The previous page (here) explains how electronic flash works on cameras with Focal Plane shutters.

Because the duration of light from an electronic flash gun is extremely short (typically a thousandth of a second or less), the time when it is discharged must be synchronised with the time when the camera shutter is fully open.

The same restrictions on maximum usable flash sync speed apply in the 21st century (even on digital cameras) as they did 50 or more years ago:
if the camera has a focal plane shutter, this consists of two curtains that traverse the frame (or "film gate") and the higher exposure speeds are achieved not by moving the curtains faster, but by progressively exposing the film (from one side of the frame to the other) through a narrow slit between the two curtains.  The narrower the slit, the shorter ("faster") the exposure.

So the fastest electronic flash sync speed is the fastest speed at which the whole of the frame is simultaneously uncovered by the two shutter curtains.

On 35mm film cameras, this was generally 1/60 sec.  (See, for instance, the Minolta SR-T 101.)  The SR-T 101, like the earliest 35mm film cameras (which were not SLRs) had two shutter curtains that travelled across the frame horizontally, thus having to cover a distance of 36mm (the width of the frame in a 35mm film camera).
Some 35mm film cameras achieved a higher sync speed by having a focal plane shutter that traversed the frame vertically.  If the shutter only has to traverse 24mm (the height of the frame in a 35mm film camera), even without moving faster than a horizontally-travelling shutter, it will complete the job faster, and the highest speed at which the whole of the frame will be simultaneously uncovered will be higher.  Thus, the Praktica VLC, for instance, had a flash sync speed of 1/125 sec.

Medium format cameras with a focal plane shutter have a problem: the frame is bigger, so even if the shutter moves as fast as it does on a 35mm camera, it will take longer to traverse the frame.  This means that the fastest speed at which the whole of the frame will be simultaneously uncovered must be slower, typically about 1/30 sec.

In this, the Pentacon Six is no better and no worse than any other medium format camera with a focal plane shutter.

(Conversely, many digital cameras have a sensor that is much smaller than the standard 35mm film frame.  Because of this, the time that the focal plane shutter needs to cross the sensor is shorter and the shutter speed at which the whole of the sensor is simultaneously uncovered is higher, often 1/200 sec.)

But there is a way to get round this problem!

As stated above, the duration of the light output from an electronic flash is extremely short, typically 1/1000 sec or less.  But if you could get a flash gun that would produce a flash that would last several milliseconds, it could start transmitting light while a narrow slit of the film was being exposed (at a high shutter speed) and continue transmitting light at the same level until that slit had reached the end of its travel and the whole of the frame had been exposed.  In the Pentacon Six, the time taken for the two shutter curtains to traverse the frame is approximately 35 milliseconds.

The effect would be to give flash sync at all shutter speeds.  And the good news is that such flashguns do exist!

They are called Focal Plane Flashguns, or "FP" flashguns for short.

Note that these flashguns take "FP" flash bulbs, which are single-use bulbs.  And they are still available in the 21st century.  They have a constant luminescence period of approximately 40 to 50 milliseconds, more than enough to provide even exposure across the whole of the frame.

BUT
FP bulbs need time to build up to their target output level.  We could call this their ignition build-up time.  This ignition build-up time (or "start-up time") is laid down in technical specifications and is 16.5 milliseconds.  They then have a burning time, which is the time during which they maintain a constant, or virtually constant, light output.  This is long enough for the two shutter curtains to traverse the full frame width (or height, depending on the camera), thus giving the whole of the frame a correct exposure, even at the top shutter speed, 1/1000 sec in the case of the Pentacon Six.

However, you can't just mount an FP flashgun on your Pentacon Six, which (like all other cameras) is synchronised to fire the flash when the shutter opens.  What you need is a circuit that will trigger the flash 16.5 milliseconds before the shutter opens.  This is called an "FP" circuit, whereas a standard electronic flash circuit is called an "X" circuit.

The Pentacon Six was designed to incorporate an "FP" circuit and an "X" circuit, and the chassis on all of them has the space to accommodate both circuits, each with its own coaxial sync socket.  However, virtually all Pentacon Sixes left the factory with only the "X" flash sync circuit installed.  This was because by the mid 1960s most photographers had adopted electronic flashguns and so had no need for a camera with an "FP" socket.  Except in one country!  The USA.

So you might find a Pentacon Six that was manufactured for the US market that has both "X" and "FP" flash circuits.  If you do, you will know, because it will have two flash sockets, under the camera throat on each side of the lens (approximately at "5 o'clock" and "7 o'clock").

Most of the Pentacon Sixes that were sold with these two circuits and contacts were re-labelled with a different name.  They were called the "Hanimex Praktica 66".  You can read more about it here: Hanimex Praktica 66 The two flash sockets can be seen under the camera throat, “X” on the left and “FP” on the right.  The flash sockets are labelled "X" and "FP" by embossing of the leatherette covering next to the corresponding socket (although this cannot be seen in this photo).   [C396_17.jpg]

But some Pentacon Six TL cameras manufactured for export to the USA also had both sockets.  You can read more about this here.

Pentacon Six TL prepared for export to the USA [C396_14.jpg] Again, the two flash sockets can be seen under the camera throat, “X” on the left and “FP” on the right.

X, F and FP synchronisation These illustrations from "Praktina Technik" by Erhard Loose and Werner Kühnel try to give a diagrammatic representation of synchronisation with "X", "F" and "FP" flash.  "F" are the old-style flashbulbs, and I did not include them in the above description, as they offer no significant advantages over electronic flashguns ("X") and focal plane flash bulbs ("FP"). The diagrams apply to the KW Praktina 35mm camera, which from the mid 1950s to the early 1960s was one of the most advanced SLR cameras on the world market.  You can read a little more about it on this website here.  "KW", or Kamera-Werkstätten in Dresden, went on to design and manufacture the Praktisix and then the Pentacon Six (although they were subsequently swallowed up into the large State-owned company, Pentacon). In these diagrams, the horizontal axis shows time ("Zeit" in German).  The vertical axis shows the amplitude (or brightness) of the flash, indicated in the shaded area.  The sloping rectangle or parallelogram is a representation of the movement of the two shutter curtains across the film gate. The first diagram ("Bild 119") is for electronic flash.  "X-Kontakt" shows when the flash is fired, which is as soon as the first curtain has cleared the film.  The fastest shutter speed for electronic flash synchronisation with the first model of the Praktina was 1/40 sec, as indicated under the diagram with the flash symbol.  With the second model of the Praktina, the Praktina FX, the electronic flash synchronisation speed was raised to 1/50 sec (presumably by using a shutter that travelled 25% faster). The second diagram ("Bild 120") is for the old-style "F" flash bulbs.  It is possible to see that the flash duration is somewhat longer, but that it is necessary for the camera to trigger the flash ("F-Kontakt") before the first curtain has fully uncovered the film, to allow time for the light output of the bulb to build up. The third diagram ("Bild 121") is for "FP" flash bulbs.  Three things are immediately obvious: the duration of flash illumination is substantially longer; it is necessary to trigger the flash (close the circuit) much sooner, before the first shutter curtain has even started to move ("FP-Kontakt"), to allow time for the intensity of the light output to build up; it is possible to use shutter speeds up to 1/1000 sec, as the level of the light output will be within acceptable tolerances during the whole of the time that is required for the slit between the two curtains to traverse the frame. Similar diagrams, slightly different in their details, could be drawn for "X" and "FP" flash with the Pentacon Six, Hanimex Praktica 66, Praktisix and Exakta 66. The caption for the FP flash diagram points out that the FP flashbulb guide number (the "Leitzahl") is greatly reduced with the high shutter speeds.  However, after a few tests it would be possible to determine the correct guide number.  Once this is known, a procedure can be standardised, choosing a standard shutter speed and varying the lens aperture dependent on the distance to the subject. For bibliographic information on "Praktina Technik", see here.

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© TRA January 2014