Re: Dragging the shutter. Technically, very well explained and useful
to know in the film world but more complicated than it needs to be in
the digital and zoom lens world. First, get rid of the TTL and auto
flashes as they complicate the picture. Next, fit the camera with a
zoom lens covering moderate wide angle to moderate telephoto. The
purpose of the zoom is to be able to frame the subject from a fixed
distance without zooming via your feet.
Then pick the highest ISO setting of your digital camera that's
consistent with your view of good image quality for the task at hand and
pick a distance. 10 feet (3 meters) works out pretty well in most
cases. Using the flash guide number or a flash meter or just plain
experimentation determine the proper flash power and aperture setting
for a distance of 10 feet. We'll assume, for the sake of discussion,
that the camera ends up at ISO 400 with the flash at 1/2 power and the
aperture at f/8. At this point you're properly set up to take perfectly
exposed photos of subjects that are 10 feet away. Your next task is to
learn to estimate a distance of 10 feet so that you can always stay that
distance away and frame with the zoom. If you're good at it and using a
smallish aperture like f/8 you can even set the focus at 10 feet and
just leave it there. If you want to drag the shutter drop down to about
1/30 or 1/15 second and take a test shot of the room. Chimp the shot
and adjust the shutter speed as required to suit your exposure taste and
tolerance of subject motion.
I agree with a darker background vs foreground but prefer only 1 stop
down. I try to control that when I can by using studio flash units as
room lights with the studio flashes set for about f/5.6 while the
on-camera flash brings the subject up to about f/8. Of course, the
range of the studio flashes is limited in large rooms and the light
drops off pretty quickly so that's where the shutter dragging comes in.
It helps fill in the rest of the room from the ambient. But the
shutter dragging works even if you don't have the room lights and is all
the more important if you don't.
But for all your planning it may happen that 10 feet just doesn't work
out in all situations. If you're forced to move closer or further
remember the inverse square law and the square root of 2 (1.4). If you
increase or decrease the distance by a factor of 1.4 the light from the
flash will need to be changed by one stop in order to maintain your
aperture. If you've picked a nice round number like 10 feet as your
base distance then a minus 1-stop distance is about 7 feet and a plus
1-stop distance is 14 feet. If you had the flash at 1/2 power for 10
feet and have to move closer move in to 7 feet and cut the flash to 1/4
power. If you have to move further out go to 14 feet and set the flash
to full power. So, you say, how is it that my flash power at 10 feet
just magically worked out to 1/2 power. Remember that ISO dial on the
camera? Maybe it shouldn't be set to 400 for your flash unit but to 500
instead. If you're not using room lights as I do you would also be free
to change the aperture instead of just altering the on-camera flash
power. But in that case you need to realize that you're also affecting
the ambient exposure and you'd have to adjust the shutter speed to
compensate.
In the film world this is a lot more complicated and needs to start out
with a light meter to work out an exposure for the ambient light as AG
has indicated. In the digital world chimping and twiddling the ISO dial
will set you free. And learning to do manual flash shooting from a
fixed distance will give you near perfect exposures and allow the use of
cheap old flash units which will allow you to buy more glass. On my 5D
I now use a Canyon 540EZ which is totally useless on Canyon digitals in
other than manual mode. Because of that they can be had for as little
as $60 from KEH despite the fact they've got the longest range autofocus
assist light of any Canyon flash unit, about the same power as a T-32
and an 8 stop manual power range.
Dr. Flash
AG Schnozz wrote:
> 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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