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Flash Systems Evolution, Features and Operation

Similar to the K mount, the Pentax hot-shoe has also evolved. The figure below shows it in its most complicated variation, with 5 contacts. It started with only two contacts however, and the others were added gradually, as more functions were offered by the bodies and the flashes.

Diagram of the Pentax flash hot-shoe

Guide Number (GN)

A flash guide number identifies the lighting power of the flash. It is measured for a particular film speed, typically ASA/ISO 100, and the units are typically meters. For a setup that will result in proper flash exposure,

GN = d * F,

where d is the distance between the camera and the subject, and F is the selected f-stop. For example, if a flash with GN = 16 is to be used to make a photograph of an object 2 meters away (d = 2), the correct f-stop, F, is 8 (since 16 / 2 = 8).

Manual Flash

Manual flashes are simplest to build and hardest to operate. They produce a fixed amount of light with every discharge, so the photographer must adjust the distance and f-stop to match the flash output to the desired image brightness. Manual flashes usually have an exposure chart on the back that matches film speed, subject distance and f-stop. Such charts automate the calculations explained in the previous section.

Manual flash calculation chart

A typical sequence for taking a manual-flash photograph is then:

  1. turn flash on,
  2. compose,
  3. set X-sync shutter speed,
  4. focus,
  5. note subject distance,
  6. find out F from the back of the flash (make sure to look in the appropriate film-speed column),
  7. select F,
  8. shoot.

A manual flash needs only two electrical contacts with the body: ground and trigger. With virtually all flash hot-shoes, ground is provided by the hot-shoe rails and trigger is the large contact in the middle of the hot-shoe. The flash discharges when the trigger contact is shorted to ground.

Some flashes feature two or more output levels, in effect offering additional (lower) GNs. This offers more compositional flexibility as well as faster recharge times. Some bodies and flashes have a PC socket in addition to the hot-shoe. PC sockets have only two wires, and thus have only trigger functionality.

Only two Pentax flashes produced in the K-mount era are fully manual: Autorobo and AF160.

Dedicated Flash

A dedicated flash is one that is capable of notifying the body when it has completed charging. The body can then respond by automatically selecting the X-sync shutter speed and possibly lighting up a flash symbol in the viewfinder.

Almost all Pentax flashes feature dedication. The letter "S" in the name indicates this feature, but some early units without "S" are also dedicated. To make matters more complicated, all of the following flash types also offer dedication in addition to other features: "SA," "C," "T," and "F."

Dedication requires an extra contact in the hot-shoe, ready.

Automatic Flash

An automatic flash is one that can limit the flash duration and therefore the amount of light that it outputs. The control function is performed by electronics contained inside the flash and guided by a light-sensitive sensor located on the front of the flash. When the sensor detects that enough light has returned back to the flash, the control electronics terminate the flash discharge. In the simplest flashes, the flash capacitor is shorted (so the extra charge is lost), but in more advanced ones (those labeled thyristor), the charge is preserved for the next flash discharge.

An automatic flash achieves proper exposure not only when GN = d * F, but also when GN > d * F.

Automatic flash operation achieves correct exposures not only in "straight-on" applications, but also when the flash head is tilted, swiveled or covered with various gels and reflectors. To perform its calculations, the flash assumes that some F (usually f/4 or f/5.6) is selected on the lens. Failure to match F results in under- or over-exposure.

Program Flash

Pentax did not make any purely automatic flashes. Instead, they added another feature, called "program flash," and gave the flashes in this category the SA designation.

Program flashes solve the problem of the photographer forgetting to set the correct F before every flash exposure. When used in program exposure mode, the flash and the body work together to set F. This capability requires an extra contact, mode, so the flash can "tell" the body what brightness it will produce. Knowing the film speed, the body calculates and sets F.

Some SA flashes have the extra functionality of signaling when proper exposure was achieved. This is done either by lighting up a lamp on the back of the flash or communicating with the body (via the ready contact) and letting it make the flash symbol in the view-finder blink.

George de Fockert has suggested that the brightness level is encoded by the flash as different frequencies. He found the following:

Table 1. Flash frequency and f-stop data.
Flash Flash Setting Frequency (kHz) F at ISO/ASA 100
AF200T Auto (red) 1 2.8
TTL 2 4
Auto (green) 4 5.6
AF080C TTL 8 8

From this table it looks like there is a simple relation between the frequencies and the selected aperture. Moreover, it looks like the TTL mode does not have a special (unique) frequency.

Through-the-Lens (TTL) Flash

TTL flashes are easiest to use. The main principle is similar to automatic flashes, but the light-sensitive sensor is located inside the body, behind the lens (hence the TTL designation). When this sensor detects that enough light has reached the film, the body commands the flash to stop discharging. Thus no matter what aperture is selected or what filters may be mounted on the lens, proper flash exposure will be achieved (assuming that the required brightness is within the capabilities of the flash). Note that a TTL flash will work in all exposure modes.

Pentax TTL flashes carry the "F" or "C" designation. TTL flash operation is achieved without adding an extra contact in the hot-shoe. George found out that the body notifies the flash via the mode contact by holding it low (0 Volts) until enough light has reached the film plane. Sam Rentmeester has suggested that the TTL flash sensor is always active and "stop discharging" notification is sent to the flash regardless of whether the flash is in TTL mode, or even regardless of whether there is a flash attached to the hot-shoe.

The Pentax LX body has a TTL-capable hot-shoe as well as a PC socket with two round contacts in close proximity. When the appropriate cord is used (4P sync cord A), TTL flash is possible using both the socket and the extra contacts.

Digital-Control Flashes

Pentax flashes that carry the "F" designation are controlled in a digital fashion (from now on we'll refer to them as "F-flashes"). F-flashes and the AF bodies are capable of second-curtain synch, contrast-control flash, and auto AF spot-beam. They all have an extra contact in the hot-shoe, digital. This contact probably tells the body that an F-flash is installed, and then a digital protocol is used for transmitting information about focal length, AF activation, beginning of travel of second curtain, etc. Other information may also be exchanged, i.e. film speed, subject distance, etc.

F-flashes receive information from the body about the focal length of the lens in use. They warn when the flash coverage is not enough, and some even zoom their heads in and out in order to match the lens' field of view.

Only the AF bodies achieve dedication with the F-flashes. However, all Pentax bodies, AF or not, are capable of controlling all non-F flashes.

Contrast-Control Flash

This seems to be a Pentax-only feature (supported only by the F-flashes). It refers to the use of two flashes: one as the main light source, and the other one as fill-in. The advantage of this feature is filling in the hard shadows that result from the use of a single flash unit.

Typically one uses an off-camera flash as the main light source and the camera's built-in flash as fill-in. It is, however, possible to use two off-camera flash units by setting the main one to contrast-control mode and the other one to "normal."

This flash mode requires that the body meters the two flash units separately. Because of that, the two flashes are fired one after the other and this forces a 50% lower X-sync speed.

High-Speed Flash

In this mode of operation (flash photography at shutter speeds higher than the X-sync), the film plane is never fully exposed, so there is no single appropriate moment when the flash can fire and light up the scene. Instead, the flash fires multiple bursts of light throughout the shutter curtains' movement and all parts of the film plane receive equal amounts of light.

The flash GN is reduced significantly in this mode of operation.

Slave Flash

Slave flash refers to an operation mode where the (slave) flash unit not attached to the body by any means is able to discharge as soon as another flash (the master) discharges. This is possible due to a special light-sensitive sensor inside the slave unit. When the body's built-in flash is not able to illuminate the entire scene, a slave flash can be set up. However, since it does not communicate with the body in a any way, no automation is possible and the slave must be used as a simple manual unit.