A few days ago, I made a video on the different units of light
and how they relate to each other, and I wanted
to include guide number in that video,
but it just didn't seem to fit, so it's getting its own video!
Let's get Undone.
[offbeat music]
♪ Gerald Undone ♪
♪ He's crazy ♪
[laid-back yet energetic instrumental]
♪
[Australian accent] Fair dinkum, matey!
How many rotations do you need?
[normally] I'm Gerald Undone, and today we're taking
a closer look at the peculiar guide number used on flashes.
Unlike the previous units we covered, and the reason
why I omitted this from the last video,
guide numbers aren't an exact measure of light power
or intensity, but rather a formula based on distance.
That formula is guide number equals f-number times distance,
typically expressed in meters, but some manufacturers
will provide conversions for feet as well.
So, let's take a look at an example guide number,
break down its parts,
and then get into some of the misconceptions.
So, if we take the Canon 600EX, which is a popular speed light,
the guide number here is 60m-- or 197'--
at ISO 100 and when zoomed to 200mm.
So, the first number is the distance,
and with Canon speed lights and several other manufacturers,
it's in the name. So, the 600EX
has a guide number of 60.0m,
the 430EX 43.0m, and so on.
And this distance is referring to the distance
from the speed light
to the subject you're trying to illuminate.
Now, the other number-- the 200mm-- is just telling you
that this guide number only applies when the speed light
is fully zoomed, because many flashes can be zoomed,
just like lenses. The more zoomed in they are,
the more focused and spotty the light beam becomes,
and thus the more intense the light becomes on your subject.
We're gonna ignore this part from now on, but just keep
in mind that when flashes have variable beam angles,
they're usually rated for their most focused setting.
Lastly, the ISO part is telling you that if you follow
the other instructions, you should be able to get
a sufficient exposure when your camera is set to ISO 100.
And that sounds like enough information, right?
And in fact, countless times I've heard sales pitches
and advice that conclude the information there,
basically suggesting that the Canon 600EX will illuminate
a subject up to 60m away when you're set to ISO 100.
But that's not true at all, and if we go back
to our original formula, we can see why.
So, if guide number equals f-number times distance,
this could also be expressed as distance-- or how far away
our subject needs to be for us to light them well
at ISO 100-- equals guide number divided by f-stop.
And so if our guide number is 60m, in order to also have
a subject distance of 60m, our f-stop would have to be
set to 1, and that's a pretty extreme f-stop.
And it's also not up to 60m away,
because ISO 100 is often the floor for cameras,
so any closer and they'd be overexposed.
So, in order for that sales pitch from earlier to ring true,
the rep would actually have to say, "The Canon 600EX
can light your subject well at full power when your subject
is exactly 60 meters away, and your camera is set to ISO 100,
and you're using an f/1 lens, and the flash is fully zoomed."
That sounds a lot less impressive now, doesn't it?
But it is more useful, because now we know
exactly how to distance our subject.
All you have to do is substitute in
the actual f-stop you want to be shooting at.
So, say you wanted to use f/4.
Well, with a guide number of 60, divide it by f/4,
equals 15m.
So, at f/4, with your subject 15m away
at ISO 100 at full power on the flash,
your subject should be well-exposed.
But remember, the 60 in that equation
came from the Canon 600EX, and the guide number is likely
to change depending on whatever flash you're using.
And if you're gonna use feet as a measurement,
make sure you use it at both sides of the equation.
So, here's another example using a different light
and using feet: the Canon 430EX has a guide number
of 43m, so 141'.
And if you wanted to shoot in f/2.8, how far away
would the subject need to be?
Well, 141' divided by 2.8 equals 50.4'.
So, if your subject was 50.4' away,
and you were at f/2.8
and your 43 GN flash was at full power,
you should get a decent exposure. Easy enough, right?
But what if you wanted to move the subject closer?
Well, you'd have to lower the power of your flash.
But the relationship here isn't as easy as it might seem,
and that's when we get into our second misconception.
In our 60m GN example, we said that when shooting at f/4,
your subject would ideally be placed 15m away.
But what if we wanted to bring our subject to 7.5m away?
Would we reduce to half power, because half of 15m is 7.5m?
Unfortunately, no.
That would be easy, but it would ignore the inverse square law,
which basically suggests that as the distance doubles,
the light intensity decreases by 4x, not 2x.
So, you'd have to take your flash power, which is usually
expressed as a fraction, and find the square root of it.
So, in our example of half power,
the square root of one half-- or 0.5-- is 0.7, rounded.
And so then you would take that and multiply it
by your distance, 15m, which would be 10.6m, not 7.5m.
And so in this case, at half power at f/4
with a guide number 60m flash at ISO 100,
you would want to place your subject 10.6m away.
OK, so now that we understand a couple
of the misconceptions and the correct math,
how do we apply this to practical situations?
Because let's face it, we're not likely to figure out
the power first and then place our subject that far away,
but instead, typically you would frame up a shot
that you like and then figure out how to light it well.
So, typically the distance would come first.
and then the power would come after.
But the problem is, this whole system is based
on the idea that you'll just stop down your aperture
in order to get the correct exposure. So, if you're using
a flash with a guide number of 48 and your subject
is 6m away, well, you'd have to stop down to f/8.
48 / 6 = 8, and then you'll get a good exposure.
But what if you want to use a particular f-stop
and want to know what power to set?
Well, unfortunately, this information
is much harder to find, and in fact,
many of the calculators online
generally only solve for aperture and distance,
but not flash power. And the reason for this
is logical-- because the power of your flash
isn't tunable to a precise degree,
and is instead forced to exist within the confines
of specific fractions, all of which are powers of two.
And so if you were to put a random distance,
guide number, and f-stop into a calculator,
the solution might not-- and likely will not--
match one of those particular available fractions.
But there is a way to figure this out using
our previous conclusions, and you can do this
on the fly as long as you know your f-stops.
Let me give you an example, and let's use our guide number
of 48 again, but this time, let's suggest that we want
to photograph a subject that's exactly 1.5m away--
which is probably more realistic than 15m.
And let's also suggest that we want to use an f-stop of 2.8.
How do we figure out what power to set our flash,
in our head, without memorizing another formula?
All we have to do is figure out our f-stop
at full power, and then move the same amount
of stops on our flash as we would on our camera.
So, if you remember back to the beginning when I said
the guide number equals f-stop times distance, well,
if our guide number is 48 and our distance is 1.5m,
then our full-power f-stop would be f/32.
48 divided by 1.5 equals f/32.
And so if we know that our full power f-stop is f/32,
what power do we need at f/2.8?
Well, luckily, every stop of aperture is equal
to a stop of power on the flash.
And so if we go from f/32 down to f/22 on our camera,
we would go from 1/1, or full power,
down to 1/2, or half power, on our flash.
And then the next stop, from f/22 to f/16,
would take us from half power down to a quarter power.
And this would continue all the way down to f/2.8, which
would tell us that our flash power needs to be set to 1/128.
And if you know your powers of 2, this gets even easier,
because all you have to do is count the amount
of stops that you moved and make that an exponent of 2.
So, to go from f/32 down to f/2.8, that’s 7 stops, and so
2 to the power of 7 is 128, and then you make that
the denominator in the flash power, so 1/128 power.
If you move 5 stops, 2 to the power of 5 is 32, 1/32 power.
And you only actually have to memorize up to
2 to the power of 7, because most flashes
have 1/128 as their lowest power setting.
And here's a trick to memorize your f-stops while you're at it:
as long as you know two f-stops that are side by side, you can
figure out the rest of them just by doubling them in pairs.
So, if you know f/2.8 and f/4, then you can figure out
the rest, because doubling f/2.8 and f/4
will get you to f/5.6 and f/8, and then double those:
f/11 and f/16, and then double those: f/22 and f/32.
But if you suck at memorizing things
and you hate numbers, there's an even easier way.
You can actually physically move the dials
for your camera's aperture
and the flash power in the same increments.
And this even works in 1/3 of stop increments, because 1/3
of a stop in f-stop is equal to 1/3 of a stop in flash power.
So, you’d basically just move the correct amount of stops
to get you down to the f-stop that you want,
and then move the same amount of stops on your flash power,
and you'll be set up correctly. Now up to this point,
everything has been based on ISO 100.
But what happens if you increase your ISO?
Well, increases to ISO also increase the effective guide
number, but you again have to respect the inverse square law.
Essentially, quadrupling the ISO is equal
to doubling the guide number.
So, if you're working with ISO 100 with a guide number of 48,
then if you moved up to ISO 400, the guide number would go to 96.
Every time you multiply your ISO by 4,
you multiply your guide number by 2.
I also have a quick note on the last element of exposure,
shutter speed, which will also tackle another misconception.
You probably noticed that shutter speed wasn't in
any of our calculations, and that's because
shutter speed does not affect the amount
of illumination on your subject from the flash.
The light from the flash is provided
at the same interval that the photo was taken,
so exposure time is kind of irrelevant.
Shutter speed will still affect the amount
of ambient light collected, though,
just not the flash’s light. So, that's it!
That's all the wonky math about guide numbers.
The idea behind this video was to clear up
some of the misconceptions, but also provide
a practical use for some of these numbers
so that you can get the correct exposure every time.
And I think for someone just starting out
in flash photography, this is a great place to begin
and a great thing to learn.
But there is a missing factor here
that I should mention to tip my cap to the veterans
of using flash: you get to know your flashes.
And this is doubly true if you have a studio.
While I do think that my stop-for-stop method
is about as convenient as it can get for calculating
multiple lighting variables, doing the math will still
be slower than a pro who just knows their lights.
And if you're just starting out, don't worry,
you'll get there too.
Eventually you'll just know that a quarter power
is the right answer for that situation.
But in the meantime, hopefully this video
will help prevent you from taking too many "test shots"
while you get to know your lights.
But that's gonna be it for me. I hope you found this video
helpful, or at least entertaining. And if you did,
make sure you leave it the old thumbs-up,
and consider subscribing if you haven't already.
But if you did not find this video helpful or entertaining,
feel free to hit the dislike button twice.
Alright... I'm done.
♪