All about electronic Flash
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Electronic Flashguns use the controlled discharge of electricity in a flash tube to make a short burst of lightning. Because the duration of the light is short, it freezes action. It's also cheaper to use and more convenient than disposable flash bulbs. Electronic flashguns use a battery or other power source to charge a capacitor. When the flash is triggered, the power in the capacitor is sent to the flash tube.
Strobe: technically, stroboscopic light is a repeating light, such as an automotive timing light or a disco strobe. In general usage, the phrase is used interchangeably with electronic flash.
PC Cord: A connecting cord for the flash. Named for the Prontor-Compur shutterworks in Germany which developed the shape of the tip.
Hot shoe: a flash mounting slot that has a central terminal so no cord is needed.
Synchronization: the timing of a flashgun and a camera so that the flash is making the most light while the shutter is open.
M Sync: a delay of .026 seconds between the flash starting to fire and the shutter opening. This allows old flash bulbs to reach their peak output.
X Sync: a xero (zero) delay. When the shutter is fully open, the flash is triggered. This is what is needed for electronic flash. Most cameras with focal plane shutters, such as SLRs, have X sync at speeds up to 1/60th of a second. If you take a flash picture with the shutter speed set higher than the X sync speed, you get a picture that is partially blacked out. Some premium cameras offer the feature of a high shutter speed for X sync. This minimizes the chance of a "ghost exposure," one which is made not by the flash but by the existing light.
Automatic Flash: an electronic flash that measures the amount of light
which has gone to the subject and bounced back to the flash. It turns the flash off when
enough light has been returned for a proper exposure. It can do this in as short a period
of time as 1/50,000th of a second.
Thyristor: a special type of switching mechanism on an automatic flash which saves the energy not required during a short flash. It simply stops the flow of power from the capacitor to the flash tube, instead of wasting it by "dumping" it. This means that a thyristorized flash gun will recycle more quickly and give more flashes on a set of batteries before they run down.
Recycle Time: how long you must wait after one flash before the flash is ready again. With a thyristorized flash the following choices make the flash recycle faster:
A TTL flash system uses a sensor in the camera to measure the flash Thru The Lens, which is more accurate when taking extreme closeups or using a long lens.
Dedicated flashguns are designed to work with a specific late model camera or family of cameras. They have additional connectors to carry information back and forth between the camera's brain (CPU or Central Processing Unit) and the flash. All dedicated flashguns set the camera to the one and only proper shutter speed for flash sync. Some also set the lens opening, on cameras which have automatic lens opening exposure systems. Most activate a flash readilight in the camera's viewfinder. Older cameras with mechanically timed shutters don't need, and can't take advantage of a dedicated flash.
Multi-dedicated flashguns by companies such asVivitar, Sunpak, Rokunar, Achiever, etc, can be adjusted to provide minimal dedication with several different brand cameras. In general, they will set the proper shutter speed, activate the readilight in the viewfinder, but will not set the lens opening.
Some flashguns, such as the Minolta 280PX, do not have a built-in exposure sensor. They can only be used on specific cameras that have a built-in sensor for TTL flash. For example, the Minolta 280PX can only be used on the X-570 and X-700 cameras.
Autofocus cameras usually have full dedication and also a built-in near-infrared light source that makes it easier for the camera to focus in low light levels. Failure to use a fully-dedicated flash wipes out many of the benefits of buying such a good camera.
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Guide Number: As you get farther away from any light source, the intensity drops off. As you change the f stop of a lens, the intensity of the light at the film plane also changes. They change at the same rate, so this statement is always true: distance times f stop = a fixed number (called the Guide Number) Every combination of flash and film speed has one Guide Number (GN). Faster films and more powerful flashguns have higher guide numbers. Often manufacturers use the GN as part of the model name of a flashgun.
You figure out what f stop to use by dividing the guide number by the distance from the flash to the subject.
If a flashgun has a guide number of 80,
When the film speed is increased by a factor of 4, the guide number is doubled. If the guide number for ISO 100 was 80, the GN for ISO 400 is 160.
How film speed changes Guide Number
Does that progression of guide numbers remind you of anything? The increase as film speeds double is just like the 1.4 times increase in the f-stop numbers on your lens:
Guide numbers can also be rated in meters rather than feet, but manufacturers don't do that too much in the USA - it makes their products seem less powerful.
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What shutter speed can I use?
If the light from a flash picture is to get to all of the film, then all the negative area must be uncovered when the flash goes off. With a focal plane shutter, at high shutter speeds only a portion of the negative is uncovered at any instant - a slit exposes each segment sequentially. That's the reason you'll sometimes see a photo properly exposed at one end and very dark at the other.
Some top-end cameras can be used with special flashguns at really high shutter speeds such as 1/4000th of a second. The special flash actually lasts long enough for the shutter curtain to complete its passage across the full length of the film.
Most cameras require that you use a slower shutter speed. On some, the highest shutter speed that is usuable with electronic flash will be painted a different color on the shutter speed dial, or marked with an "X."
Here's a rough guide to camera types and the highest shutter speed for flash sync (a slower speed is always safer)
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What fits what:
Which flash is needed: (Most cameras will work with all lower levels of dedication, they just won't give you all the features of the more sophisticated dedication system.) The specific models listed are for illustration only, this does not represent a specific endorsement..
B: dedicated, shutter speed only
C: dedicated, shutter & f-stop
D: dedicated, TTL or Program
E: Autofocus special flash
Specific Flash Recommendations
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