Before the modern electronic flash there were flash bulbs and before that, trays of powdered magnesium. The less said about those early flash attachments the better.
An electronic flash is seen by many people as just another bright light so they tend to think it will behave like sunlight and magically illuminate a distant scene. The trouble is that the rules are based on where the source of light is. The light intensity decreases as the distance increases from the source. Simple physics.
This is not significant with sunlight as the sun is so far away (maybe 150,000,000 km) and any place on earth is effectively the same distance away from the sun. It is very significant for the flash unit on a camera as it is comparatively very close to the subject. Step back a pace or two and everything changes. The light from a flash unit of a certain intensity measured at one metre will measure a lot less at two metres and much less still at three metres. The light fall off is at the rate of the square of the distance increase from the source. That rule applies to any light – sun light, flash, floodlight, candle light, etc.
Small cameras will only provide some flash illumination for subjects up to maybe ten feet away (3m), bigger accessory flashes may reach as far as something like thirty feet (10m) or more.
An electronic flash unit has batteries to supply the power, circuitry to raise the low battery voltage to a high (and dangerous) voltage to charge the flash capacitor. A trigger supplied from the camera body at the correct time causes the flash tube to turn the power stored in the flash capacitor into light. Some units just keep supplying power to the flash tube until the flash capacitor is empty, others will receive a turn off signal and stop the flash tube from producing more light than is necessary.
Electronic flash has a duration of typically 1/1,000 sec to 1/50,000 sec. The faster speeds occurring when the automatic circuitry in the flash shuts off the flash early to avoid overexposing a close subject. The colour temperature of the flash tube is normally 5,500? Kelvin and is considered as equivalent to sunlight. The surface of the sun is about 6,000? Kelvin.
Compact (viewfinder type) cameras usually have a shutter inside the lens that at some stage of its operation at all speeds will be fully open. This is the instant the flash unit is fired. Compact cameras will therefore be able to synchronise the flash unit at all available shutter speeds.
SLR (Single Lens Reflex) cameras usually have a focal plane shutter that is the size of the negative and situated close up against it. The early designs operated like two roller blinds. Later designs use a vertically travelling set of blades that simulate the roller blind operation. At shutter release the first blind fully opens, and when the desired shutter time is over, the second blind follows to block the light again. Exposure time is determined by how long it takes for the second blind to follow. At very fast shutter speeds the second blind follows so closely that the exposure is actually only a very narrow slot travelling across (or down/up) the frame. The instant when the first blind is fully open and just before the second blind is about to start to follow is the only time the flash will correctly cover the film. With a Single Lens Reflex camera this means 1/60 to about 1/250 is the fastest shutter speed for electronic flash synchronisation (it varies for different brands and models, you have to read the camera manual).
Some newer camera & flash combinations can use the flash unit at shutter speeds of up to maybe 1/4,000 sec because the special flash unit will fire many times very quickly as the focal plane shutter slot moves to keep the travelling slot correctly exposed. In this instance the exposure at any one spot of the negative will be 1/4,000 sec, but the difference in time from top to bottom (if that's the direction the shutter travels) will be 1/250 (or whatever the maximum sync speed is for your camera). This could give strange results when attempting shots of fast moving objects (straight lines becoming bent etc). These special high-speed sync modes limit the flash output so the flash range is much less than normal.
The Guide Number quoted for a flash unit shows its output capability. This number will be metre or foot related for a film speed of usually ISO 100. The bigger the number, the more powerful the flash output, and the greater "reach" it has. In manual mode, the Guide Number divided by the distance gives the aperture required. For a Guide Number of 30 (metric), at a distance of 5 metres, the camera aperture should be 30 divided by 5 equals 6. There is no f/6 so open to the next lower stop number (lens more open) at f/5.6. Similarly if the distance was 2 metres then 30/2=15, you could try f/16 but better to use it more open at f/11 or the intermediate stop between the two, if available.
The Guide Number will vary with a flash unit that has a zoom head, or if a diffuser is used to change the spread to a wider angle. Guide Numbers are designed to allow for the reflection effects in an average room. Used outdoors the Guide Number should be reduced by a factor of 1.4. If you double the film speed, i.e. use ISO 200 instead of 100 then you have to increase the Guide Number by a factor of 1.4. If you went from ISO 100 to ISO 400, the Guide Number is doubled. Automatic modes remove the need to worry about these calculations.
Flash units can be lumped into three types, Manual, Automatic and Dedicated. (longer descriptions start after the next few paragraphs).
Manual is a flash unit that only provides the full output flash each time and you have to do all the thinking and set the camera to suit the flash unit.
Automatic usually means self-auto and will automatically shut off the flash output when the light detector in the flash unit senses that enough light has been output. You have to do some of the thinking and set the camera to suit the flash unit's setting.
Dedicated means that the flash unit is designed to work only with a certain brand or model of camera. It can be same brand or an independent manufacturer with an appropriate adapter module. There is usually automatic communication between the flash unit and the camera to make it easier to operate. This makes a complex camera/flash outfit become a simple to use Point-&-Shoot. Compact point-&-shoot camera built-in flash units are considered dedicated. Some thought is required (i.e. what mode to use for what situation). There really is no camera/flash unit arrangement where no thinking at all is required.
Fill Flash is the technique of using the flash unit combined with ambient light to remove harsh shadows from the subject in the sun. It can be used to brighten the front of a strongly back lit subject or brighten a subject that is in the shade when the background is bright. Professionals use it frequently, amateurs hardly at all. To prevent "flat" washed out photos, the fill in flash should be weaker than the ambient light.
Bounce Flash means the flash unit is pointed to a nearby ceiling or wall or reflector to make an indirect path for the light from the unit to the subject. This can produce a softer light with no harsh shadows in the background. The flash intensity is reduced considerably and suitable corrections have to be made. Manual flash users have to open up the lens by about two f/stops. An automatic or dedicated flash unit will take care of these adjustments as long as the head swivels and the sensor still points to the subject. The quality of the light will be altered if the colour of the ceiling or wall is not white.
A specially made umbrella is often used to diffuse the flash more for softer shadows, either the flash is aimed into the umbrella and the reflected light is aimed at the subject or the flash is fired through a translucent umbrella to diffuse it.
Also a soft box can be used to diffuse light for those soft light effects. To make these setups work correctly a large studio flash is usually needed, as the normal camera attached flashes are not powerful enough to overcome the light loss in umbrellas or soft boxes. Soft boxes can range in size from maybe hand size to about person size where the box may be 2 metres tall and 1 metre wide.
A chart or circular "calculator" will be on the flash unit allowing you to set the aperture depending on the speed of the film and the distance of the subject from the flash unit head. The "calculator" does the Guide Number calculations for you. Some control of the flash strength can be achieved by putting pieces of translucent plastic (cut up milk bottles or ice cream containers) in front of the flash unit head. If you own a flash meter you can measure the changes without wasting film.
Studio flashes are normally manual but usually have a variable output control. The effective strength is set by varying the flash distance from the subject and also by varying the flash unit output control. A flash meter is essential when using studio type flash units. Another feature usually found on studio flashes is a modelling light. This light is a normal lamp and will be aimed at the subject and be bright enough so that the flash units can be aimed and moved to eliminate awkward shadows and get a better setup without having to fire the flash and wait for a developed result. Polaroid or digital cameras are often used to test how a setup is working before committing the image to maybe expensive film stock.
Using a manual flash unit with only one flash strength makes fill in flash very tricky. For the distance to the subject you have to find the aperture for the camera from the calculator on the flash unit. Then you set the camera to one f/ number higher (aperture smaller). Now you measure the ambient light and find the shutter speed that goes with the already set aperture, minding you don't exceed the camera flash sync speed. This means the ambient light gives correct exposure and the flash light gives under-exposure. Varying combinations of ambient light, film speed, flash strength, distance, lens aperture available and the maximum flash sync shutter speed have to be considered and in some cases fill in flash may not seem possible. If it seems that the flash unit is not powerful enough, try it anyway, as lesser values of fill in are better than none at all. If the flash unit is too powerful then some lessening of the flash output with pieces of translucent plastic or sheets of paper can be done. Lots of experimenting and keeping good notes will help.
A manual flash used for bounce illumination is also tricky, you have to experiment a lot and maybe waste some film. The best method is to obtain a flash meter and just by test firing the flash you can find the aperture required.
Usually there is an adjustment on the flash unit to have either full flash (just like an old manual) or automatic with possibly a choice of one or more auto settings.
As an example, my old Vivitar 273 (vintage 1977) has M(manual) & Yellow & Red & Blue as the 4 switchable possibilities. There is a circular calculator where you have to set the film ISO speed. My unit shows Yellow works from 3 to 25 ft, Red works from 2 to 18 ft, and Blue works from 2 to 13 ft. When I select ISO 100 for the film, then the calculator shows the Yellow setting needs f/4, Red is f/5.6, and Blue is f/8. All automatic flashes will have some form of “calculator” on the back, some are more easy to understand than others.
You have to make intelligent selections of flash unit setting and camera aperture based on desired maximum distance and the depth of field required. It's easier to do than read about it. If the flash unit gives some indication (read the manual) that the flash "failed" then you may have to select a larger aperture setting or move closer to the subject or use a faster rated film or fit new batteries.
Fill flash with an automatic flash unit involves finding the speed/aperture setting for the ambient light, then making sure the shutter speed is at or slower than the flash synchronisation speed. Apply the next lower aperture number on the flash unit auto setting and fire away. Example, if the camera setting ends up at f/11 at 1/60 then the flash unit should be set to f/8. Some juggling of settings may have to be done. The best way is to take a few test shots with the correct settings, then take a few more with wrong settings, making notes on what you did. You may find one of the "wrong" settings may prove to be just fine. If using too fast or too slow films there may be no way that the speed/aperture settings will fall into a range that allows the flash to give a proper fill in. If this happens, try anyway and see what happens. Take notes!
Bounce flash is also fairly easy with automatic flash. Usually you just twist the flash head up so that the light will bounce off the ceiling and down onto the subject. You must realise that the path of the light is now longer so reducing the distance limit of the flash, also if the ceiling is not white or is too high the result will not be satisfactory.
The flash output is stopped when the light is sufficient for the exposure,
this is done by measuring the light coming back off the subject with a
through-the-lens (TTL) sensor that reads the reflection of the light coming
off the actual film surface. Be aware that this sensor is most likely a
centre-weighted type, later cameras may have more elaborate flash sensors
with multiple segments. Experiments may have to be done to see the effect
with an off-centre subject or when using the camera on its side.
Some newer flash units have a "pre-flash" sequence that uses the camera's ambient light (Canon) or flash sensors (Nikon) to help calculate the correct exposure and strength for the following main flash.
If the flash unit gives a "failure" indication, decisions have to be made about moving closer, maybe fitting new batteries to the flash unit, using another lens with a larger available aperture or trying again with a faster film. If using the camera in aperture priority mode (in my opinion the best way to use any auto SLR) and you get "failure", then you have to select a larger aperture (smaller f/number) or the rest of the steps in the previous sentence. If using colour negative film, quite often a "failure" will turn out just fine due to its good exposure latitude.
Dedicated Fill Flash is very easy, just set the flash unit to the fill
flash mode. You will need to read the manual. My flash unit manual is bigger
than the camera manual and is a lot more complicated to drive once you
get away from fully automatic mode. Usually some adjustments can be made
on the flash unit so the fill ratio can be made to go from the normal default
1:1 (usually too bright) to some lesser amount like 1:2 or 1:3 or maybe
as far as 1:16 or even 1:64 in a whole series of small steps.
Bounce flash is similar to the automatic mode, you have to understand that the range will be reduced because of the longer path of the light to the subject and losses when reflected off a ceiling or wall. Also colouration will creep in if the ceiling is not dead white.
All the normal flash and fill in functions can be retained even when the flash unit is "off-the-camera", that is, connected by a dedicated cord but held closer/further away, out to the side or up or down. This often gives more dramatic lighting with more "shape" to faces due to the more interesting shadows. The extension cable is essential if trying macro photography when the subject is close to the lens and the flash unit can't point down far enough to cover the subject properly or the end of the lens or its lens hood casts a shadow. Most flashes won't be reliable closer than about two feet (0.6m) from the subject. Check your manual about this.
With dedicated flashes being used in TTL mode usually you can connect a few units together and with the appropriate cables the multiple units will behave as though they are only one flash. The camera measures the flash light received and signals the flash(es) to turn off when enough light has fallen on the film.
A very useful accessory is a slave trigger unit. The most usual type fits on the bottom of the flash and is powered by the flash. It triggers the attached flash when a pulse of light is detected from up to maybe 30 metres away depending on type of trigger. The light may be from your flash or someone else’s so care needs to be taken. Units can be found on the gadget racks at camera stores. Usually any brand trigger can fit any brand flash.
A more modern and useful accessory is a remote TTL control trigger unit. To control the flash unit correctly the TTL control unit will usually have to be of the same brand as the flash. An appropriate flash unit can be turned on by the start of the main flash then turned off by sensing the end of the output from the main flash unit. This means TTL flash control without a cord connecting the second flash. This also means that this smart unit can be used with any camera that has a flash on it. You could use a Pentax SLR with its small pop-up flash and trigger a bigger Nikon unit with the remote TTL function to get more flash range or better lighting angles. There is nothing amazing or tricky happening, the camera, no matter what type, will use its TTL flash circuitry to determine when to turn off its flash. If other light is being added from other flashes started at the same time then the TTL circuitry will just turn off the built-in flash that much earlier. The remote TTL sensor sees the end of the master flash output and then turns the slave units off. An example of this device is the Nikon SU-4 remote TTL controller.
Macro Photography with a flash can get as complicated or as simple as you wish. With a manual or self-auto flash consideration has to be made of the effective aperture of the lens when focussed closer than normal. When the lens is focussed closer, the lens effectively moves further away from the film and moves the aperture away from the film thus making the chosen aperture appear smaller. Allowance has to be made for this change. Measuring with a flash meter will only give an aperture reading for a normally focussed lens, you will have to calculate the new aperture and make allowances.
With a fully dedicated TTL flash setup macro use is very simple. The flash will keep outputting light until the internal camera TTL flash meter detects enough light and then turns the flash off. If you extend the lens or change the aperture it doesn’t matter as the TTL circuitry is measuring the light reaching the film surface. I regularly use a bellows unit on the camera where the lens is disconnected from the body so the body has no idea what the aperture is. The flash works perfectly every time as the TTL light measurement takes care of everything.
In the old days when I had a completely manual SLR and using a manual flash I basically gave up on macro work as I was too lazy to do all the calculations necessary and too mean to waste some film to test the setups. Now with an all automatic dedicated flash setup I can attack any macro setup with confidence.
Nikon has an arrangement where the flash unit emits up to maybe 16 invisible pre-flashes just after the mirror goes up and just before the shutter starts moving. These flashes are reflected back off the front surface of the shutter and are used to try and determine in which of the five segments the subject is situated taking into consideration the focus distance information supplied from the lens. When the logic has worked out where the subject is then the shutter is released and the main flash fires. The TTL flash measurement is now made only using those flash meter segments that the logic has determined are relevant. The pre-flash sequence is very fast and does not cause any delay in taking the shot. Link to Nikon metering page.
Canon fires the test pre-flash before the mirror moves and uses the camera’s normal ambient light meter segments together with information about the focus area selected to work out the relevant segment and the flash exposure required. Then the mirror goes up and the shutter fires and the main flash occurs, controlled by the exposure determination made during the test pre-flash. No link as I've lost the site where I found a good Canon metering description. The Canon EOS-1V brochure has a vague description about E-TTL flash upon which the above is based.