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Flash Photography (part 1)
Note that this articles is a detailed discussion on how flash works. If you really want to get the most out of your flash read on. However if you just want a basic introduction to flash, specifically if you use a compact or bridge camera, then read our Flash on Compacts section.
Introduction
Flash photography can be a major source of confusion even for experienced photographers. After learning that exposure for ambient light is controlled by aperture, shutter speed and ISO, flash photography adds a whole new element to the mix - an artificial light source which can be independently controlled. What's more, when we tell you that the shutter speed doesn't really come into play when controlling flash exposures you may wonder if it's worth the bother re-learning what you thought you knew when you could just stick with shooting in natural light only.
Many people are put off flash due to bad experiences with harsh washed out photographs with demonic looking subjects glaring at the photographer with bright red eyes. You would be forgiven for thinking that flash photography was the route to evil and not worth bothering with. However, nothing could be further from the truth. Once you have learned how to control flash you will discover that it can be used to create fantastic images from portrait to still life where you have full control over the lighting; your very own sun in your camera bag!
Built in Flash Versus Separate Flash Units
Before you get too deep into this chapter we should explain that for best results with flash you really need a separate flash unit. Ideally this would be a flash where the head can be tilted and swiveled to different positions so that the light can be bounced of walls and ceilings.
All those bad flash photos you've seen with harsh light and deviled red eyes are probably the result of a built-in flash. Due to the physical size of cameras a built-in flash is located very near to the lens of a camera meaning that the light is going directly at your subject and reflecting straight back. This leads to very harsh, unflattering light and frequently to the dreaded "red eye".
Red eye is caused by reflections off the subject's retina (the back of a person's eyeball) and happens because in dark conditions their pupils are open wider. If you shine a light directly at their eyes (which you will do if your flash is very close to the lens) then you will illuminate the blood vessels in their retina which is what's recorded on your camera. Yuck!
Many built in flash units have ways of reducing this effect, for example by flashing twice, once before the actual exposure to cause the subject's pupils to shrink to minimise reflections from the retina. Even where these are effective though a built in flash will still produce an unflattering direct light source giving images with very little depth and unpleasant shadows.
The one time that your built in flash may be useful is in sunny conditions when the direct flash can light a dark subject against a bright background. This might sound illogical but using the flash in sunny conditions is not as daft as it sounds. See the Fill-In Flash section below for details.
Basics of Flash Exposure
There are two elements to a picture taken with flash:
- The flash lighting
- The ambient lighting - i.e. the light that is already present exluding the flash, such as sunlight or interior lighting.
In some pictures the ambient light may not really register and the flash will be dominant. In other pictures the reverse is true. We discuss this below under "balancing flash with ambient light".
When composing a picture on most cameras, even if the flash is turned on, your camera meter will be telling you what the exposure should be as if flash wasn't there. In other words it is metering for the ambient light. Through the lens flash metering takes place as the picture is taken not while you are composing it. This means that you only need to pay attention to your light meter if you are interested in how the ambient light will appear in your shot.
Sync Speed
Before we get into how exposure is controlled with flash you'll need to understand something called "Sync Speed" or "X-sync". When you take a photograph with flash the camera opens it's shutter and then the flash fires. This requires some synchronised activity between the flash and the camera; the flash should not fire until the shutter is fully open or the image will only be partly illuminated. For this reason your camera will have a flash sync of x-sync speed, this is the maximum shutter speed at which your camera can take a shot and guarantee that the shutter will be fully open. This varies from camera to camera and is usually somewhere in the region of 1/60 to 1/500 second.
Despite the fact that these shutter speeds may seem quite slow it is still possible to freeze subject motion with flash. Lets' imagine that you are in fairly dark conditions and the flash is the main source of light, your camera's flash sync speed is 1/60 which is quite slow and you want to freeze the motion of someone jumping (it's just an example). As the flash is only illuminated for a very short period of time, usually less than 1/1000th second and much less than the shutter speed, you will find that the flash itself will freeze the motion as the person will only be illuminated while the flash is lit. Therefore their motion will not register.
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This photo of falling candy canes was taken with a slow shutter speed 1/8th Second. As the flash was the only illumination and was only lit for a very brief period the candy canes are frozen in mid fall. |
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This is the same shutter speed as the shot above, 1/8 Second but this time the shot is illuminated by room lighting and there's no flash. The candy canes are therefore reduced to a total blur.
Note that in order to get a correct exposure at 1/8 Second in this lighting I also had to open the aperture somewhat - hence the difference in depth of field between the two images. |
Not an artistic pair of shots by any means but they make the point.
Some cameras can sync at very high speeds when coupled with certain flashes, this is called high speed sync and is useful outdoors when high shutter speeds are necessary due to the ambient light levels, this will be covered later in the chapter.
The Effect of Shutter Speed on Flash Exposure
For the most part the effect that shutter speed has on flash exposure is nil, nothing, zero, zilch, nada, a great fat zip etc etc. What are we saying here? Well changing your shutter speed won't affect the exposure of a picture illuminated solely by flash because the flash is only lit for a very short time. Making your shutter speed longer won't affect the flash as it will only be on for a brief period whether the shutter is open a long time or not.
In order to change the exposure caused by flash you can do one of the following:
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change the ISO setting
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change the aperture of the lens
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alter the flash power output (manual flash)
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change the distance between the flash and the subject
However to add more confusion into the mix some flash systems when in certain automatic modes will set the flash exposure how it deems to be correct regardless of any of the above.
As we said above, it is important to think of any flash picture as two exposure elements in one; ambient light (i.e. light not coming from the flash) and flash light. Ambient light may be daylight or artificial light in a room where you are taking the pictures. Before the exposure the camera's meter is measuring the ambient light and not the flash light. The trick to successful flash photography is to know how much flash and how much ambient light you want in your picture.
To help you understand flash exposure it will be best to start by going back to the very basics of flash and assume a totally manual flash unit. Bear with us, we know that for the most part you won't be using your flash in its manual mode but it is useful to understand the basics so that you can understand what the flash is doing when it is in an automatic mode.
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Photographs
This is a site about photography so I'm sure you are expecting to see plenty of pictures.
For now, why not take a peek at the flickr galleries belonging to the two authors of this site.
Colin's Flickr Page
Phil's Flickr Page
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