Автор: victory13
Дата: 14-09-12 10:05
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Manual Strobe Control
Table of Contents:
1. Guide numbers, strobe power, and the Guide Number formula
2. Adjusting the GN formula for metric system or different film
speeds
3. Guide numbers in practice
4. Balancing strobe and ambient light
1. Guide numbers, strobe power, and the Guide Number formula
When you can't rely on TTL strobe control, what are your
alternatives? Manual control is really the only other way to control
your strobe. So let's jump right in.
Here's the basic situation: your strobe dumps all of its light while the
shutter is open. There's no other way the system can work. Click here
if you want to know why. So how can you control how much light
reaches the film from your strobe? Basically, there are three ways.
One way is with your lens aperture. You have to set the shutter speed
for a range that syncs with the strobe, but the aperture is completely
under your control. The smaller the aperture, the less strobe light gets
to the film. The second way is by changing the distance between you
and your subject. I don't know if you remember high school physics or
not, but the intesity of light falls of like the square of the distance from
the source. The same applies to your strobe light. If you take a picture
from twice as far away, your strobe will have one quarter the effect.
Three times farther away? One ninth the effect. You get the idea. So
you can change the distance between you and your subject to change
the effect of the strobe. The third way is possible only with more
complex and expensive strobes: you can dial back the strobe power.
Some strobes come with a knob where you set how much light you
want from the strobe: full power, 1/2 power, 1/4 power, 1/8 power,
etc.
But let's ingore that power dial for a minute, and pretend that we have
a fixed-power strobe. How do we know what aperture and shutter
speed to use for any given picture? First of all, forget the shutter speed
for now. As long as it's within the sync speed range for your camera,
you'll be OK. Then how do we know what aperture to use? The
answer is to use a formula that applies to all strobes, and a number
called the Guide Number (GN). The guide number for your strobe
should be stated by the manufacturer. If the manufacturer is Ikelite,
that Guide Number will probably be reliable. If the strobe was made
by someone else, you'll probably have to take their guide numbers
with a grain of salt; my experience is that many manufacturers overrate
their guide numbers, sometimes dramatically. This is probably a
marketing ploy.
The guide number is a number that tells you what aperture setting to
use for the strobe-to-subject distance of your picture (or vice versa).
The formula is
GN = aperture * distance.
The higher the guide number, the stronger the strobe. For example, the
guide number for my Ikelite SubStrobe 200 is around 40, in feet for
ISO 100 film. The GN for my Ikelite SubStrobe 50 units is 28. Why is
the guide number for the SS-200 only 1.4 times as much as the GN for
the SS-50, when the SS-200 is supposedly 4 times as powerful as the
50? Because the 50 concentrates its light into a relatively narrow cone
with an opening angle of around 70 degrees. The SS-200 spreads its
light out into a 100-degree cone. That means the 200 can "cover" a
much wider lens than the 50. Since you've got more area to cover, the
effective brightness of the light goes down.
So how do we use the Guide Number formula? How about I show you
some examples. In all the examples below, let's just assume I'm using
my SS-200, with a guide number of 40. Its actual guide number is 38,
but 40 makes for easier calculating.
Let's say my subject is 5 feet from my strobe. To calculate the proper
aperture, I re-arrange the GN formula from above like this:
aperture = GN / distance.
Now I solve for the aperture: aperture = 40/5 or f/8. If the subject is 3
feet, I set it to 40/3 = f/13. Now f/13 isn't a regular stop, so I'd shoot
one at f/11 and another at f/16, and hope for the best.
But what if I have set my aperture to some value (say, for proper
background exposure, or depth of field, or whatever) and I don't want
to re-set that? Can I use the guide number formula to tell me what
distance I need to be from the subject? Sure, but I need to re-write the
formula again:
distance = GN / aperture.
Let's try another example. Let's say I've picked 1/60 sec and f/8 for my
exposure settings. I have the same GN 40 strobe. Where do I need to
be relative to the subject? Use the formula: distance (in feet) = 40/8 =
5 feet away. If I had my aperture set to f/11, I'd need to close in to
40/11 = 3.6 feet.
There's an easy way to remember all of this: if you have the aperture,
divide it into the guide number to get your distance. If you have the
distance, divide that into the guide number to get an aperture.
Hey, wait a second! In the section on setting shutter speeds and
apertures, we talked about how you have to set both of them in order
to get a good exposure. Why are we only talking about apertures in
this section on strobes? Doesn't the shutter speed have something to
do with it? No. As long as the shutter speed is in the strobe sync range
for your camera, the shutter speed is irrelevant as far as the strobe
goes. If you really want to know why, click here.
Are we clear on how to use the guide number? Then let's look at how
to adjust the guide number for other situations than feet and 100-speed
film.
2. Adjusting the GN formula for metric system or different film
speeds
Why do manufacturers say things like "in feet for ISO 100 film" when
they give you a guide number? Because of the guide number formula.
Let's think about the GN-40 strobe we've been using all along. If I
used a GN of 40 and told you the answer was in meters, what would
happen? Well, you'd do that last calculation and decide that the right
distance was 3.6 meters. Uh, converts to 11.7 feet, when we know the
right answer was 3.6 feet.
If you want to think in meters, you need the guide number in
meters. For my SS-200, the guide number in meters is 12. Do that last
calculation, and you'd get the proper distance as 12/11 = 1.09 meters.
Get out your calculator; 1.09 meters = 3.53 feet, which is pretty close
to 3.6 feet.. Aha! Now we're on to something. We didn't change the
strobe, so the subject distance shouldn't change, no matter what
distance units we use. But then the guide number has to change, right?
Cool. Converting a "feet" guide number to "meters" is pretty simple:
divide the "feet" guide number by 3.25 and you'll get the "meters"
guide number. If you have a metric guide number and want it in feet,
multiply by 3.25.
What about film speed?
If you switch from 100 speed film to 200 speed film, you need half the
light, right? So if I set my distance & aperture based on the guide
number for 100 speed film but I'm using 200 speed film, I'll get twice
the light I need, right? And that's bad, right? So how do we fix it?
I don't want to get into the long hairy explanation (you can read it here
if you really care), but the solution is to multiply the guide number by
the square root of 2 (i.e. 1.4) each time you double film speed.
Alternatively, you can multiply the guide number by the square root of
the ratio of the two film speeds in question. That's probably easier to
remember.
Want an example? OK, let's do the 5-foot subject distance example at
ISO 200. Our old guide number was 40. Our new guide number will
be
40 * sqrt(200/100) = 40 * 1.4 = 56.
Then we apply the GN formula, so that a 5-foot-distant subject would
then imply an aperture of 56 / 5 or f/11.
Is that right? Sure. We had f/8 before, with 100 speed film. With 200
speed film, we need half the light. Thus we need to increase our
aperture number by one stop, which decreases the light reaching the
film by half, which is our actual goal. So what's one stop "down" from
f/8? Look on your camera: it's f/11. Bingo!
Another example? OK, what if we went to ISO 50 film? Then the new
GN is 40 * sqrt(50/100), or 40 * 0.707 = 28. Then at 5 feet, we'd need
an aperture of 28 / 5 = 5.6, or f/5.6. What's one stop more light than
f/8? That's right: f/5.6.
3. Guide numbers in practice
All of this is awfully complicated. How does your average mortal do
these calculations under water, while setting up a photo? I have no
idea; I'm not your average mortal. :) Seriously, that's way too much
math to do underwater. No one does that. Strobes come with an
exposure chart that allows you just to look up the answer on the chart.
Match up film speed, strobe power setting, and distance, and the chart
tells you aperture.
My Ikelite SubStrobe
200 has a rotating
wheel on the back of
the strobe (fig. 1). You
line up the strobe
power setting over the
film speed you're
using, and look on the
other side of the wheel
for aperture/distance
combinations that will
work. For instance, at
quarter power and ISO
100 film, my SS 200
wheel tells me these
combinations will
work:
5 feet (1.5 m) f/4.0
3.6 feet (1 m) f/5.6
2.5 feet (0.75 m) f/8.0
1.8 feet (0.5 m) f/11
1.25 feet (0.38m) f/16
We didn't talk about power settings, so now is a good time. Some
strobes have power settings that let you tell it "I want a full blast," "I
want 1/2 of a full blast," or 1/4, 1/8, etc. The GN of the strobe does
Fig. 1: A sketch of the guide number wheel
from my SubStrobe 200. The inner wheel (the
black part) rotates. You line up you strobe
power setting with the ISO speed of your film
on the bottom, then the top ring gives you
aperture & distance combinations.
depend on the power setting. Similar to the film speed situation, the
new GN is the old one times the square root of the power setting. That
is, if we set the strobe on 1/2 power, our GN 40 strobe becomes a GN
= 40/sqrt(2) = 40/1.4 = 28 strobe. Does that "square root of 2" sound
familiar? It should, if you read digression #1. The same reasons
behind aperture numbers are behind the guide number changes. If you
want the real nitty-gritty on exactly why the power setting affects the
guide number only to the square root, read digression #3, where I
derive the guide number equation from physical principles.
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