Hot-Shoe Flashes (Canon 420EX) - is light controlled by output strength or duration?

Started Nov 10, 2012 | Questions thread
Duncan C Veteran Member • Posts: 7,664
Re: Hot-Shoe Flashes (Canon 420EX) - is light controlled by output strength or duration?

Quantum Help wrote:

Some speedlights work bothways.

Manual mode is one that combines both factors. It delivers a given amount of energy based on what you programmed it for. If you gave it a setting of say, 1/16th power. The duration will be for 1/16th of the time that a full power flash will put out and since it is putting out a given amount of power, it will also be of a shorter duration.

In Auto Mode, where the sensor controls the light output, it starts off at a full power discharge and is abruptly turned off when the sensor reads that it has enough light for a good exposure.

TTL metering sends out a pre-flash (a weak flash to determine the amount of reflectance in the subject. It reads that light reflected back to the camera, then computes the amount of light to be generated for the exposure. This is sort of a manual flash output.

Your statements are not quite right.

All electronic flashes work by fully charging the capacitor, starting a full-power flash, and cutting off ("quenching") the flash output when the desired amount of light has hit the subject.

There are a range of different ways to compute the amount of light needed. As you say, manual mode is a fixed amount of power, determined in advance.

Auto mode uses a light sensor on the flash to measure the light returned from the subject. The flash decides when enough light has been emitted, and quenches. The discharge is still a full-power discharge that is cut off suddenly.

Digital TTL mode varies slightly between camera brands and the generation of the camera, but basically boils down to using low-power preflashes to calculate the amount of light needed, followed by a full power discharge that's cut short, just like in the other 2 cases.

High speed sync is yet another variation. In that mode, the flash electronics lets out a very rapid series of very short flash pulses. It's still starting a full-power discharge, but in the case of high speed sync, the pulses are so short, and so fast, that the xenon gas in the flash tube is kept "lit" at a lower intensity, for the duration that the flash is firing. The intensity of light output fluctuates VERY rapidly - so rapidly that the fluctuations don't affect the picture. (The flash pulses at multiple kilohertz - multiple thousands of times a second. I want to say somewhere around 20 KHz, but that's from hazy memory, and I could be way off.)

All of the above modes are variations of the same thing: Fully charging the capacitor, Starting a full discharge of the capacitor, and then switching it off quite suddenly. The differences are in how the amount of light is computed, and whether the flash fires once or multiple times.

In contrast, some (most?) studio flashes work by charging the flash capacitors less than fully for a lower power flash. When the camera triggers, the capacitor completely dumps all it's charge, but there's less power in the capacitors so the total light output is lower. The flash duration for these flashes actually gets LONGER for lower power, since the xenon gas in the flash tube does not get fully ionized at lower power, and so it isn't as good a conductor.

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