Flash 101 (Technical Bits)

What is flash?

It might seem obvious, but before we delve too much into how to use flash, we first of all need to address exactly what flash is. Fundamentally flash is an additional light source that a photographer adds to a scene. Unlike a continuous/hot light source, like a lamp or light in your house, flash is a strobe light. It is a single burst of light that exists for only a short period of time.

Shutter speed and flash sync speed

The concept of flash being a strobe is extremely important to bear in mind, because it has a large effect on our choice of shutter speed and aperture. The first thing we are going to look at is shutter speed and indeed just how a shutter works. Most of our cameras today use a focal plane shutter, called such because the shutter resides just in front of the focal plane (sensor/film). The shutter actually consists of two curtains (front and rear) that move horizontally across the focal plane.

In “rest” position, the front curtain sits in front of the focal plane, blocking all light from reaching the sensor/film. When you depress your shutter release, the front curtain moves across the film plan, exposing the sensor to light and after a fixed time, which is your desired shutter speed, the rear curtain now moves across the focal plane, once again blocking all light from reaching the sensor. At this point, the exposure is complete and both curtains reset to the rest position.

So, for a fixed period of time, when the front curtain is open and the rear curtain hasn’t yet closed, the sensor is exposed to the light coming through the lens. So far, this is exactly what happens when shooting using available light. But how does this tie into using flash?

Well, one thing we have to ensure is that when the flash actually does fire, that the sensor at that time is exposed. So the front curtain must be open and the rear curtain must not start to close before the flash has fired. If the flash fires as either of the curtains are opening or closing, or indeed if the exposure is finished (rear current closed) then the flash will be out of synchronisation with the shutters and the exposure will be affected. This affect can be anything from a visible line across your picture, one half exposed correctly (the part exposed to the flash) and the other underexposed, to underexposure of the complete scene.

As such, your camera will have a maximum speed at which it can synchronise to your flash, called the flash sync speed. For most cameras with mechanical focal plane shutters this sync speed is usually around 1/250. Older cameras sync at slower speeds (1/125) and indeed the very high end cameras or cameras with electronic shutters, can sync at speeds of 1/500 and above.

Effect of shutter speed on flash

Ok, so we now know there is an upper limit of shutter speed that can be used with flash, but is there a lower? Well let us imagine we are in a completely dark room, with no available light what so ever. Without using flash or any other light source, it is impossible to obtain an exposure. If we left the shutters open for a second, minute, hour, day, week or even year, the shot would still be black as there is no ambient light.

Now let’s introduce flash into the scenario. Remember flash only happens for a small fraction of a second. So if I had the shutter open for a second, a minute, an hour, day or week, it makes no difference at all. The whole exposure happens for the instant that the strobe existed. For the remainder of the time, no extra ambient light registers in the exposure. The point being, that the shutter speed has no effect what so ever on the exposure, assuming its slower than the max sync speed of course.

This is worth re-enforcing again. Shutter speed, assuming it’s slower than the flash sync speed, has little or no effect on exposure!! Ok, this does assume a dark room with no extra ambient light, but for explanation purposes I think this is OK to assume for the moment. So if I shot at 1/125 or 1/250 or indeed 1 second, 2 seconds or even 30 seconds, the exposures will be identical!!

But, with all that said, how often do we shoot in conditions with no ambient light? I would guess the answer is not often. So in fact, shutter speed can have an effect on exposure. If we would like ambient light to register as well as flash, we can use slower shutter speeds, or where we don’t want natural light to register, we us higher shutter speeds.

High Speed Sync

Imagine we are outdoors on an exceptionally bright day and for some reason, we need or want to use flash. Can you see a potential issue? Well the problem is this; if it’s very bright out, we need to either use much faster shutter speeds and/or stop down the lens so as to limit the amount of light through the lens. The problem is that we have a maximum speed in which we can control the normal operation of the focal plane shutter. It’s just not possible at the moment to have shutters that have a full open/close operation at the fastest shutter speeds of today’s cameras (1/8000).

So, rather than the front shutter opening the whole way before the rear shutter closing, the rear curtain follows closely behind the front curtain. So at any point in time, only one portion of the sensor is exposed. This has obvious impacts for flash. For the single burst of flash, at that point in time, only one portion of the scene is exposed, so only one portion will be subjected to flash. This is a flash sync problem!

Modern flashes, overcome this problem by using a high speed sync mode. Rather than firing just once during an exposure, the flash will fire multiple times as both curtains move across the sensor. The multiple strobes are imperceptible to the human eye and look like a single flash, but they do occur. So the advantage of high speed sync mode is that you can use far higher shutter speeds in conjunction with your flash, but it does come at the expense of a loss in the output power.

Flash Power, Guide Numbers and the Inverse Square Law

How much light does a flash produce? Hopefully if you’ve read this far, you’ll have asked yourself this question by now and it seems like the right time to address this. A flash is what is considered a point light source. Anything close to the flash is exposed to more light than anything further away. Again, this is a very important concept. Flash can only light once distance correctly. If you’re closer to the flash than this distance, but still in shot, then you’re going to be over exposed. If you’re further away than this “correct” distance, then you’ll be underexposed. This is a really common error that so many photographers make. They will have different subjects at different distances from the camera/focal plane and then use flash. It is impossible to light them all evenly.

Inverse Square Law

So how dramatic is the difference? Well believe it or not it can be anything from a huge difference right through to a very subtle difference and it all depends on how far your subjects are from the flash. There is a basic law in physics, called the inverse square law. This states that the strength of light is inversely proportional to the square of the distance from the light source to the subject. A bit of a mouthful I know! Read back over it again.

In simple terms, if you move a subject twice the distance from the light source, it will receive a quarter of the light! If I move it 3 times the distance, it will receive 1/9th of the light. This is a dramatic fall off and one you need to be aware of when using flash, or indeed any other point light source.

Guide Numbers

So, we know the light gets less powerful the further we are from the light source. One way to compensate for this is to open up our aperture, so as to allow more light in. Another approach could be to increase our ISO. A final option is to increase the output power of the flash. But obviously how much we can boost the power is related to how big and powerful the flash is. As the effective range of the flash varies with the camera aperture and the film speed, both of these factors must be taken into account when calculating the distance at which the subject will be adequately illuminated.

• Guide Number = Flash Range * Aperture
• Aperture = Guide Number / Flash Range
• Flash Range = Guide Number / Aperture

So at any fixed aperture, the larger the guide number of the flash, the larger the range of the flash. When we introduce ISO into the equation, it does get a little bit more complicated. Increasing ISO effectively increases the power/GN of the flash. At each ISO, we multiply the GN by a compensation factor, which is the square root of the ratio of the new ISO to the original ISO 100.

So at ISO100, our compensation factor is 1. At ISO200, we now have a ratio of 200:100, or effectively 2:1. The square root of 2 is 1.4; hence the compensation factor is 1.4. If we take my SB800 as an example, who’s normal GN is 125 @ ISO100, it now becomes 175 @ ISO200. If we were to go to ISO400, we now have a 4:1 ratio, so the compensation factor is now the square root of 4, which is 2. Hence the GN goes from 125 to 250. Basically, because of the inverse square law, you need two ISO increments to double the guide number.

• ISO100 – Factor = 1
• ISO200 – Factor = 1.4
• ISO400 – Factor = 2
• ISO800 – Factor = 2.8

In practical terms, this means that with a higher ISO, even the smallest amount of flash can help illuminate and lift a scene, because effectively your flash is considerably more powerful.

OK, so if you have read this far, now I think it is time to move on to the practical part of this tutorial.