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Lesson Info

Flash Fundamentals

So I know at this point we've been up to a fairly comfortable point for a lot of people which is natural lighting, and then once you start adding in a flash, it's something that I don't do a lot of myself. I've had great interest in the past and I've used it in many many situations, and I do have to admit that when you add flash into everything else that we've been talking about, it's really the most complex area of photography is when you start mixing flash and natural light, amongst everything else. So we're gonna take it one step at a time. First off, let's talk about the way that artificial flash photography works. Now back in the old days, we had these box cameras and they have lights where we could fire the flash, and the light was pretty simple: it was either off or it was on (chuckles). It would fire this flash, and it would strike our subject, and we would record that light. If it was either too powerful or not powerful enough, well, we could move the camera forward or back, o...

r we could move our subject forward or back. That was the range of what we could do, which was a much simpler time I think. Well the other thing that we could do is we could adjust the aperture setting and that would adjust essentially the power of the flash or how much light we're recording. So there was very few limitations as far as what we could do or there were quite a few limitations. Next up was we had flashes where we could control the power. We could say, "Oh, let's put it at 1/2 power and 1/4 power," and so we'd have that addition to just simply moving back and forth and the aperture itself. Then flashes and cameras developed automatic flash. This is where flash would figure things out for you. The way it did this is with a light sensor and they put this light sensor in the flash unit itself. So when you took a photograph of a subject what would happen is light would go out from the flash, it would hit the subject, it would bounce back to the light sensor, and the light sensor would have to make a determination of was that the right amount of light, if it wasn't, let's send some more light. Now it actually does this at the speed of light, so quickly. So if we could slow down time really slowly what's happening is it sends light out, it senses it, and it's still sending light out at the time, and it kinda tells itself when to turn off in that situation. So it worked out okay, and a lot of flashes still these days have a simple automatic all-built-into-the-flash automatic setting, but they found that it doesn't work real well in a lot of situations. For instance if you're shooting with a very telephoto lens and flash fires this big area and just a little bit of a light from your subject bounces back, it's gonna say, "Ah, it's not bright enough," so it fires off more flash. What it ends up doing is your subject gets being horribly overexposed because it wasn't a predominant thing in the frame. It wasn't filming in the frame, essentially. So the next development in flash was known as TTL flash, and it's what's used today. So now what happens is, rather than measuring the light at the flash level, it measures it through the lens. Now the way that modern cameras do this is it sends out a test flash, and then checks the exposure, and asks whether that seemed like the proper exposure, and then it fires the real flash, and it does this so quickly, we can't even see that there was two flashes, and so it's a very very fast process that it's doing this, so that's how TTL flash works. In some situations TTL flash is a great system to use. If you are at a social event, and you're going around taking pictures of individuals and people at tables and groups of people and every situation is a little bit different and you wanna get through and take some (snaps) decent exposures very quickly, TTL flash is a very good system to use. If you're setting up a scene and you're putting the tripod, and your lights here and there, and you want things very precise, and you have control over the the situation, TTL flash is a disaster because it changes every time. Like I just moved the box over here and it changed the reflection of light, and now it changed the power of the light. You want manual control in that case. So there is a pretty quick change that photographers make from shooting TTL to just get me over to manual where I have full control, and it kind of is the same philosophy as shooting manual exposure. When you know you have a little bit of time and you wanna have full control of exactly the settings, that's when you wanna go to manual exposure. But the beauty of manual exposure and flash is that it's the exact (snaps) same, every single time, and you don't have to worry about that variable changing. You can worry about composition, timing, and all the other things involved in photography. When we talk about flash, we have talk about the inverse square law, and that basically states that light traveling twice the distance has 1/4 the power which is something we talk about, light fall-off. Light is much more powerful close to the light source and much weaker as we get further away. If we illuminate a square area and then we ask the flash to fire twice as far, what happens? Well it's twice the width and it's twice the height, which means it's four times the area, and that's why it is 1/4 of the power that it was at half the distance, and so it falls off in power quite a bit when you do that, so twice the distance has 1/4 the power. What that means when we are setting our camera up and the apertures and so forth is that when we have an object that moves this distance, we're gonna need a different aperture setting. If it's relatively close to the camera, we might be at 5.6, but we're gonna need to open up to 2. when it moves twice the distance. It's interesting to see how much light falls off. Back when I was shooting the time-lapse at Mount Hood, I was playing around with my friend at flash and if you notice this flash, this is a normal flash, and it illuminates everything right in front, and then it just falls off to absolutely nothing in no time flat. This is one of the most important things to know about anyone who's first getting into flash photography is it's good for the things that are right there in front of you, but anything that gets further away, it just doesn't have much impact at all. So there's a very narrow band in this photograph that is properly illuminated by flash. It's just there's little area, about a one-foot wide area around where that flash is on the ice that's properly illuminated. So yes, you can illuminate the penguins right there in front of you, but it's not gonna do anything to the penguins in the background or the mountains way way in the background. When we talk about flash we need to talk about how our shutters work. Now this I have the feeling is going to change over the next five years, but for right now, this is the way it works. Most all of our cameras have focal plane shutters that we talked about before that have two curtains. We have the first curtain that's gonna open up, and then the second curtain that's gonna close. When we add flash into the mix what's gonna happen is the flash is gonna fire, but it has to fire when it can illuminate the entire sensor. So let's open up our first curtain, there's our exposure, the flash fires, and then the second curtain is gonna close, and so when we go to faster shutter speeds, it goes to a scanning process. If we fire a flash at that time, we're gonna have a problem. So when we go to really fast shutter speeds, it just becomes an even smaller slit in that case. So the shutter speeds, as I said, it's a terrible name. The speed of the shutter doesn't change. It changes the timing of the first versus the second one. So when it comes to flash synchronization, it usually triggers (snaps) right when the first shutter has completely cleared the sensor and it's totally open, and then the second one, curtain will close, depending on what your shutter speed is set. So with a high-speed shutter, if you were to, I know you used to be able to make this mistake, if you set 1000th of a second on your camera what would happen is the flash would fire as soon as the first curtain was fully clear of the sensor, but on 1000th of a second, that second curtain is already starting to come down and close it off, and the photograph that you would end up would look like this. Those were always very easy photographs to diagnose in that, oh, 1/3 of the frame is illuminated, flash fired, you had your camera at too fast a shutter speed. Nowadays, they generally won't allow you on a camera to do this, unless you're hooking up your cameras manually and the camera doesn't know what the flash is doing. And so when flash fires, it's interesting to understand how it fires. If we were to measure it over time, a 30th of a second, relatively a short period of time, and to understand the light intensity, what happens when a flash fires with a relatively low-powered flash is it fires a quick burst of light and then it does nothing. The flash happens in a flash. It's that quick. Now if we crank up the power on the flash a little bit and we have medium power, well, it's almost the exact same thing, it just lasts a little bit longer is the difference between it. If we were to turn it up to really high-power, it's not that the flash is that much more powerful, but it fires for a longer period of time, so that's the difference between a low-powered flash and a high-powered flash within a single system. Now as you go from one system to the next, there's gonna be some variances, but on one particular flash system, that's what's happening when you change it from a low power to a high power is it just takes longer and it's there for a longer period of time. So if you wanna fire your shutter speed at a fairly short time span, like 1/25th, or 1/250th of a second, the entire flash has time to fire in that shutter speed. So you can fire a flash and using fast shutter speed at the same time, but there's gonna be a limit as to how far you can go. With most cameras, it's gonna be somewhere in the 125 to 250 range. It varies a little bit from camera to camera and the shutter systems that they have, and so your shutter speed doesn't affect your flash power. You can set this at 250th of a second, or a 30th of a second, or 30 seconds, the flash ends up being the exact same power. The flash will be controlled by the power itself, and so flash duration is gonna effect the flash power. Slight little diversion here into the red-eye effect, so this has been a problem in the past with people who have built-in flash on their cameras. So you're gonna get this red-eye effect when the flash and the lens are very close together and it happens for this reason: when the flash fires, it illuminates the retina in the back of the eye, and then if the camera's viewing angle, where it's lens is, is close to it, you can actually see this in the lens and this is what's causing the red eye in the pupil. Now there's a couple different ways of getting rid of this. One way is the flashes that fire a burst of light, and that burst of light causes the pupil to constrict, and now there is a smaller opening for which to see the retina in the back of the eye. Now this is also one of the more annoying ways to shoot a photograph because you shine a bright light into your subject's eyes that has nothing to do with lighting, it's just reducing the red eye. A much better system of reducing red eye is to simply get the flash off the camera so that you can't see the retina in the back of the eye being illuminated by it. This is why you don't have the problem under normal daylight conditions it's 'cause you are not the source of the light in those situations and why you don't have that problem in studios is they're not shooting with the light directly behind them, in that case.