FP stands for Forward Point (forward in time)

Discussion in 'Lighting Equipment' started by frank_skomial, Jun 15, 2004.

  1. Seems a lot of confusion about fast flash sync FP. Some call
    it "Fast Pulse", some call it "Focal Plane", but what it really is?

    Everything happens at focal plane, so naturaly FP is called "Focal
    Plane" and fits many people well. Fast Pulse perhaps makes more
    sense, and fits many more people. How about Forward Point? - that
    depends how deeply you are willing to understand it.

    When we talk about high speed flash synchronization, perhaps FP
    means something else?

    I think FP is an electronic implementation of the Forward Point
    flash synchronization.

    Where FP came from?
    In older days, it was possible to have very fast flash
    synchronization, faster than the camera x-sync speed, with a slow
    burning single use bulbs. This was done with single burn bulbs that
    contained a magnesium powder and blasted one shot of light. Since
    the burning required some time from ignition to full brightness, a
    Forward Point (forward in time) was used for this type of
    synchronization. The FP voltage signal was sent to start burn the
    bulb at the time when SLR mirror started moving up. For electronic
    flash there is an X-Sync method. X-sync trigger signal is usually
    sent to electronic flash when a focal plane shutter is open wide -
    presumably as wide as the width of the film frame (on horizintal
    travelling focal plane shuttere) - this is for single shot lighting.

    Faster than camera x sync shutter setting results in shutter
    openning narrower slot, much narrower than the frame width (or frame
    hight on vertical travel shutters). The shutter narrow slot sweeps
    across the film plane, but at any time only a narrow part of frame
    is exposed to the light.

    Magnesium burning bulbs provided bright flash lasting longer than
    the time needed for the shutter slot to move across the film frame.
    This way entire frame was exposed, and the shutter was faster than X
    sync. The Forward Point (forward in time) was needed to accomplish
    this without electronic flash involved.

    When single shot electronic flash is used and shutter is set faster
    than X sync, only a narrow portion of the film is exposed. (this is
    usually prevented on modern electrionic cameras). It is
    technically difficult (or impossible) to prolong duration of an
    electronic flash to cover entire time event of narrow shutter slot
    traveling across wider film frame. Once the high voltage capacitor
    is shorted through the flash bulb by the trigger signal of the
    shutter, it just blasts too fast to cover entire film frame.

    To implement electronic equivalent of the old FP method, it was
    necessary to design flash units that produce multiple flash blasts
    (strobes) is sequence and adjucent to each other, thus effectively
    prolonging the duration of the flash to cover time required for
    narrow shutter slot travel time.

    What happens in some Nikon and Canon flashes that have FP (also
    called a high speed sync or something like it). When camera is set
    to speed faster than the X Sync, and the flash is set to FP, then
    the flash computes how many strobes it needs to cover entive frame
    with consecutive blasts. Max power of each strobe in sequence is
    limitted to allow power for the rest of the remaining number of
    strobes to make, so the maximum range of the flash unit is reduced
    by the number of flashes needed.

    I think FP stands for electronic implementation of Forward Point
    technique, a technique that allow faster than X Sync shutter speeds
    with flash. This is now basically a technique that allows to
    substitute one blast with multiple electronic strobes that are
    adjucent and (hopefully) equal in strength.

    I think if you explain FP as a focal plane, you will not prompt for
    additional questions. If you explain FP as Fast Pulse, perhaps no
    one will ask more. However, if you try explain FP as Forward Point,
    certainly this could trigger an avalanche of questions. As long as
    it works and people are happy, let FP be called anyway you wish.
  2. I found an additional good place for this subject:
    http://photonotes.org/articles/ eos-flash/#fp You may need to scroll to PF section.

    The site has it: "Such bulbs produced light quite rapidly and sustained their light output for the full duration of the shutter opening. They were called FP bulbs." ...but it does not explain the meaning of FP. There was a reason why the bulbs were called FP, that is the Forward Point synchronization, and that is why some cameras, e.g. Pentax 6x7, have second sync socket marked FP for use with these bulbs. By the way, DO NOT USE the FP socket for your electronic flash synchronization, though it is unlikely you will find FP socket on modern cameras. Electronic flash in FP mode uses X Sync socket.
  3. FP has always stood for Focal Plane. In the days when flashbulbs were king, cameras like the Speed Graphic let you choose whether you wanted to use a focal plane shutter or a leaf shutter. Leaf shutters could always synchronize with flashbulbs at any speed, but if you wanted to synchronize with a focal plane shutter at high speed, you needed to use FP bulbs.
    Your invented "Forward Point" terminology makes no sense for flashbulbs. ALL flashbulbs require that the "fire" signal be sent to the bulb at your "forward point" before opening the shutter. Unlike electronic flash, bulbs take awhile to start burning and producing light. Most cameras don't have a different sync setting for M versus FP bulbs. The much more common M bulbs can't sync with a focal plane shutter at high speeds, because they reach peak brightness and fade very quickly. If you want to sync with a leaf shutter at high speeds, you can use any bulbs, including the common and cheap M bulbs. If you want to sync with a Focal Plane shutter at high speeds, you have to use the slower-burning FP bulbs. FP bulbs have no advantage over standard M bulbs unless you're using a Focal Plane shutter.
    Your invented "Forward Point" terminology also makes no sense because those aren't the words that ordinary English-speaking people use to describe something that happens before something else.
    Your invented "Forward Point" terminology is not what appeared in the manuals and literature of the day.
    If you want to invent a "backronym" and call FP sync "Forward Point" sync, go ahead, just like I sometimes have said EOS stands for "Extremely Overpriced Stuff". But just don't claim that that's anything like a recognized meaning of the term.
  4. Thanks for the link to my site. However, I'm afraid I have to agree with Richard on this one.
    "Forward point" may be a valuable mnemonic for you, but I've never seen anything in any
    literature anywhere indicating that this term has ever been used for "FP" flash. Even a
    Google search brings up just one relevant hit - a post you made here last month.

    Additionally you state that, "Everything happens at focal plane". I'm afraid that this is not
    strictly true. As Richard points out, many medium format and large format cameras use
    leaf shutters in the lens; nowhere near the focal plane.

    Out of curiosity, do you have any sources to back up your contention that the term
    "forward point" has historical basis?
  5. And thus Myths and Legends are made........
  6. Yes, I have printed instructions for bulbs that require "Forward Point" synchronization. Printed in 2 languages, English and an Oriental language I cannot read. Instruction is more specific, specyfing values for the Forward Point in milliseconds. For class M bulbs required 20 msec, for class S requires 30 msec, and for class FP requires 40 milli seconds. The FP bulbs were not invented for focal plane shutters. They were manufactured as an economical alternative for then non-existent or expensive electronics. Then the Forward Point synchronization sockets were built into some cameras with focal plane shutters to accomodate readily available bulbs. Some older leaf shutters had 2 sync sockets, FT sync and X sync. The leaf shutters are open more or less all the time during exposure so Forward Point synchronization is not necessary, and FP socket was dropped from leaf shutter lens designs. However, the Guide Number of the bulb (per the instructions) largely depends on the leaf shutter speed, e.g at 1/50 sec guide number is 4 times larger than at 1/1000 sec.

    I am not inventing anything. As I stated: "that depends how deeply you are willing to understand it"

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