Manual Strobe Control
Table of Contents:
 Guide numbers, strobe power, and the Guide Number formula
 Adjusting the GN formula for metric system or different film speeds
 Guide numbers in practice
 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 fixedpower
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
strobetosubject 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 SS200 only 1.4 times as
much as the GN for the SS50, when the SS200 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 SS200 spreads its light out into a 100degree 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 SS200, 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 rearrange 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 reset 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 rewrite 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 100speed 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 GN40
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 SS200, 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 5foot 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 5footdistant 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.
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. 
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 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 nittygritty 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.
4. Balancing strobe and ambient light
All of this theory is great, but how does it help you take pictures with your strobe on
manual control?
Too little strobe light. Notice how the eagle ray and the
background are reasonably wellexposed, but the ray lacks color. This is because I
was 5 feet from the ray when I took the picture, and the aperture was set to require me to
be 3.6 feet from it. 
Too little background light. Notice how the turtle is well
exposed, but the background is too dark. This is because I forgot to remeter the
background, and didn't have the shutter open long enough. 
Ah, just right. The background is a nice bright blue, and
the foreground is welllighted and colorful. This exposure is pretty much dead on. 
The goal of exposure control is to get a properly exposed
subject and background. You don't want a whitehot overexposed
subject on a dark background, and you don't want a colorless, flat subject on an otherwise
properly exposed background. What you want is a properly exposed background with
just enough strobe light on the subject to restore its color and make it stand out from
the background a little. That desire is why you're controlling your strobe
manually, so it's about time I show you how to do it.
Basically, you have to start with a properly exposed
background, and then balance the strobe light against that. So look in your camera's
viewfinder after the shot is framed up, and use the meter reading to set your aperture and
shutter speed. Mostly you won't get to change the shutter speed very much; it will
have to be in the sync range for your camera. It's likely that you'll be stuck in
the range between 1/60 and 1/125, or perhaps even down to 1/250 if you have a modern SLR
in a housing. So set something reasonable in that range, and then set the aperture to fit
the shutter speed and expose the background properly.
Then it's time to go to work.
You have two options at this point: change your distance to
match the strobe's power setting, or change the power setting to match your
distance. Obviously, if the picture is framed up just the way you want it, changing
distance is not going to work. Most of the time you will adjust the strobe's power
setting to match the distance you have selected. This means you have to estimate the
strobetosubject distance in order to get the right power setting. Even if you
change your distance and leave the strobe's power setting alone, you will still have to
estimate your distance to the subject in order to get a proper exposure. This skill
takes some effort to develop, but everyone can get good at it pretty quickly.
If your strobe doesn't have a power setting, you will be
forced to change your distance. That's why it's important to have multiple power
settings on a strobe. That's one of the reasons I like Ikelite strobes; the SS200
has 4 power settings: full, 1/2, 1/4, and 1/8. You can almost always find a power
setting that will work with the picture.
What happens if you realize that the strobe just doesn't
have enough power to light your scene at the distance and aperture setting you have
chosen? In this case, you have to open up the aperture, which will mean going to a
faster shutter speed in order to keep the background exposed properly. Hopefully, your
camera will have enough latitude in strobe sync speeds that this is possible. Similarly,
if you realize that your strobe's lowest power setting is too powerful, you'll have to
stop down to a smaller aperture, which will force a slower (i.e. longer) shutter
speed. Again, hopefully your camera will cooperate.
It's worth mentioning that the distance that counts is not
the lenstosubject distance, but the strobetosubject
distance. So if you have your strobe positioned way out away from the camera, be
sure to remember that and estimate the distance by looking from strobe to subject, not
from you to the subject.
Next Chapter  Back to Exposure 101 