This is the oldest and most fundamental way to control electronic flash.  Early consumer photographic electronic flash units typically had only full power output.  So, exposure was principally controlled by the cameras aperture, sensitivity (ISO), flash to subject distance, and light modifiers.

Studio type flash units use manual exposure control.

Newer small electronic flash units and studio flash units have added multiple power levels settings to increase usability.

Digital cameras, with instant feedback on exposure, have only increased the popularity of this exposure control method.

Despite this being an "old" way to control flash, it remains a vital and effective method to control flash exposure.

A light sensor was added to electronic flash units.  This sensor measured the amount of light reflected from a subject to control exposure.  The sensor could vary the amount of time the flash unit was on to allow more or less exposure.  This control was very quick, almost instantaneous, so an exposure could be adjusted as it actually occurred.

This sensor used was typically an electronic device called a thyristor, hence the name "thyristor automatic".

Flash units typically had a choice of a few automatic modes.  The user selected an ISO and a mode (often color coded).  This mode indicated an aperture to be used and a range of distances available for proper exposure.  This aperture was then set on the camera.

When used where the sensor on the flash could be pointed at the subject, this exposure mode is very effective and likely as accurate as more complicated systems.

The Honeywell Strobinars of the 1960's popularized this exposure control method.  But, many current high quality flash units like Metz often retain this exposure control method and this method is returning to some newly designed flash units, like the Canon 580EX II.

This method of exposure control is also still viable, even with modern digital cameras.

As far as exposure control, this may be the apex of development.

In a TTL system a sensor in the camera body points toward the light sensitive area and controls the flash.  This allows the sensor to be in the most advantageous position for exposure control.  Extreme wide-angle lenses, extreme telephoto lens, extension tubes, macro lenses, filters, etc., can all be intrinsically compensated for with TTL exposure.  The exposure control is almost instantaneously just like the "traditional automatic" mode above.

This type of system was first offered in the mid 1970's by Olympus.  In just a few years almost all the major SLR manufactures had TTL flash metering.

Camera manufactures were pretty set with their individual versions of an accurate and flexible flash metering method, namely TTL.  But, along came digital sensors.

Unlike film, digital sensors reflect little light.  This one aspect means that the very accurate and mature TTL systems in place for over two decades had to be abandoned.  TTL systems rely totally on reading the light reflected off the image gathering area.  So, if there is little light being reflected, TTL just does not work.  So, for digital cameras the TTL method was basically DOA.

Enter the ETTL ⁄ iTTL type flash metering.

These systems rely on a preflash to control exposure.

The flash unit fires a low powered preflash in advance of the actual exposure flash.  This light is then gauged inside the camera with a sensor and the exposure set for the quickly following actual flash exposure.  In SLR type cameras this sensor is typically in the viewfinder (unlike with TTL where in SLR type cameras the sensor was behind the mirror and pointing back at the image gathering area).

This gives many of the upsides of TTL metering, but not all.

The preflash has all of these downsides compared to TTL

• Many humans and animals with very quick reflexes blink with the preflash.  So, when the actual exposure occurs the subject has closed (or partially closed) eyes.
• The exposure parameters are actually set a bit in advance of actual exposure.  Typically this is of no consequence, but at times it could be an issue.
• The actual exposure is delayed to accomplish the preflash.  Again, typically this is of no consequence, but at times it could be an issue.
• The preflash frustrates the use of traditional flash accessories such as optical slaves.

With so much computerized technology now available, camera manufactures have welded all sorts of nifty new features onto the newer ETTL ⁄ iTTL systems.  These nifty new features, such as robust and convenient central control of several flash units, were never as developed with TTL metering.

But, remember those nifty features are not the nut of this discussion.  This brief overview is totally with regard to the basic exposure control afforded ETTL ⁄ iTTL and similar systems, not the nice new add on features.

But, bottom line ETTL ⁄ iTTL systems are a convenient, efficient, and effective way to control flash exposure.

About the thyristor: en.wikipedia.org/wiki/Thyristor