I wanted to give a little more explanation of "dragging the shutter"
for those who are still a bit glazed over. May my application notes
here make you totally comotose.
Pretend with me that you are at a party somewhere. A restaurant,
church dining hall or some other venue that is larger than your
typical closet. My pretend room has no windows and is lit like a
typical restaurant where you can barely read the menu so you end up
ordering today's special--hoping that e.Coli from the lettuce doesn't
kill you.
At ISO 400, a "proper" exposure would be something like 1/15 at F5.6.
Too slow, so you use a flash. If you use an "auto-flash" you will
need to manually set the aperture. The flash power will vary
according to distance, but the aperture needs to be set. Now, my
particular flash says that for ISO 400, to place the aperture at
F5.6.
Ok, here's where things get interesting. What is the maximum
flash-sync speed of the CAMERA? The OM series was 1/60, but a modern
vertical shutter will go up to around 1/250 or more.
Let's assume a maximum sync speed of 1/250. If you set the shutter
at 1/250 and the aperture at F5.6, the room, without flash, will be
4-stops underexposed. Inotherwords, black with tiny dots where
lightbulbs or shiny bald-heads are. This also means that there will
be a huge fall-off from the flash. People in the correct distance
are lit nicely, but they are essentially in a dark cavern.
What we do in this circumstance is to start thinking of "fill-flash"
vs "flash-lighting". With "fill-flash" we primarily use the ambient
(room) lighting and only suplement with the flash to bring out the
subject or get rid of shadowed eyes or stop motion.
So, we need to alter our base exposure. We keep the aperture at the
recommended F-stop (F5.6 in this example) and slow down the shutter
to bring up the background exposure. Remember these rules:
- Aperture controls Flash-Exposure
- Shutter Speed controls Ambient-Exposure
- ISO controls the effective flash strength.
Ok, If I want a straight "flash-fill, I need to bring my exposure
down to F5.6 and 1/15 a second. The flash will light the subject
(actually will probably overexpose by up to a stop) and the
background will be perfectly lit. We do this ALL the time in outdoor
portraiture where I use fill-flash. This is fine when you are trying
to achieve as flat of lighting as possible.
But what if you want to slightly darken the background to bring out
the subject? Well, let's assume for our party shot that you want the
background to go down two-stops. This makes for a very nice 3D image.
If my "correct" ambient exposure (non flash) is F5.6 at 1/15, you
subtract two stops from that. But wait! Before you touch the
aperture, you must remember that the aperture controls the flash
exposure too! Don't touch that dial! You must restrict yourself to
just adjusting the shutter-speed. Two stops from 1/15 is 1/60. So,
my two-stop underexposure for the background is F5.6 at 1/60.
When you take the picture, you get a nicely lit subject from the
flash and the background is usually warm without any harsh shadows.
Hold on, you ask! What if my ambient exposure comes out to, say
1/500 at F5.6? That's too fast for the max flash-sync speed! This
happens when shooting outdoors. This is where you have a couple of
options. You have to either make your flash BRIGHTER so you can use
a higher F-stop like F8, or you adjust your ISO. When doing outdoor
fill on a bright sunny day, this is a real issue.
At ISO 100, the Sunny-16 rule says that your ambient exposure is
going to be F16 at 1/125. If my max flash-sync speed is 1/250, that
means I can open the aperture up to F11. For a typical portrait where
the camera is 15 feet away from the subject, you will need a
FLAME-THROWER of a flash to punch out enough light at F11. Forget
trying to do with with ISO 400 print film in your OM camera!
So, your options start to get a bit restricted with focal-plane
shutters. A lens-shutter camera usually allows flash-sync to 1/500.
This extra stop of leaway is enough to make it possible to do outdoor
fill flash portraits without hauling arc-welders around with you.
I hope this helps.
AG
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