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Guide Numbers

One of the big questions before we started this workshop when we announced it that was, are you going to talk about guide numbers, what's a guide number, and so that we're going to talk about guide numbers and then as promised, I'm going to show you a secret built in light meter that some speed lights have, and we're going to use a canon camera to trigger a nikon flash, and it will meet her everything correctly, and they were going to use a nikon camera to trigger a cannon flash, and it will mean your everything correctly, so we're about to do that. First, we want to camp out on the guy numbers and explain what guide numbers are and guide numbers were used back in the day before flashes had read outs and tt almeida ring 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 reall...

y don't, but let me explain what it is, and then I'll explain why, well, why they're useful 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 an s o value usually I s o one hundred for example, if we have a good number of eighty that means at twenty feet at f point for you'd get a proper exposure because twenty times four equals eighty okay, so if you want to do math twenty times four equals eighty there's your guide number distance multiplied by your aperture value at this s o that's what you've got numbers so that means is if you have a good number of eighty on your flash and you have a different flash has a guide number of one hundred that means the flash that has a guide number of one hundred is more powerful the higher the guide number the more powerful it is because if you're at one hundred you could be a twenty five feet away twenty five times four is one hundred right and so that's that's how good numbers work is it's not rocket science it's just this little this times this equals this and that's what it is if you increase your eye so did two hundred then this would change to a different distance but that's how good numbers are or you could say a good number of eighty that f eight you could be a ten feet because ten times eight is eighty and so you could just do all kinds of math here so you could say is well my guide number at I saw one hundred eighty I want to shoot it five feet what should my aperture value be? And you could do the math and figure that out? I'm not smart enough to figure that math out, but you could figure out what your aperture value is doing that now a lot of older flashes have like this little slide rule kind of thing on the side where you khun set the 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 in that little calculator on, so if you look at older flashes, you can see that there's this sort of dia lee 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 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, then you don't maybe you don't trust the tomorrow levin look through the eyepiece method of meeting 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 cannon people, you don't okay, so how this works is if you hit mode on your nikon speed light and again, not all mike on speed lights have this there's a mode that says g in that stands for guide number so not all nikon flashes have this, but the nine hundred nine tens do and here's what you can dio remember when we talked about media ring for light with a light meter, we had to put in to values and our meter told us a third notice? This is the same thing we're putting into values to find the third, so it works the same. So what we're going to 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 could see it, I'm going to put in f seven point one, okay, that's telling me that I can shoot at fifteen feet away and I'll get a good exposure. If I take this change, my aperture value down to four point five, four point that fifteen feet that still gonna work, so 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 four point six feet and I'm going to get a good exposure and this little scale here says that f four this flashes going to expose correctly from about three feet all the way up to about thirty five feet at and I esso value of four hundred if I go in here and change the I s o value, which I can do by going into the menu on it okay and hold it, go down to the menu where I go to my isil wouldn't change this to eight hundred so now our light is more sensitive to light now look, we can go all the way up to about sixty feet, so we just increase the range of this flash sorry guide number calculator is just sort of doing all the math for us telling us how far away we can shoot in adjusting the power of the slash show that is a way that you can adjust your flash if you have that built in so you can put in the ice so the aperture value and then just put it to the distance if you have a tape measure or something where you can sort of get it close right and you'll get a good exposure ok, so that is something that's available if you don't have a light meter and you have a nikon flash or there's a lot of flashes actually that have a guide number a calculator on it. If you have a cannon flash, you can actually buy a little portable guide number calculator iphone, aps have them etcetera. So that's one way to do that so good numbers are really cool. So on a cannon flashing, we 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 flash here at the very bottom of the flash, I'm in manual mode, by the way, you see this this range right here from one point seven all the way up to sixty feet. When I push my show to release half way, you'll see that the little dot shows up that says, this will work at two point three feet away, so it's roughly the equivalent of a guide number meter telling us what the distances 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 two point three feet at this value if I increase my eyes so so I'm going to hit my eyes so I'm going to increase this to let's say, eight hundred 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 twenty feet away? Well, I could just use this to push my exposure, going to put that up to, say thirty to push my meter halfway look at that twenty feet. I just got lucky. So if I actually did that little bit too far eights of full power that's thirty 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. Now watch what happens when you take this off my flash off my camera now I have no scale, it goes away. Why does it go away? Because this doesn't allow you to input the aperture and s o so it doesn't have to parts of the equation to calculate the third it goes away, okay, so that and a nutshell are is a guide number it's not something that I really use but it's something that's available it's not super character. A complex but that's how it is. We have questions about the gun. Yeah, we do have some questions about the guide number. First of all, who knew, you know, know, about the good numbers of question is from pablo? Does the guide number assume that you're using one hundred percent of flash power? It does? Yeah, unless you're using the guide number of calculator. So what it does on the calculator that we have built in you can put in the ice so in a range, and it will adjust. So that's the benefit of that yeah, assumes that you're at full power. I have a question from it. Tore bond. What is the relationship between the guide number and what seconds? Oh, that is the awesomest question ever. So what seconds measure roughly the power potential of a flash it's usually used for studio lights. Um, I actually have a video on this because this is have fairy. Uh, this is a question that people ask all the time. How many watts seconds do I need to get x amount of power from my flash? Let me tell you what what seconds are inside of all flashes, speed, lights, studio strobes it doesn't matter there's a thing called a capacitor but that capacitor does is it stores energy it's like a gas tank in a car it just stores the energy when a car it's gas in the flash its power when you tell your flash to fire that capacitor all at once it sends the energy to the flash tube through some circuits and the flash fires and you get flash okay in a car when you hit the gas pedal, the gas goes through the fuel filter and engine and fuel injectors and all that kind of stuff and it's ignited by the spark plug and explodes and then you get power from your car. So the reason I'm talking about the car is that what seconds don't equate to power in the same way as gasoline doesn't equate to how many miles a car will go so you could have a car that holds twenty gallons of gas and you could have a truck that holds twenty gallons of gas and if you have a prius it's going to go a lot farther than your ford f one fifty all right? So it's the wide seconds measures the potential the power that's held. So there are all kinds of things that happened between that capacitor and the flash tube including the flash tube itself and so lights that are more expensive half mohr efficient circuitry and flash tubes and so there's something that's happening that energy because we know physics we know that energy can't be destroyed or created it's just transferred to something and so when that energy comes from the batteries into the capacitor and then you say go that energy has to go somewhere and so normally that energy is either translated into light or heat and if your flash isn't very efficient you'll get more going to heat then you will tow light I'm getting way scientific, but the point is lott seconds measure the power potential ah guide number is a little bit more accurate because it's actually they've measured at this distance that this aperture setting at this s o you're going to get a proper exposure and so it should be consistent time to time so a guide number we'll show you exactly how many feet you can get um out of your flash if you just do the math, you can figure out how many feet which aperture they're going to get so lot seconds really measure power potential. So how many watch seconds are these little flashes? I think there are around fifty one seconds around there may be a little bit more or less um I believe that's about right? It might be a little bit higher than that um so there isn't a direct correlation between except to say that what seconds shows the power potential guide number is how many feet you'll get using this equation depending on the aperture value okay, it's sort of inverse square live when you start getting into that stuff um and so you know, don't worry too much about that just know that I hired the guide number is the more power you gonna get out of your flash? The higher the watts seconds our mohr flash, the more power you're going to get out of your flesh cool a question from photography by mario does this formula the good numbers is that work for outdoor portrait ce yes, outdoor indoor doesn't matter um, so but what this doesn't account for is you might want to shoot it ten feet of f ate that I so one hundred and you're thinking you're going to get a proper exposure, but what if you have a ton of ambient light? It might not you might not be too over ready to overpower this son, so this assumes that you're just measuring, like from the flash that's what that assumes I'm gary green from scottsdale and blew our images both have questions about zoom okay blew our images asked what impact a zoom factor have with the guide number calculated calculations and give the green asked, does the guide number assume a particular zoom setting on flash? It does, and here's where manufacturers if you start digging in on google on guide numbers, what you find is there is no specific zoom that flash manufacturers used to establish your guide numbers, and so sometimes there are arguments you know, coke versus pepsi, dodge versus forward, et cetera about flashes because you might have brand a that uses zoom of two hundred millimeters, which is going to really give you a lot of power but not very widespread, and brandy uses a zoom of thirty five millimeters, which gives you a really broad spread of light but not so much power, and so one of them might be more powerful than the other. But they're measuring apples to oranges and so it's sort of, um, they don't always tell you what zoom they used to determine their guide numbers and so that's why god numbers aren't sort of like what seconds there's sort of a ballpark figure that you can look at s so if you're looking at a nikon and checking the guide numbers against other nikon flashes, that's going to be a good representation of the relative powers of those flashes or mike cannon to cannon or quantum to quantum it's when you start crossing over to different brands, looks like I'm swimming different brands that you might start seeing some apples to oranges, stuff and it's not published anywhere what zoom is. And so you sort of have to one of the things I always recommend your if you're close enough to a great rental company, then rent the gear first, try it for a weekend, see how it works and then spend a few hundred dollars because you might spend twenty five bucks to rent something and find out uh, not for me. Great. Another question from daphne from israel is can I use a cable for off camera flash if I use a cable for off camera fashionable that show the guide number calculation that depends on what kind of cables there are two kinds of cable one is a cable that sort of expensive and on the top of your camera used this night concert to get close to the the camera on top of your hot shoe it's called hot shoe because there are a bunch of different pens on here that send and receive information to and from your flash so there's communication coming from the camera and to the camera. Ok, so there are cables that actually have all of those different. They have pins and cables that conduct all that electricity you're getting the full benefit and so essentially you're flash still thinks it's on your camera so you'll get you know that that same thing, so normally those cables cost eighty dollars around there if you're just using a sink cable then no so it has to be the the hot shoe adapter flash that conducts all the electricity and forgot what those cables were called but you'll know because they're not cheap all righty should we ask one more let's go one more and then I want to show the secret stuff it's really cool. Okay, I am scar as shooting in a studio. What type of scenario would you need to have one thousand watts per second strobes or higher? What? Okay, um it's not uncommon. So if you're shooting for example with the seven foot parabolic umbrella at a distance of fifteen feet to get a large group shot you need power. He needed one thousand one seconds of power maybe even more than that for shooting a car. If you're using there are soft boxes that are thirty feet by twenty feet huge for shooting, no commercial stuff. You need lots of power if you're shooting architectural stuff sides of buildings if you're need to shoot with a medium format camera that has that aperture are yeah picture of maybe thirty two and an ice over twenty five you need lots of light so that's you know something that not every person is going to need but there are a lot of applications we need lots of light we shoot a lot with four by six foot soft boxes, so there bigger than me, physically on, and then they have multiple baffles, and we put grids on those things, and we just get this. Wow, amazing light, it's, like you used those you're like, huh? But you have to have a lot of juice to illuminate that all evenly. So those are the kind of situations that you wouldn't. I want one of those likes, yeah, look at stuff like what, uh, studio work, like any liebowitz does, um, or jason jarvis, that kind of stuff where you're using lots of juice to illuminate things. Very specifically, you'll see that, or goto anyone of our videos, and you'll see us using twenty, twenty, four hundred one second lights, or twelve hundred seconds. It's. Quite a bit, actually.