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Flash Photography (part 2)
Manual Flash
Manual flash is where the flash output is controlled by the user and not automatically set by the flash or camera. Many flash units come with a manual mode where the user can set the output at different settings, e.g. full power, 1/2 power, 1/4 power and so on occasionally even down to 1/128 power.
{For those curious, flash power is adjusted by altering the length of time the flash is lit for. Half the power means halving the time - the light output level does not actually change but this is not important. Just thought you'd like to know!}
Let's imagine a scenario where you are taking a photograph indoors with a camera with an x-sync of 1/200 and you are going to use the flash as your primary source of light (i.e. you are ignoring ambient light). Your camera settings in manual mode in this example are ISO 200, 1/100 at f/8 with the flash on 1/2 power. Your camera's light meter is telling you that the photograph will be underexposed but you ignore this because you know that the light meter doesn't know anything about the flash and it's only measuring the ambient light.
You take a picture and on reviewing the image you see that it is overexposed maybe by about a stop so what do you do to remedy the situation? Well, changing your shutter speed will not affect the flash exposure at all as described above so you ignore that setting. You could do one of these things:
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Close the aperture by one stop to f/11; reducing the amount of flash light hitting the sensor
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Change the flash output (power) to 1/4; reducing the amount of flash light hitting the subject
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Change the ISO speed to 100; making the sensor less sensitive to the light hitting it
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Move further from your subject (see below)
Those are the only things that will affect the flash exposure. Regarding changing the distance between the flash and the subject - doubling the distance between the light source and the subject will quarter the intensity of the light hitting the subject; this is the inverse square law referred to in the Exposure section. Therefore to half the power of the flash you need to move back about 40% the initial flash-subject distance.
Now that we know what elements can be used to change the exposure caused by a flash let's move on to automatic flash systems for now. We will come back to manual flash when we take the flash off the camera later in the chapter.
Through The Lens (TTL) flash metering.
Different camera manufacturers have implemented different systems here; Canon's current system is ETTL-II (Evaluative Through The Lens), Nikon has iTTL; so it is not really possible the describe all the different systems in detail but the basic concepts are all the same.
TTL metering relies on a flash being "dedicated". This doesn't mean that it is keen on its job with a strong work ethic, it simply means that it is designed to work and communicate with a particular camera or range of cameras. Nikon and Canon flash units are dedicated to Nikon and Canon respectively (no surprise there) but third party manufacturers like Sigma and Metz also produce flash units that are designed to be dedicated to particular camera systems. This means you can own a Canon camera and Sigma flash and they will be able to communicate and work automatically providing you bought a model that is "dedicated".
In these automatic systems flash output is controlled purely by the camera. In a basic 'through the lens' system, the camera monitors the light bouncing off your subject and when it decided that there has been sufficient, it cuts the flash. In some systems, a low power pre-flash is fired just before the shutter opens. From this the camera can take a light reading and compute exactly how much flash light is going to be needed after the shutter has opened. The camera is automatically setting the flash exposure to a level it deems to be correct based on the distance to the subject and the aperture and ISO settings you have set.
Summary of events during exposure with TTL:
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As the shutter button is depressed the flash releases a "pre-flash" burst of light.
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The camera measures the light returned and determines how much flash light the subject needs.
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The shutter opens
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The flash fires
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When the required light (as measured in step 2) has been released the flash ceases outputting light.
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The shutter closes.
What this means is that you could take 2 pictures with the same ISO and same distance from the subject except the first may be at f/8 with the second at f/16 and the flash exposure will be the same as the flash will automatically provide more light for the second picture to make sure it is sufficiently lit.
This may take some thinking about as it contradicts normal photography experience. You may be wondering what the point of changing the shutter, aperture or ISO might be when using automatic flash... The answer is ambient light. Changing those exposure settings will change how much ambient light registers on the image and this will allow you to achieve a creative balance between ambient light and flash light. More on that shortly.
You may also be wondering (having the enquiring mind you no doubt do) how you can change the flash exposure if it is getting it wrong. Well - this will depend on your system but the answer on most systems will be "Flash Exposure Compensation". This is often distinct from normal exposure compensation in that it does not affect the ambient light exposure at all. Just the flash output. So if the scene contains someone in a white dress for example and the flash is being fooled by this you can increase the flash output by using flash exposure compensation.
Balancing Flash with Ambient Light.
There will frequently be occasions where you don't want your flash to be the only source of light in a photograph be it indoors or out. Indoors you may want to show the ambient light coming from a candle or room lighting, you may even want to hide the effect of the flash to some extent in order to make the lighting appear more natural. Outdoors you may simply want the flash to fill in some shadows.
The effect that ambient light is having on a scene is easy to control especially if you are using TTL flash. You can leave the flash to its own devices and use the normal exposure controls (shutter speed, aperture and ISO) to control the ambient light.
For example, if you want the ambient light to have a significant effect on the lighting you will pay attention to the camera's meter and adjust the camera exposure settings to get a correct ambient light exposure or thereabouts.
Alternatively you may with to deliberately darken the background by underexposing the image while letting the flash automatically set its power to correctly light the foreground.
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To darken the sky this image was taken with exposure compensation set to underexpose by 1 stop while Flash Exposure Compensation was set at zero.
Flash was to camera left connected by dedicated hot shoe cord that enabled the camera to retain TTL flash metering with the flash off camera. |
Different cameras operate in different ways. Some cameras will allow you to work in aperture or shutter priority and use exposure compensation to adjust only the ambient exposure. Others may darken the flash if you use exposure compensation. If your camera has a separate Flash Exposure Compensation setting then the chances are that the standard exposure compensation will not have any effect on the flash exposure at all.
Fill-in flash is just another method for balancing ambient light with flash. Shooting outdoors in bright sunlight can cause harsh shadows on a subject's face, the flash can be used to "fill in" these shadows to provide a more flattering look. You may need to reduce the flash output by using flash exposure compensation (or manual flash) in order to make the lighting appear natural. Some cameras will do that for you automatically if the ambient light is bright enough.
<Examples to follow>
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