Flash Photography with the Minolta Direct Autoflash Metering System

The introduction of the PX series of flashes with the Minolta Program System made using flash easier than ever before.
Image Copyright Minolta

Before the introduction of the X-700 in 1981, photographers had used either manual calculation for flash, or more recently, auto flash units that had a small sensor inside them that measured the reflected light from the subject, and instructed the flash to shut down when exposure was adequate.

However, auto flash units were not perfect, as even on advanced flash units the number of apertures that could be used were few, normally three or less. This limited the capacity of the photographer to select a particular aperture for a creative purpose. Additionally, the sensors on the units did not match the coverage of the lens being used, and being fixed on the flash unit, were unable to assist with exposure if the sensor was blocked, for example by a diffuser.

Finally, auto metering was near useless when a photographer was using filters that affected the amount of light hitting the film, or was doing macro photography, where extension rations resulted in less light reaching the film.

The release of the X-700 with its Direct Autoflash Metering System, commonly known as TTL (for "through the Lens") flash metering system, was the first step in an evolution of flash photography, making it easier and more accurate than ever before.

Cutaway image of a X-700 with Motordrive 1, 35-70mm f/3.5 MD zoom, and 280PX Flash
Image Copyright Minolta

When switched on, the flash capacitor would commence charging. Once charging was completed, the flash would send a signal to the camera, instructing it to switch to the X-sync speed of 1/60th second. This was nothing new, as the Minolta X series flashes had been doing this for several years.

However, upon firing the shutter button, everything was different. Rather than the flash metering the reflected light itself, the camera had an extra silicon photocell adjacent to the mirror box that assessed the amount of light reflected off the film, enabling it to determine when sufficient light had been emitted for correct exposure. Upon determining this it could instantaneously advise the flash to shut down.

In order to put this new technology into effect, Minolta had to design a range of new flashguns that incorporated the TTL technology. This range of flash units, designated as PX flash units included the 360PX, 280PX, 132PX and a completely new dedicated ring flash, the 80PX.

The PX flashes incorporate two extra pins on the flash mount. One is the automatic flash sync pin that alerts the camera that the flash is fully charged, and automatically sets the camera to flash sync and activates a blinking “flash ready” signal in the viewfinder. The second pin is the TTL pin, via which the camera tells the flash to cut the output once sufficient light has emitted for correct exposure.

Analysis shows that the flash provides a 2Hz charge of approximately 2.8v that flashes the "60" in the viewfinder to provide the "flash ready" signal. This is done via the dedication pin (left pin). The TTL Pin (right pin) maintains a constant 1 volt current which drops to 0v immediately the flash is fired. When the camera determines the exposure is sufficient a 22 millisecond 2.5v signal is sent from the camera via the TTL pin to the flash, and the flash will cease emitting. If there was insufficient light from the flash for a correct exposure the camera will wait one millisecond before sending the 22 millisecond 2.5v signal to the flash. If the flash had not used the entirety of its charge upon receiving the 2.5v signal from the camera, it will send a 2.8v current to the camera at 8Hz via the dedication pin to flash the "60" in the viewfinder at 8Hz for half a second to signal correct exposure.

It is noted that the FDC lamp and correct exposure 8Hz flash in the viewfinder will fire even if the film records too much light for a correct exposure. If for example, the flash is to close to be sufficiently throttled down by the TTL signal, the film may be overexposed but the FDC lamp will still light, as the flash will have received a 2.5v signal from the camera prior to using its entire charge.

Thanks to Robert Hoehne for his assistance in determining the exact process used by the camera and flash to manage the exposure.


Advantages of the Direct Autoflash Metering System

While in the summary above some of the advantages of the Minolta Direct Autoflash Metering System were briefly discussed, the key benefits of the system and their practical applications will be detailed below, using examples from Minolta literature from the period.
Flash Range
Image Copyright Minolta

As discussed earlier, one of the disadvantages of auto flash units was that they had a limited number of aperture selections, normally three or less, and often only one. Given that the settings were designed for use with multiple lenses, it was often unlikely that one of the settings would match the minimum aperture of the lens a photographer wished to use. This meant that the photographer was forced to use a smaller aperture than might be his preference, resulting in reduced flash range.

The use of TTL metering eliminated this shortcoming, enabling photographers to get flash range previously unavailable to them without tricky manual flash calculation. In the image above the smaller crop shows the result at an aperture of f/5.6, compared to the well exposed main image, shot at f/1.4

Macro Flash
Image Copyright Minolta

One of the difficulties with macro photography is that long extension through a bellows or extension tubes results in light loss. This loss of light, and the fact that the photographer often wants to shoot at smaller apertures to increase depth of field, often means that flash is necessary with macro photography.

This problem had historically resulted in photographers having to carefully measure flash to subject distances, and then perform manual calculations to determine the appropriate aperture. Naturally, this method was difficult and time consuming. The new Minolta Direct Autoflash System resolved all of these problems, making macro photography a breeze.

The capacity of the system to meter directly off the film means that no calculations are required and the photographer achieves freedom to concentrate on composition.

Lens and Flash Filter Use
Image Copyright Minolta

When shooting with lens filters with a traditional manual or auto flash system the photographer had to be aware of the filter factors applicable to each filter used, and adjust the exposure accordingly. With the Minolta Direct Autoflash Metering system, this problem was resolved. Naturally given the flash output was measured through the lens, the exposure automatically compensated for any filters used.

In the image above the shot was taken in the middle of the day with a neutral density filter to reduce the ambient light to a level below the sync speed (effectively underexposing the background slightly). Then the flash has been used with an orange panel from the Panel Set for the 360PX, to simulate sunset. The camera has ensured that the flash output was sufficient to ensure a correctly exposed photo, with the final result being very impressive.

Multiple Flash Use
Image Copyright Minolta

When shooting with a multiple flash set up, photographers had traditionally had to use complex calculations or expensive flashmeters to ensure well exposed photographs. However, with the Minolta Direct Autoflash Metering System this was no longer required.

The Minolta system included cables and a three way splitter designed to provide TTL control of multiple flash units, enabling quite complex flash arrangements to be set up with TTL control for perfect exposures every time. In the photo above, the image shows the difference achieved between using a multiple flash set up as opposed to a single flash.

A Schematic of the Minolta Direct Autoflash Metering System
The schematic above shows an example of the ways the system can be connected to achieve the results desired by the photographer. These connections are made through the use of special cables. Not all cables in the system are for the control of flash, and to assist with the understanding of this a brief description of these different cables and their uses is provided below:
  Cable OC
The Cable OC is the most commonly used cable in the system, being the cable designed to take a flash off camera (hence "OC"). This cable can be connected to a 360PX through a plug on the side of the flash, enabling simple off camera use for basic flash techniques. It also is the cable used with the Power Grip II to connect the camera with the grip. It is the only cable that communicates directly with the camera and accordingly is a requirement for any off camera flash arrangement.
  Cable EX
The Cable EX is used as an extension for any of the cables in the system. It may commonly be used in series to connect a flash that is some distance from the camera to the camera.
  Cable CD
The Cable CD is used to connect two PX series flashes together. When connecting multiple units the photographer can use a triple connector (refer the schematic above) or simply connect the various flash units in series. The Cable CD can be connected from the plug on one 360PX to another flash, permitting this set-up to be used.
  Off Camera Shoe
The Off Camera Shoe is used with the various cables to connect to PX flash units. It has a tripod socket on the base, and a socket for connection to the system cables on the front. While the 360PX has a socket in the side for a cable to connect to, the simpler 280PX and 132PX do not have this socket. Accordingly these flash units require the use of an Off Camera Shoe to be able to be connected to the system. It is noted that while the 360PX does not require the use of a shoe for connection via the cables, it is often preferable to use the shoe in order to provide the flash with a point for tripod mounting.
  Off Flash Sensor
The Off Flash Sensor is a cable that connects the flashes to the camera, but without TTL. It includes a small sensor that enables Auto flash control with an off camera flash. However, it is not a part of the Direct Autoflash Metering System as it does not use TTL
  Cable MD
The Cable MD is used to connect the X-700 with Motordrive 1 attached to the Power Grip II, enabling use of the shutter release on the grip. The cable also activates the viewfinder display when the shutter release is partly depressed.
  Cable AW
The Cable AW connects a camera without the Motordrive 1 to the Power Grip II. While the connection is actually to the cable release socket on the camera, the cable is called a Cable AW because the manufacturer envisages that such an arrangement would always involve the use of an autowinder for automatic film advancement.
  Cable FB
The Cable FB connects the Power Grip II with a Multifunction Back to enable flash control in programmed interval photography. When using the Multifunction Back to manage interval photography that requires flash, the Cable FB sends a signal to the Power Grip II telling it to wake up the flash prior to the moment of exposure ensuring that the flash has time to reach full charge.
Shortcomings of the Direct Autoflash Metering System
The Minolta TTL system designed for the X-700 and X-500/570 was, as detailed above, a huge step forward in the use of flash in photography. However, particularly when compared to more modern TTL flash systems, the Direct Autoflash Metering System has some drawbacks. Some of the key areas that I consider are drawbacks include:
  Expense and Availability

The cables required to build a studio flash system using the system are relatively rare, and can be expensive. In the used market the cables average about US$20-$25 each, and given each cable is approximately one metre long when extended a simple studio set-up can be an expensive proposition.

One alternative is to use after-market flash cables, or to extend existing cables. Altrex has made a cable for the X-700 that is often available second hand. This cable actually has a second shoe fitted so that it can easily be used with any PX series flash. Best of all, the cable used is a flat cable (like that used for telephone extension leads) and accordingly it can be simply extended. The photo below shows my Altrex Duo-Sync Cable (the black cable) which has been cut and standard telephone connectors installed.

The Altrex Duo-Sync Cable for X-700 with home made extensions enabling up to six metres of cable to be laid. The nature of the Altrex camera connection means a Cable OC can also be run from the hotshoe. Total cost of this was approximately $20
It is noted that Minolta advise that operation may not be correct if a total of more than six cables are used, of any description. I assume in this respect it means the maximum cable length should not be more than 6 metres.
  Lighting Ratios

Unlike many later flash systems, there is no capacity to use lighting ratios with the Minolta Direct Autoflash Metering System. Minolta advise that if this is required the best method for using lighting ratios with true TTL is simply to use distance to adjust the ratios of the various light sources.

The signal to shut down the flash units comes when sufficient light has hit the film. If one flash is closer to the subject, and both units shut down at the same moment, the closer flash will have a greater effect. In this respect, placing one flash 1.4 times the distance to the subject than the other will result in a one stop difference. Placing it 1.7 times the distance will result in a 2 stop difference.

I know it is a simple issue, but it is true that placing cables all over a studio is just inviting someone to trip over them, pulling down your expensive gear. My advice? Be carefull!

I find the Minolta Direct Autoflash Metering System to be of immeasurable assistance with macro photography and general photography. The capacity to use a telephoto lens such as a 200mm and know that the exposure has been based purely on the light hitting the film, and not an arbitrary sensor angle, provides me with great confidence.

For home studio use, the system is less useful, and a traditional approach using a flashmeter and manual flash control is probably better for the management of fill ratios. That said, the TTL system is very easy for an amateur photographer to use, and does not require the purchase of an expensive flashmeter while providing very consistent results.

Overall, the Minolta Direct Autoflash Metering System was a huge step forward for photographers in 1981, and even today provides excellent results to those who use it intelligently.

Thanks to Robert Hoehne and Andrew Duncan for their assistance in the preparation of this page.

The Minolta Direct Autoflash Metering System.
Image Copyright Minolta
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