[Lord Lucan]

My question is about FP flash sockets on focal plane shutter cameras, and while this sub-forum is for film SLRs, and theoretically you could have an FP socket on a DSLR, I believe the LX was the last Pentax with one.

In the case of X sync, triggering is by a microswitch (or equivalent) actuated when the first shutter curtain reaches fully open. At sync speed (or lower) the camera focal plane shutter will be fully open at that moment and then no doubt for at least a few milliseconds more to give time for the flashgun to react, shine, and then fully extinguish. If it did not fully extinguish before the second curtain were released, the trailing part of the film would be exposed more than the leading part - hence the importance of instantly turning off the discharge of electronic flashguns as soon as correct exposure is reached and while the shutter is still fully open.

OTOH, my understanding is that the FP socket is for special FP flashbulbs that burned "slowly" at a steady rate for (I guess) around 1/20 second. Wikipedia says that "FP" stands for "Flat Peak" (not "Focal Plane" as I had assumed) - referring to the steady burn. I know that FP bulbs allowed focal plane shutters to be used above sync speed even up to highest speed because even the narrowest focal plane slot would be lit throughout its travel across the film because of the relatively long burn of teh FP bulb.

My question is : how is the FP triggering done?

In the case of the FP socket, the flash bulb surely needs igniting before the first curtain starts to move, because a flashbulb will not reach full brightness instantly - it needs to ramp up and only the peak is flat. How is this slight delay arranged? Does it rely simply on the time that the first curtain starts to move being more than the time the bulb takes to fully ignite? Otherwise the camera design would somehow need to introduce a short delay to starting the first shutter curtain when the shutter button is pressed, which I find hard to believe was done in those mechanical shutter cameras, for both technical and useability reasons.

[UncleVanya]

Are you talking about a PC socket? My K-3 has that. I'm not sure I am following you.

[stevebrot]

My question is : how is the FP triggering done?

On most cameras that support it, by a set of conductive points in the shutter mechanism.

I find hard to believe was done in those mechanical shutter cameras, for both technical and useability reasons.

Time to start believing. I have an Exacta VXIIa (Varex IIa) that supports M, FP, and X sync. It was made in 1957 and is very much mechanical in every aspect. The Pentax Spotmatic also supports both X and FP and instructions on how to adjust the points are in the PDF manual available at the Pentax Manuals Web site:

Pentax Manuals

Link to PDF service manual (page 31):

http://pentax-manuals.com/manuals/service/spotmatic_sm.pdf

The service procedure makes it clear how the two work.


Steve

[Lord Lucan]

Time to start believing. I have an Exacta VXIIa (Varex IIa) that supports M, FP, and X sync. It was made in 1957 and is very much mechanical in every aspect. The Pentax Spotmatic also supports both X and FP and instructions on how to adjust the points are in the PDF manual available at the Pentax Manuals Web site:

I meant that I found it hard to believe that the shutter would be deliberately delayed to allow the flash bulb to get going; because people want the shutter to fire asap when they press the button. Indeed it is not deliberately delayed, as your link to the Spotmatic service manual shows.

Thanks for that link, and it answers my question. The X trigger is by means of the first shutter curtain fully opening, as expected. The FP trigger is by means of the mirror reaching its almost fully up position (3mm short of it in fact) which will be slightly earlier, giving time for an FP flash bulb to get going. The X trigger actually requires both the mirror and the shutter contacts to be made. The diagrams on Page 51 (the last) are the most telling, the last diagram being the circuit diagram.

---------- Post added 03-16-18 at 04:41 PM ----------

Are you talking about a PC socket? My K-3 has that. I'm not sure I am following you.

A Prontor/Compur [PC] socket is the physical miniature co-axial socket to connect an external flash via a lead, and most DSLRs have one, and some older film SLRs had two (possibly even three). However, the PC standard AFAIK does not define how exactly it is used, and there are at least three types of use : M, FP and X (as Stevebrot mentioned). On modern cameras only the X type is used, being for modern electronic flashguns. The M and FP types were for two different types of glass flash bulbs which naturally modern cameras do not support. Ther is no physical difference between M, FP and X PC sockets, only the electronics behind them, so on cameras with more than one they are marked (eg on the LX and KM). My father had a 1950's camera with a single PC socket but a switch that changed it between M and X types. Hope that helps.

[stevebrot]

OTOH, my understanding is that the FP socket is for special FP flashbulbs that burned "slowly" at a steady rate for (I guess) around 1/20 second. Wikipedia says that "FP" stands for "Flat Peak" (not "Focal Plane" as I had assumed) - referring to the steady burn. I know that FP bulbs allowed focal plane shutters to be used above sync speed even up to highest speed because even the narrowest focal plane slot would be lit throughout its travel across the film because of the relatively long burn of teh FP bulb.

Milliseconds to peak is the key consideration where flash bulb sync is concerned, with length and consistency of burn being important when using focal plane shutters at greater than x sync speed. In contrast to electronic flash, things are sort of fuzzy where bulb flash is concerned. For example:

• FP and M sync provide essentially the same lead time
• While 20ms is the nominal time to peak for both M and FP bulbs, actual times may be as little as 5ms depending on brand
• Using M bulbs with M or FP sync may provide usable guide numbers at focal plane shutter speeds as high as 1/500s depending on bulb type. More expensive FP bulbs allowed 1/1000s.
• M bulbs with X sync usually required a step down in shutter speed. On the Spotmatic, that meant 1/30s.

I started using bulb flash for school functions. Electronic flash was expensive back then and bulb flash was still very common. I was glad when I was able to retire the flash gun, but still respect the tech and sort of get the wicked grin when reading threads on this site where people are trying so hard to duplicate the "Hollywood" look in broad daylight using HSS. With bulbs, that was the norm and easily attainable.


Steve

(...flash bulbs are still available...perhaps there might be a market for a wired shutter release for FP sync )

[UncleVanya]

I meant that I found it hard to believe that the shutter would be deliberately delayed to allow the flash bulb to get going; because people want the shutter to fire asap when they press the button. Indeed it is not deliberately delayed, as your link to the Spotmatic service manual shows.

Thanks for that link, and it answers my question. The X trigger is by means of the first shutter curtain fully opening, as expected. The FP trigger is by means of the mirror reaching its almost fully up position (3mm short of it in fact) which will be slightly earlier, giving time for an FP flash bulb to get going. The X trigger actually requires both the mirror and the shutter contacts to be made. The diagrams on Page 51 (the last) are the most telling, the last diagram being the circuit diagram.

---------- Post added 03-16-18 at 04:41 PM ----------


A Prontor/Compur [PC] socket is the physical miniature co-axial socket to connect an external flash via a lead, and most DSLRs have one, and some older film SLRs had two (possibly even three). However, the PC standard AFAIK does not define how exactly it is used, and there are at least three types of use : M, FP and X (as Stevebrot mentioned). On modern cameras only the X type is used, being for modern electronic flashguns. The M and FP types were for two different types of glass flash bulbs which naturally modern cameras do not support. Ther is no physical difference between M, FP and X PC sockets, only the electronics behind them, so on cameras with more than one they are marked (eg on the LX and KM). My father had a 1950's camera with a single PC socket but a switch that changed it between M and X types. Hope that helps.

That helped a lot. Thanks!

[Ontarian50]

I'd never heard of FP bulbs being called "flat peak". As FP sockets and sync settings were only ever seen on focal plane shutter cameras, it seemed to make sense it meant "focal plane".

If you trace the flash sync connections through a traditional camera, like an old Spotmatic, you find the FP sync hooked up to the mirror mechanism. When the mirror's tripped, a few milliseconds before the shutter starts moving, an FP bulb gets enough time to light up and get to that "flat peak" (apparently). Ideally, it's burning bright right through the whole time the shutter is moving - so you can use flash sync speeds higher than the sluggish 'ol 1/60th you get at X sync.

The X sync socket is wired to the first curtain drum, and it makes contact the instant that first curtain has opened fully - at 1/60th, the second curtain hasn't started closing yet. So you've got that tiny moment to capture the blink of the strobe.

As for modern DSLRs having an FP sync setting.... well ... they do.

The higher end DSLRs and flash units support what is commonly called HSS, or high speed sync today. Through electronic trickery, the flash pulses the flash tube so it stays lit for a few milliseconds longer than normal - so you can use it with a higher sync speed than normal. The penalty is all that pulsing results in a lot less brightness and flash range.

Olympus started using this trick way back on their OM-3 and OM-4 cameras, along with special flash units that could pulse their tubes. This was Olympus' response to the high-speed sync (1/250th) focal plane shutters coming out from Nikon. Since Olympus were still using horizontal cloth shutters in the OM-3 and OM-4, it allowed for a claim to higher sync speeds - even if it proved to be of limited use.

---------- Post added 03-17-18 at 11:02 AM ----------

I should also point out that while FP sync sockets, or FP sync settings are often found on older SLRs, the bulbs themselves seem to be very, very rare.

I have in a store room, a big box of various flash bulbs. I've got several sizes of bulbs, right from the big ones the same size as household incandescent lamps, all the way down to the tiny AG-1 size. I've got flash cubes, Magicubes, flash bars, flip flashes. I've got bulbs in clear, blue, and even a box of dark tinted infrared bulbs.

But I don't think there's a single box of bulbs marked FP. So I don't think the benefits of FP sync were as widely appreciated back in the day - at least not around these parts.

[stevebrot]

I should also point out that while FP sync sockets, or FP sync settings are often found on older SLRs, the bulbs themselves seem to be very, very rare.

I concur. I can only remember seeing them in camera stores and remembering that they were not cheap. M-class bulbs, OTOH, were widely available wherever film was sold.


Steve

[smilesnkisses]

It was triggered electr-mechanically.
A mechanical switch closed an electrical contact.
This was for almost every camera of that era.
M sync delayed the shutter (first curtain 0.01885 seconds as M bulbs reached peak brightness after 0.02 seconds. This allowed the FAST(est) shutters of 1/400th sec to capture the brightest segment of the brn and then start clsing the 2nd shutter at 0.02125 seconds. This balanced the rise/fall.
A 1/30yh sec shutter would start at 0.1875 and close at 0.05208 seconds getting 95% of bulb light (after the pre-peak moment)
1/25th sec opened 0.01875, closed 0.5875 capturing 98% of light after peak. Slower exposures added only ambient light capture

V was just a self time first pre 0.01875 delay

F/FP reached peak in 0.005 sec (vs M bulbs were 0.02 secs to peak) thus a DIFFERENT mechanical contact - electrical circuit was needed, and thus a different socket.

X sync is 0.0025 if i remember, but easier set with contact at full opening of first curtain.

Sorry to tell you but YES there was a built in delay from shutter release to when curtain (1st) started to move
M 0.01875 sec
F/FP 0.005 sec