Hmmm. Dr. Flash avoids all this auto mode and TTL and flash
compensation mumbo-jumbo by shooting only in manual mode on both camera
and flash. It's just plain easier that way.
There are a few simple rules to learn about shooting flash.
1) When shooting electronic flash there are always two exposures. The
first is the ambient light exposure which is affected by ISO, shutter
speed and aperture. It's what you'd get by using your light meter but
with the constraint that the shutter speed used must not be faster than
the sync speed for the camera. The second exposure is from the flash
which is controlled only by ISO, the flash's power output and the
aperture on the camera. Shutter speed has no effect at all. Even if
you use your Pen F which can sync at any shutter speed the flash
duration is still much faster than the fastest shutter speed on the
camera. So the Pen F will make life easier for fill flash (since it
allows controlling the bright ambient light with the shutter) but that
1/500th shutter speed won't affect the flash exposure one bit.
2) The "depth" of your flash exposure is maddeningly limited by the
inverse square law. To get a handle on that it helps to keep a few
numbers clearly in mind... 1.4 (the square root of 2), 2 (the square
root of 4) and 4. Now, here's how to use those numbers for some flash
exposure insight. Assume your subject is 7 feet away and you have
correct flash exposure for that distance. By the time the light from
the flash has reached 10 feet (7x1.4) it's only gone 3 feet farther but
is already 1 stop down. By the time the light reaches 14 feet (7x2) the
light there is already 2 stops down. By the time the light has gone 28
feet (7x4) the light is down by 4 stops which is getting quickly toward
black, or no effect at all.
Applying what we just learned we can see that, if flash exposure is
applied to a near subject then that subject is going to be about the
only thing that's properly exposed. You don't have to have another
subject much closer to the flash in order to get serious overexposure.
You don't have to have another subject much farther from the flash to
get serious underexposure.
One help (not a solution) is (as discussed here somewhat already) is to
"drag the shutter" which means to use a slow shutter speed to assist
with illuminating the scene from the ambient light. That does help
light the background and is independent of distance from the flash.
However, the foreground flash exposure still suffers from the effects of
the inverse square law. But note that, if you're shooting a subject at
10 feet rather than 7 feet, light loss doesn't hit one stop until 14
feet and doesn't hit two stops until 20 feet. The main point is that
the farther you can get from your subject the more even the illumination
will be for other subjects ahead of and behind your main subject. But
there is no free lunch. As you move your flash farther from the subject
you lose light power and the tiny flash head begins to look more and
more like a point source of light (like the sun). Just like the sun,
point sources of light cast rather harsh shadows. The solution for that
problem is to turn your tiny flash into a very big and broad light
source. The easiest way to do that is to bounce off a (relatively)
neutral ceiling or wall or both and use higher ISO and bigger apertures
to compensate for overall light loss. And, if you want to go whole hog,
sprinkle some slave flashes around the room to help fill the background.
Doing this with TTL is a bit problematic since you don't really know
what the camera is going to do. But doing it without TTL on film either
means that you need a flash meter or else a digital camera so you can
quickly evaluate the effect of any given exposure.
If I were going to do what you're about to I'd use the Pen F and its
fast shutter speed sync in daylight (for fill flash) and the OM in the
dark (for the larger film format). I'd also get a flash meter and a
different flash unit that offers good manual control with multiple power
levels. Ken's Vivitar 285 is a nice flash unit offering both manual and
auto modes but it's about a stop less powerful than a T32 or the modern
system flash units for the major brands. I recommended to Moose last
year that he add a Canon 540EZ to his gear which he did. The 540EZ was
Canon's cat's meow TTL flash... for film. It don't know how to do TTL
on digital so they are not in high demand except by the newbies who
don't know it don't know digital TTL. :-) You can buy one in BGN
condition from KEH for about $80 or as low as $60 with patient waiting
on ebay (assuming the newbies aren't there bidding the price up to $140)
Of course, you don't have to have a 540EZ. It could be some other,
similar flash unit. It doesn't do TTL except on Canon film EOS cameras
and it doesn't do auto mode on anything. But it does have 7 power
levels, a bounce and swivel head and power zoom on the head from
24-105mm plus a 17mm pull out diffuser (which also makes a handy bounce
card). Get a meter and a more versatile flash.
Dr. Flash
On 5/19/2011 10:42 AM, Walters, Martin wrote:
> In spite of the eloquence and expertise of Dr Flash and others on the
> list, flash remains something of a "black art" for me. As I rarely use
> flash, my usual approach is to simply put my non-Oly dedicated flash on
> TTL and fire away. For info, the flash is a Braun 340 SCA (full TTL
> with the correct adapter shoe), in auto it has two selectable F stops
> (F4, F8), a swivel/bounce head with zoom (28-50-100). The filter set
> includes a diffuser (coverage for 21mm), ND4, and coloured filters.
>
> So, I'm taking a flash 101 course, which looks to cover pretty much all
> the topics. I'm in an interesting position as the only film shooter in
> the group (we're 8-10). Some of the CanNikon gang appear to have serious
> gear, and some of those camera/flash combinations are probably more
> intelligent than I am.
>
> After a couple of courses it's obvious that I'll have some challenges
> and some of the concepts will be theoretical rather than practical
> (e.g,, flash compensation, though the instructor lent me an EOS to work
> with; varying sync speed). My dedicated flash has some significant
> limitations, especially no or only very basic power control (filters on
> the flash). That being said, the instructor was quite intrigued that I
> had full TTL capability both on and off camera.
>
> One of the things we've looked at is compensation (both exposure and
> flash). So, this weekend I'm planning to spend some time test shooting
> to see what happens with various setups under TTL. In the meantime, I'd
> be grateful for some info on how exposure compensation on an OM2N
> affects TTL flash. I assume that it will affect both metering and flash,
> which is controlled by the meter (I believe modern N*kon bodies, for
> example, treat the two independently). Is this correct? However, that
> brings up another question: if exposure compensation is active, how does
> the OM actually compensate, when it automatically fires as 1/60 with a
> dedicated flash?
>
> As a third question, AG and Dr Flash have talked about underexposing the
> background by 1 or 2 stops. One method mentioned was to set the aperture
> for 1/30 exposure, and letting the OM fire at 1/60. I understand the
> concept - and here my ignorance is showing - but does this work in
> straight auto or in TTL as well?
>
> In anticipation of suggestions to get a different flash.... Yes, I've
> thought about a T32, though it appears only slightly more flexible than
> the Braun, and a Vivitar 285, which doesn't do TTL but allows control of
> flash intensity. However, the lazy side of me doesn't want to throw away
> TTL.
>
> As far as sync speed is concerned, I could always get out my Pen F,
> which syncs at all speeds. Of course, no TTL, and life is probably too
> short to deal with other challenges!
>
> Martin
>
>
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