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# Guide Numbers

Lesson 16 from: FAST CLASS: Speedlights 101

Mark Wallace

# Guide Numbers

Lesson 16 from: FAST CLASS: Speedlights 101

Mark Wallace

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## Lesson Info

### 16. Guide Numbers

Next Lesson: Built-in Meter

Lesson Info

Guide Numbers

and guide numbers were used back in the day before flashes had readouts and t tl metering and built in there. There was a way to say How far away does my flash need to be from my subject at a certain aperture value at a certain bias. So to get a proper exposure and guide numbers, you don't really need to use a guide number for calculating exposure. You really don't. But let me explain what it is, and then I'll explain why. Why, why they're useful? Um, it's a, uh it's a calculation. So a guide number equals the distance from the subject multiplied by your aperture value F number at and I eso value. Usually I s 0 100 For example, if we have a guide number of 80 that means at 20 feet at F 200.4, you'd get a proper exposure because 20 times four equals 80. Okay, so if you want to do math 20 times four equals 80 there's your guide number distance multiplied by your aperture value at this is so That's what you got. Number is so that means is if you have a guide number of 80 on your flash, an...

d you have a different flash as a guide number of 100. That means the flash that has a guide number of 100 is more powerful. The higher the guide number, the more powerful it is because if you're at 100 you could be a 25 feet away. 25 times four is 100 right? And so that's That's how good numbers work. It's It's not rocket science. It's just this little this times. This equals this, and that's what it is. A lot of older flashes have, like this little slide rule kind of thing on the side where you can set the I S O and the distance, and it will tell you what your aperture value is using this formula. And so that's how photographers used to set exposure on their flashes. They would just use the guide number and that little calculator on. And so if you look at older flashes, you can see that there's this sort of dialling thing and you can get those dials. David Hobby has a great article on strobe ist dot com that talks about guide numbers and that ah, little dial. But the thing that I think I'd numbers are really useful for is understanding how powerful flash is and understanding sort of the distances you need to be from your flash. And one of the neat things is, if you want to shoot in full manual mode, okay? And you don't have a light meter and you don't, maybe you don't trust the tomorrow. Levin, look through the eyepiece method of metering light. What can you do? You can use the guide number to calculate exposure. And if you're a Nikon person, you have a guide number calculator built into your flash. Sorry, can in people you don't. Okay, so how this works is if you hit mode on your Nikon speed light and again, not all Nikon speed lights have this. There is a mode that says G in that stands for guide number. So not all Nikon flashes have this, but the 909 tens do. And here's what you can dio Remember when we talked about metering for light with a light meter, we had to put in to values and our meter told us 1/3 notice. This is the same thing. We're putting in two values to find the third, so it works the same. So what we're gonna do here on this Nikon speed light is I'm going to put in my aperture value, So right here. Can we see this yet? We can see it. I'm going to put in F 7.1. Okay, That's telling me that I can shoot at 15 feet away and I'll get a good exposure if I take this change my aperture value down to 4.54 point that 15 feet. That's still gonna works. I put in how many feet away I want to be. It's changing what this flash output is. Let's say I want to go to a specific distance from somebody can put in the distance. So I'm gonna go down here too. Oh, say 4.6 feet and I'm gonna get a good exposure. And this little scale here says that F four, this flash is going to expose correctly from about three feet all the way up to about 35 feet at and I eso value of 400. So on a canon flash only show you something that it does have. It doesn't have a guide number calculator, but it does have something that's pretty cool. If we look on this cannon flashier at the very bottom of the flash, I'm in manual mode. By the way, you see this? This range right here from 1.7 all the way up to feet. When I push my shutter release halfway, you'll see that the little dot shows up that says this will work at 2.3 feet away. So it's roughly the equivalent of a guide number meter telling us what the distance is from the subject that will work. Okay, now this flash, because a cannon flash is smart enough to know what the aperture value and the I S O is from the flash. So if I go in here notice this is at 2.3 feet at this value, if I increase my I s o. So I'm gonna hit my eyes. So I'm going to increase this to, let's say, 800. Now, look, that just jumped up to seven feet because my sensitivity is much higher. And so this in manual mode, with my flash on my camera, this will tell me how far away my flash should be for my subject to get a proper exposure. What if my subject is 20 feet away? Well, I could just use this to push my exposure. Put that up to say 32. Push my meter halfway. Look at that 20 feet. I just got lucky. So if I actually did that little bit too far, eighth of full power, that's 30 feet. That's too much light. Dial that down. You can see that moving as I adjust my power. And so that is the equivalent of a guide number calculator.

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