by Ali on August 29, 2007, 05:58:00 pmBy Ciaran Whyte ( www.thewonderoflight.com)
Flash is probably one of the most powerful and versatile tools available to the photographer. After high quality optics, I would stipulate that a flash gun be the next most important purchase any photographer should make.
Without a flash, a photographer has to work with available light. This in itself isn't necessarily a bad thing, but available light can limit our exposure and indeed creative possibilities. With the introduction of flash, the photographer is now in complete control of the lighting within the scene, which opens up the number of creative choices available to us.
But what exactly is flash and how do we use it effectively? Unfortunately with the additional creative possibilities that flash gives us, also comes technical difficulties. In order to produce pleasing results with flash, we need to understand what exactly it is, how it works, how flash affects exposure, when and how it's used and how to control it. So if you're new to flash, there is a learning curve, sometimes a slow frustrating one. But stick with it! Once you've grasped the concepts, it won't be long before you've mastered the use of flash. Hopefully these tutorials will go some way to assisting you in understanding flash photography.
By the way, this tutorial already assumes you have a good grasp of basic exposure. If you haven't then I suggest you read through my exposure tutorial here, or other similar material online.
Rather than bore everyone with everything in this tutorial, I've split it into a few sections. The first section deals with the technical bits relating to flash and the second second section deals more so with the practical elements. The remaining sections looks at real examples of flash in use. A full index of what is included in the tutorial is also listed below.
Flash - The Technical Bits
What is flash?
Shutter speed and flash sync speed
Effect of shutter speed on flash
High Speed Sync
Flash Power, Guide Numbers and the Inverse Square Law
Inverse Square Law
Guide Numbers
Flash - The Practical Bits
A Flash Gun - Direct Versus Bounced.
Direct flash
Bounced flash
Diffused flash
Fill Flash - Back Lighting
Fill Flash - On Camera Flash - Direct, Bounced and Diffused
Off Camera Flash
Fill Flash - Off Camera Flash
Overpowering ambient light with flash
Fill Flash - Side Lighting
Flash - The 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.
Flash - The Practical Bits
At this point, I assume you're either familiar with the technical bits, or simply don't care. The more you know about flash, the easier it is to use it, but leaving that aside, the next thing we need to address is why we use flash. There are so many uses for flash and once you start using it, you're really only limited by your own creativity, the number of flash units you use and the power of them. Keeping things simple for the moment, let's just look at a single flash unit and its' uses.
Primary source of illumination
My guess is that most people use flash in this mode, most of the time. This is where the shooting situation is dark, so there's not a lot of ambient light, or certainly not enough for a useful exposure. So we use flash to light the scene and subjects.
As a fill light, reducing contrast
Another very typical use of flash is when there is a lot of contrast created by the existing available light. This can happen on very sunny days, where the part of the scene in the light is quite bright and the part in the shade is substantially darker. There is a large dynamic range in the scene and it contains a lot of contrast. In this case, we can use the flash to light the shadows and reduce contrast.
Another example of this, is where a subject maybe backlit and in silhouette against the light source. By using flash, we can illuminate the silhouette and light the subject so as it is now longer a shadow against the bright background.
This is what is referred to as fill - a secondary light source used to balance light in a scene.
Squirt/Wink flash
A small squirt of flash at minimum power is a technique commonly used in portraiture. It adds a small amount of light to the shadow regions, reducing dark shadows under the eyes and more importantly adding catch lights to the subject's eyes.
A Flash Gun - Direct Versus Bounced.
Most cameras today come with a small in built flash that sits above just above the lens. Personally, I think this flash is more or less useless with only very limited power and as its position is fixed, it offers very little in the way of creative possibilities.
In reality, if you really want to be creative with flash you'll need to buy a separate flash gun that sits into your camera's hot shoe (some people call this a hot shoe flash). Ideally this flash gun should have a head that both tilts and rotates; allowing you to bounce the flash off various surfaces rather than always having it fire directly at your subject.
Direct flash
Direct flash is what most people, without a hot shoe flash, are pretty much stuck with. The flash sits directly above the lens, in parallel to, indeed in very close proximity to, the focal plane. The result of this flash is a very harsh, unflattering, flattening light. As the flash is directed towards the subject, there will be no shadows on the subject itself, leading to it looking flat, rather than 3 dimensional. But shadows are cast and unfortunately they are cast behind the subject, which is generally unpleasing to the eye. One other disadvantage of direct flash is the horrible effect of red-eye. As the flash is more or less in line with the focal plane, the camera picks up the light hitting the back of a persons' retina, which leads to red eye.
Bounced flash
With a hot shoe flash, we can tilt the flash head upwards or indeed sideways so as to bounce the flash. This has the effect of softening the light falling on the subject that comes from the flash. Another significant advantage is that rather than having the harsh shadows behind subjects, caused by direct flash, we can now throw the shadow out of shot, below and behind the subject or indeed to the side, depending on the direction of the bouncing. One thing to note here is that the intensity of light hitting the subject has been reduced as the light now has to travel a longer distance before reaching the subject.
Diffused flash
One final trick we can do is to diffuse the light coming off the flash, by using a diffusion dome. This diffuses and softens the light even more and when used in conjunction with bounced flash, can create very soft lighting with virtually no shadows.
Fill Flash - Back Lighting
Meet Teddy. Teddy came around for a portrait session and I thought it would be wonderful to shoot him against a window, using it as a backdrop. Putting my camera in Matrix (evaluative) metering mode and on aperture priority, I dial in my required aperture and let the camera take care of the rest.
As you can see from the scene, there is a large dynamic range in the shot. The camera has done it's best to not blow out too much of the window detail, but because of this the teddy is too much in shadow. The camera tried to more or less average the exposure between the window and the subject.
One way of "fixing" this, is to put my camera into spot metering mode and meter off Teddy's face. When I take the shot now, Teddy is perfectly exposed. But in order for the camera to lift Teddy from the shadows, my exposure choice has dramatically blown out the window detail behind him. The camera just doesn't have the dynamic range to expose both objects correctly!
The lighting setup for two shots above is shown below.
This is where fill flash comes in. With fill flash, we should be able to add light to the scene, lifting teddy from the shadows whilst still retaining detail in the background. Effectively what I want to do is meter for the background and then use my flash to light the foreground, balancing both light sources to each other.
Fill Flash - On Camera Flash - Direct, Bounced and Diffused
The simplest way to use fill flash is simply put my camera into aperture priority mode, switch it into matrix metering and my SB800 flash into TTL mode (in fact it should switch to TTL-BL). This is a fully automatic mode, where the camera in conjunction with the flash makes all the decisions relating to exposure and flash power. The above two modes combine to give you a balanced fill flash, so both camera and flash try to work together to maintain the exposure in the background and use the flash to light the subject, which is what we want
The exposure here is obviously much better. Teddy has been lifted out of the shadows and the background detail has been retained. Exactly what we wanted! This is a super result from a pure automatic mode, making it almost fool proof. For me however, there are some problems with this shot.
The flash is directed straight at Teddy, so he's a little flat and washed out, with no shadows. There is also a catch light directly in the centre of the eye, which is unappealing. A very simple fix to this is to use bounced flash. By tilting the head upwards, approximately 50 degrees, I can bounce the flash off my ceiling and light teddy using this softer light. So leaving the camera on the exact same settings, where it is making most of the creative decisions, I simply tilt the head upwards and click the shutter.
Exposure wise, both scenes are more or less the same. However, the second shot is far more appealing. There is modeling/shadow on the face and it no longer looks flat and washed out. We've also lost the harsh centered catch light in the centre of the eye.
When shooting outdoors, I sometimes hear photographers say that it's just not possible to use bounced flash. Whilst I don't necessarily subscribe to this idea, I do agree that it can be difficult at times to find surfaces which can be used to reflect the bounced flash. Personally I always carry a hand held reflector with me… but I digress! So, let's imagine that it's simply not possible to bounce the flash and we are forced to shoot it directly. In this case, a diffusion dome comes in extremely useful. As discussed already, a diffusion dome softens the light considerably and is perfect for situations such as this.
In fact, we can combine the diffusion dome and the bounced flash methodology to give us even softer pleasing shadows.
Hopefully you'll agree that from a lighting perspective, this is probably the most pleasing? The shadows are soft, the exposure is good and the lighting is balanced between window light and flash. All positive points!
Just so we're clear on the lighting setup for the last four portraits of Teddy, I've enclosed a lighting diagram below. In all cases Teddy was back lit by the window and lit by an on camera flash. However, the flash has been used direct, bounced, diffused and different combinations of the three.
Off Camera Flash
So far, the flash examples I've shown have all been with the flash mounted directly on the cameras hot shoe. However, to really push the boat out creatively, we really need to mount the flash off camera in a different position. This opens up the number of creative possibilities but also adds an additional level of complexity. For one, we need to find some way of firing the flash when we depress the shutter. A simple way is a PC sync cord, which goes into your camera's sync port and connects directly into your flashes sync port. This obviously assumes both camera and flash have one, which is not always the case. With this method, apart from a signal to fire the flash, there is no other transfer between camera and flash, so you can no longer use a lot of the automatic modes.
Another solution, is for a TTL hot shoe adapter, which effectively extends your hot shoe, by attaching to the hot shoe on your camera and then to the bottom of your flash via a cable. With this method, the camera and flash still have the ability to talk to each other, so auto exposure modes can be used successfully.
Some flash systems use wireless flash modes, like the CLS (creative lighting system) offered by Nikon. With this method, you can use one flash, to control and fire the other flashes. This system can get pretty advanced and very complex, very quick. You can also buy third party infrared triggers which can be used to communicate between camera and flash. Again, the draw back of these is that like the sync cord, they are used simply to fire the flash. No data transfers are sent to and from the flash/camera, so no automatic modes can be used. The flash power has to be controlled manually.
A final mode, the one that I used and prefer is a wireless radio trigger. I personally use Pocket Wizards, but there are cheaper, alternative third-party brands. One pocket wizard sits on my cameras hot shoe and the other plugs into my flashes sync port. With these I can place my flash anywhere at all and fire it from my camera.
Fill Flash - Off Camera Flash
With the flash positioned to the camera's right (subjects left), I now have one issue relating to how I control the output power of the flash. The only solution for this is to use a hand held light meter, which can meter both flash and ambient light. So I now switch both my camera and flash into manual mode, where I have complete control over shutter speed, aperture and flash output power.
For the shot below, I used the hand held meter to meter for the ambient light in the background and dial in the correct aperture and shutter speed. The flash at this point is still turned off.
We are more or less back at the beginning, in that Teddy is underexposed. However in this case the exposure, rather than trying to average between the back light and Teddy, it is based entirely on the window, so Teddy is completely underexposed.
At this point we now introduce flash. As everything is manual, I'm now able to decide exactly how much flash power to use, depending on what ratio I want between the window light and flash. For the moment, let's look at a 1:1 ratio. I turn on the flash, fire it and take a meter reading. Based on the flash reading and the previous ambient light reading, I calculate the difference in stops between flash and ambient. I then dial down or up the power on the flash accordingly, so I have the same flash intensity as I do ambient light.
Obviously as the camera is off to the side, we now have a lit side of the face and a shadow side. This ratio of light and shadow can be used to produce many different effects and lighting styles (broad lighting, short lighting, Rembrandt etc.) which I won't be discussing just for the moment. But one thing that is very noticeable is how harsh the flash is. Can you guess why? Yes, it's because the flash is directly pointed at Teddy albeit, off to one side.
A simple fix for this is to bounce the flash. In the last shot, I bounced the flash and used the diffuser dome to soften the shadows even more. The more astute of you may notice that in this shot, Teddy looks a little pinker than he has in previous shots. The main reason for this is that the wall in my house is Red and as the flash bounced off that, it changed the colour of the light that was cast.
Overpowering ambient light with flash
Using the same setup as the previous off camera flash shots, I can try something different with Teddy. Rather than having a 1:1 ratio between ambient light and flash, I can also change this ratio. For this shot, I meter once again for the background, but rather than dialing the correct values for aperture and shutter speed into my camera, I decide to underexpose the scene by closing down my aperture and stopping down by two stops. I adjust the power of my flash to match this new exposure and take the shot.
The flash is now the main source of illumination and I've made it look like the shot was taken late in the evening rather than at mid-day. Simply by changing this ratio, the position of the light relative to the camera and the harshness/softness of the light, we can pretty much create the lighting that we want.
At this stage Teddy is pretty bored with posing the way I've had him posing. So let us change things around. For the next series of shots, I'll place Teddy side on to the window, so that this time, one side of his face will be illuminated by natural light.
Fill -Flash Side Lighting
At this stage Teddy is pretty bored with posing the way I've had him posing. So let us change things around. For the next series of shots, I'll place Teddy side on to the window, so that this time, one side of his face will be illuminated by natural light.
Placing Teddy beside the window, I use my hand held ambient light meter and meter for the lit side of his face. This is to ensure that I expose for the highlights and avoid blowing out any detail.
The problem we have here, a problem faced by most photographers shooting outdoors or with ambient light, is that there is quite a bit of contrast between the lit side of the face and the shadow side, approximately 2 whole stops! For some photos, this contrast can add to the mood, but normally we would like to reduce it.
A simple approach we can use is to use a reflector, off to the subject's right/cameras left to throw light back on to the shadow side of the face, reducing contrast.
This method is extremely effective and more importantly, easy to control. In fact, I would always recommend that people start off with reflectors, so as to understand light a little more, before introducing flash into their arsenal.
The reflector has done a really good job here to reduce the contrast and lift the shadows. It's easy to control the amount of reflected light that you want by simply moving the reflector closer or further away from Teddy.
One disadvantage of reflectors though, is that the ratio of available light to reflected light is always going to be less than 1:1. This is due to the inverse square rule! However, by using flash instead of a reflector, we can control the ratio precisely.
So now, with an off camera flash, placed on the opposite side of the subject to the window, we are able to light the scene as we desire.
Flash to available light ratio: 1:1
Flash to available light ratio: 0.75:1
Flash to available light ratio: 1:0.75
Ok, Teddy is bored and the batteries in my flash need charging, so I'll finish this tutorial here. Hopefully I've given you just a little bit of insight into the technical issues with flash as well as some of the practical uses. Remember, once you get more into flash photography, the creative possibilities are endless.
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