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Re: [OM] OM2/OM4 question: fill flash

Subject: Re: [OM] OM2/OM4 question: fill flash
From: VELUWEH@xxxxxxxxxx
Date: Tue, 24 Feb 1998 11:46:57 -0500
Much has already been said about this subject, but not the complete
story, and I've also read some inaccuracies (NO manual flash is
certainly not the only option you have and NO you better stay away from
the exposure compensation dial because that only complicates it).

A fill flash is nothing else than a combination of ambient light and
flash light, where both light sources contribute to the exposure. Fill
flash is most commonly used to reduce or even completely dissolve
extreme contrast differences that can't be handled by film, but there
are other applications. Conditions where it may be wanted include:
A - Outdoor backlit portrait shots on a sunny day resulting in extreme
contrast differences. W/o fill flash you can either meter on the
background resulting in a silhouette picture, or (spot) meter on the
person resulting in a burnt out background and a dull foreground, or
average both readings resulting in an overall wrong exposure. Such a
condition asks for a normal flash exposure of the foreground while the
background is normal exposed by ambient light.
B - Heavy shades on a sunny day resulting in heavy contrast
differences. W/o fill flash details in the shadow parts are lost. This
condition, unlike the previous one, asks for a subtle mix of ambient
light and flash light, because you don't want to dissolve all shades
(what would happen with standard flash) but just reduce contrast. So
you must find a way to reduce the influence of the flash (underexpose
it) while retaining correct overall exposure.
C - Colour casts when shooting under trees, coloured canvas (like
[party] tents) or fluorescent light can also be dissolved by fill flash.
D - Heavy overcasted skies resulting in dull light and dull colours
that can be brightened up by flash; with the flash off camera you can
even create shadows.
E - Portrait shots that are already properly lit but where a fill flash
can produce shimmers in the eyes.

The OM System supports, depending on what combination of camera and
flash you're using, four different flash modes:
1 - Manual (guide number based) flash, possible with any OM camera and
any flash, including non Olympus flashes;
2 - Computer flash (aka "normal auto"), possible with any OM camera and
any computer flash, including non Olympus flashes; Olympus flashes that
support computer flash are the T20, T32 and T45, but not the T18 or
F280;
3 - TTL flash, possible with selected OM cameras (the 2[N,Sp], 3Ti,
4[T(i)] and 40[aka PC]), all four Olympus T flashes and the F280;
several third party manufacturers have also developed dedicated flashes
(or adapters) for the OM system;
4 - Super FP flash, possible on the 3Ti and 4T(i) and the F280;
basically, this is also TTL, but at higher speeds than 1/60, reducing
the GN.

Flash modes 1, 2 and 3 are typical examples of shutter speed priority:
you MUST use a shutter speed of 1/60 or slower. Manual and computer
flash modes involve setting automatic cameras to manual mode.

Now all four flash methods can be used for both normal flash and fill
in flash; the ways in which it can be done differ, depending not only
on the method, but also on the desired effect. Where condition A (the
backlit portrait) asks for a full flash, the other conditions demand
flash under exposure. Flash under exposure can be accomplished for all
modes by using a larger distance than the GN calculates for the
selected aperture (that is, moving out of the flash range). But
obviously this is not the most practical way.

In TTL mode, you must stop down the aperture untill the automatic
metering indicates 1/60 sec or slower; otherwise it won't fire. From
there on, the more you stop down (the slower the  shutter speed
indicates), the less the influence of the ambient light will be (since
the actual exposure will be 1/60 sec). So for minimum flash effect you
should stop down to a value that gives you a shutter speed close at
(but not faster than) 1/60 sec. When you're afraid even at this setting
the flash influence is to big, you can either move further away, or
reduce the flash power and therefore its working range by putting an ND
or diffusion filter on the flash (a white paper or handkerchief will
also do). This would be the place where you would be looking for that
fill in flash ratio switch that isn't there...

Computer flash under exposure is, in theory, the easiest of all. You
just set an aperture value on the flash that is 1 or 2 stops smaller
than the value set on the lens. So when the lens is set at F16, an
aperture setting on the flash of F11 or F8 will do. Simple, provided
your flash supports these aperture settings... But let's first look at
two examples: condition A (the backlit portrait) and B (the shade
reduction).

For the backlit portrait shot (A), set the camera to manual 1/60, and
meter on the sunlit background and set the aperture on the lens to give
proper exposure for 1/60 sec. When you're not satisfied with this
aperture because the depth of field is too large, or when a small
enough aperture is not even available on the lens, neutral density
filters may come in handy, but remember that when you use them you must
continue with the settings on the flash as if a smaller aperture
(larger aperture value) was set on the lens. Obviously using a slower
film would be better than filters.
Now since this particular fill in situation asks for full flash effect
(since ambient lighting effect on the main subject is [close to] zero),
you just set the selected aperture on the flash and that's it. So the
aperture settings on lens (calculating any ND filters) and flash are
equal. Situations like this are probably well-handled by TTL or FP
flash.

Situation B, the shade reduction shot, is handled differently.  First
of all, you don't want a full flash exposure - don't want all shades to
disappear. Second, you must be cautious that the highlighted parts of
the shot are not over exposed due to the added flash exposure.
Set the camera to manual 1/60, do an average metering (including the
high lights and the shadows) and set the aperture accordingly. You can
also (spot) meter on the high lighted parts, but than you must shut
down the aperture more to compensate for the added flash effect. Now
set the aperture on the flash 1 or 2 stops smaller, to achieve flash
under exposure.
Still, the effect of these shots are hard to predict because there's no
modeling light, and the human eye has larger contrast amplitude, so
it's best to experiment and take several shots using several flash
exposure compensations (0, 1, 2, 3).

Now what if the aperture you want on the flash isn't present? Not very
unlikely, since 1/60 second is ridiculously long in sunlit situations,
asking for very small apertures. Many computer flashes, including
Olympus' T flashes only have two or three settings, in the medium
aperture range (2-8). Compared to some other flashes, The T's are very
poorly equipped because the three settings are consecutive (2.8, 4, 5.6
for 100 ASA). The Metz 32 MZ-1 has three settings than increase in two
stops (2, 4, 8 for 100 ASA).

But now what if you need F11, or even F16?

The answer is: get yourself a second hand Metz 45 CT-1 hammer head
flash, available everywhere. This immediately gives you 5 consecutive
apertures (2.8, 4, 5.6, 8, 11 for 100 ASA). The best news is, however,
that its computer aperture capabilities can be extended by using the
Mecamat external sensor (a bit harder to find), giving you 11
consecutive apertures, ranging from F2.0 to F64. A MUST if you're
planning to do lots of fill in flashes, and YOU want to be in control
and not some TTL electronics.

Happy 'flashing',


Hans van Veluwen
veluweh@xxxxxxxxxx

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