since the top sync speed
for electronic flash is 1/60s. If this is not fast enough to give the
desired background exposure, an adjustment to the f-stop will have to be
made which will alter the foreground exposure. The only way to compensate
for this is to adjust the flash to subject distance which will alter the
image size unless one is using a zoom. All of the above can certainly be
accomplished, and even rather quickly if one is experienced at it, but it
doesn't compare to the "modern" exposure systems that Peter was referring to
in his post. Canon, Nikon, Minolta and Pentax all offer exposure and flash
systems that will accomplish all of the above automatically. They can do it
because they use a little thing called matrix metering. Even switching an
OM-2 or OM-4 to Auto will not accomplish this, since Olympus uses average
metering for TTL. You will have no way to control the foreground to
background ratio. I am extremely sad to say that only manual control can
accomplish this in the OM series cameras. We Oly users can only dream of an
OM-5 which will incorporate these very desirable features.
> Robert wrote:
> There was a long discussion on this recently on the list. I had
>believed
> that using the exposure comp dial to adjust fill would work, but
>it was
> pointed out to me, I now believe correctly, that this will not
>work if
> you require your ambient exposure to remain constant. The EV
>will be
> affected if you do this even though you will get more or less
>flash.
> ---------------------
>I've given this some thought myself, and I don't think ambient exposure
>would be affected. Assuming you set the "proper" aperature for a 1/60s
>shutter speed (which is what the camera sets automatically), using the
>exposure compensation dial won't (shouldn't ?) affect either aperature
>or shutter speed, only flash output. The flash output will be based on
>aperature, which the camera has no control over, and film speed. Thus
>ambient exposure (f<x> & 1/60s) should remain constant. Make sense?
>
>-Stephen Kudlacek
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