I think Huw has it right. This has always been my understanding of how the sync works in a leaf shutter.
In any case one only needs speeds faster than 1/125 s. when ambient light levels are very high, as in bright sunlight, or if one were using an extremely low power flash, perhaps to stop action. With normal strobes in ordinary room light all at conventional levels, it would be unusual that the strobes would not be at least four stops brighter than ambient.
" In other words, the delay you refer to is not something independent of the shutter blades, but is actually part of the same system. This is a direct mechanical linkage and provided the shutter is not faulty there is no way that it can fire the flash before this point."
There is NO misunderstanding with how a shutter works. What you have posted is exactly what I said - but, it is a delay from the time the shutter is first triggered - whether is directly mechnically linked or not. On one of my older lenses, I have several sync positions for different types of flash bulbs plus an "X" sync for electronic flash. I assume that a different gear train is engaged to vary the delay to trigger the flash bulb sooner (shorter delay) than at "X" sync in order to let the flash bulb reach peak illumination when the shutter is fully open - and take into account how long it is illuminated. Change types of flash bulbs sync delay is changed to account for time it takes the bulb to get to peak illumination and how long the bulb actually burns.
My only reason for posting is to alert Prime that IF he uses a fast shutter speed with an electronic flash under certain circumstances he may underexpose the film. So, if he gets under exposed film with a correctly set f/stop and flash power setting, the cause could be because of a high shutter speed - and not some mysterious force from the planet Saturn, the moon being out of alignment with his horoscope sign, a problem with his camera, or not wearing his aluminum helmet at the proper angle to protect himself from aliens....
At this point - I think we're beginning to beat that proverbial dead horse.
Steve, I'm sorry if you think I have misinterpreted you, but your initial post on this subject did very explicitly state that, 'leaf shutters have a built in delay (for electronic flash - X-sync) to time the flash to compensate somewhat for the shutter not being completely open when the flash is triggered'. This is simply untrue, because the way that leaf shutters work means that the blades are always fully open when the flash is triggered.
You went on to say that, 'the faster the shutter speed you are using with a flash, the greater the exposure problem...' Well, this is true, but only because the flash may be too long in duration for the open time of the shutter. It has nothing at all to do with the acceleration/deceleration times of the shutter blades. It would be true even if they could open or close instantaneously, and in fact the problem would be worse if that were the case.
You also said, 'the larger the shutter, the greater the problem because of greater mass, etc.'. I'm sorry, but no. Large shutters do indeed take longer to open and close, and this is why, for example, Copal #3 shutters have 1/125th as their fastest speed. But it does not affect flash sync.
Finally you said, 'my advice is to expose at 1/60th second or less using an electronic flash with a leaf shutter'. I'm sorry if it upsets you but I think this is bad advice, and it is the main reason I posted a follow-up. Only when using very high flash powers is it necessay to use slower shutter speeds, and that only for the reasons stated earlier, and it would be true regardless of what type of shutter was in use. For most handheld units you can safely use shutter speeds up to 1/500th with leaf shutters almost all the time, without any danger of the shutter itself cutting off the flash before it is fully discharged. For a few handheld units and for studio flash at higher power settings it may be necessary to stick to shutter speeds of 1/250th or possibly 1/125th and slower. To use speeds of 1/60th or slower whenever using flash is quite unnecessary, and squanders one of the main advantages of leaf shutters.
Gee...you know we're both writing in English but not communicating. I'll try waving my hands on this one.
First let's define "delay." That is the time from the moment the shutter is released until the flash is triggered. The flash on a leaf shutter is NOT triggered immediately upon shutter release, so by definition there is a delay. OK??
When you release a leaf shutter, there is a gear train connected to a spring drive gear mechanism that provides flash synchronization. This gear train is the "delay mechanism" AND the sync mechanism in the shutter. Gears have to turn a certain distance (delay) before the flash is triggered (trigger mechanism). If you would like confirmation of the sync gear train, please get a service manual for a Compur or like shutter and you will see the part numbers for the flash synchronization gears, etc.
The flash is not triggered by the blades reaching the open position but by the gears and how far they have turned, which MOSTLY corresponds to the position of the blades being fully open - but, does not EXACTLY correspond to the fully open position at ALL shutter speeds. There are inaccuracies inherent within timing of the gear train trigger mechanism versus shutter speed setting.
Hence the information from Stroebel about the flash being measured as 59% less at 1/500th than at 1/100 or less - something YOU seem to want to ignore - actual tested, and documented results by a recognized expert in the field of photography and photo science. Not an opinion, not conjecture, not speculation - documented facts.
As for exposing at 1/60 or less - yes that's still my advice if you are using manually set power levels and NOT an automatic flash with a quench circuit - AND you have a shot that cannot be easily duplicated. When you talk about long flash exposure times only at full power you are both correct and somewhat wrong. If the flash uses a quench circuit then yes, the flash duration gets rapidly shorter than the longest duration at full power because you are using a fully charged capacitor, and stopping the current when the appropriate exposure has been reached.
With this type of flash operation, flash speeds can be as short as 1/10,000 of a second. Most flashes with quench circuits will not go faster than 1/10,000 as when the flash time gets shorter than that, you can have a problem with reciprocity (failure of the law of reciprocity) just like you do with long exposure times.
Now, if you use a flash that allows power variation (1/2, 1/4 power, etc.) the duration is not effected nearly as much as the quench circuit limiting the exposure, because you are only discharging a partially charged capacitor. Using a flash in this mode rarely approaches a flash time of 1/1000 of a second.
My personal flash usage is mainly with large power packs (3000 watt/second ratings) and multiple flash heads in architectural photography. And yes, I have to run nearly everything at full power with multiple heads being used on a lot of occasions where I am not attempting to only provide fill for daylight, but fully illuminating an entire scene.
I rarely use a hand-held unit with thyristor quench circuit. As I stated previously, I have noticed under exposure using 1/250th of a second or higher shutter speeds and my strobes - hence, my comment about "your mileage may vary." Meaning, your personal equipment AND usage may NOT create this problem. Is that clearly understood? This is not a hard and fast edict for everyone. It is a problem that I have had and have documented as being related to the shutter speed. Not and equipment problem, not an exposure miscalculation, not a strobe problem, the shutter speed was too short.
You are correct, not everyone will have this problem, but my point is if you are doing flash photography using leaf shutters - YOU SHOULD KNOW ABOUT THIS AS A POTENTIAL PROBLEM. Your approach seems to indicate that it will NEVER be a problem. This is forum allows educational opportunities - I chose to point out a documented anomoly inherent within leaf shutter / electronic flash usage under certain circumstances.
My advice should have been - test your equipment under a variety of operating conditions, including power levels, and shutter speeds as underexposure is a real potential problem with certain flash / shutter speed combinations. Does that suit you somewhat better?
And, I too am sorry if you don't like my recommendations which are based on both photo science documentation AND personal experience - and MY ONLY POINT BEING (again, apparently ignored by you) - is that:
>>>IF<<< someone has underexposed flash photos
>>>AND<<< they are using a high shutter speed with a leaf shutter,
>>>THEN<<< it may be BECAUSE of the high shutter speed,
AND NOT AN ACCIDENT, OR MISCALCULATED EXPOSURE.
I'm truly sorry IF YOU don't get that simple message.
Hi Steve. While it is true that your last post said nothing new, it simply being the most amazingly detailed reiteration I've ever seen, I do have a very slight problem with your 1/60 sec recommendation. I think that Kodak's recommended X-sync speed of 1/125 is probably a little better at lessening the possibility of ambient-light 'ghosts' than 1/60 sec would provide. Depends on the amount of ambient light, so your milage may vary ;-)
Having said that, I have an old shutter that just isn't up to snuff any more, and needs a speed of 1/50 sec or longer to work right. When the temperature drops below 50 F I can set it on anything; it slows down to the point that it gives second-long exposures every time.
I personally like to use flash in a manner that emphasizes natural light as much as possible, so when a flash is needed I typically have many-second-long exposures, and fire the flash manually, or (rarely) paint the scene with light. But in the end I don't use the flash much, so I will always concede any point made by someone more experienced with flash than I.
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Thanks Steve for the detailed explanation. I have a great deal of respect for Stroebel too - even if I don't have any problem of synchronizing even at 1/500 s. But that is for macro work and short distances with automatic flashes.
Steve, after this reply I am not going to waste any more time on this point, so you can have the last word if you wish.
You refer to the gear train and main spring drive mechanism - this is what drives the entire shutter - blades, as well as the flash sync. These last two are connected mechanically in a way which does not allow slippage or variation in the position of the blades at the point when the flash is triggered - unless, as I said before, the shutter is faulty. And I am not talking about general wear and tear, but a serious fault. Apart from this circumstance it is not possible for the flash to trigger before the blades reach the fully open position. Like it or not, this is a fact.
You keep on citing Stroebel, but nothing you have quoted is at all inconsistent with what I have been saying. Problems occur when the shutter begins to close before the flash has finished (and, yes, I know that flash output is not always a good approximation of a step function). This is true for all types of shutter, not just leaf shutters, and, contrary to your original reasoning, would be true even if the shutter were capable of opening or closing instantaneously. It is also rather obvious.
I can quite imagine the 3KJ packs you use would create a problem with speeds over 1/60th, but your advice related to electronic flash in general, without that qualification - a wholly unreasonable restriction. Furthermore, the problem has nothing whatever to do with the explanation you gave. Leaf shutters will indeed sync correctly at any speed, but sometimes a flash may be too long for the shutter speed selected - a point I made clear in my original post on this thread.
With that, I am out of here.
" It is not possible for the flash to trigger before the blades reach the fully open position".
HuwEvans, you are very much mistaken. As Stroebel has it - "Flash-synchronized shutters are designed to close the electrical circuit to the flash at the appropriate time so that the shutter will be fully open when the flash reaches its peak light output". It is for this very reason that the flash is triggered BEFORE the blades are fully opened so that the light peak - comming later, i.e. with a delay - can reach the fully open blades (hopefully, but not always). Stroebel speaks clearly about this, even figures showing the delays are provided by him.
"Like it or not this is a fact".
Without a more complete rendition of the quotation (until I get home and can look it up), I'm hesitant to accept Stroebel's quote, as he might be speaking of M-synced flash (the setting used for magnesium-wire flashbulbs). Bulbs must be triggered before the shutter is opened to allow the burning reaction to get going.
That being said, I think that shutters are made with a little slop in the flash-triggering mechanism. The greatest opening force, applied through the shortest and most controlled lever arm, is applied to the blades to get them moving. At the end of the opening sequence the lever arm is lengthened (to provide more velocity) which means the opening force is much diminished, and the mechanism relies on the inertia of the blades to achieve the full-open position. I suspect (without evidence) that if the blades had much friction, as they might in a horizontal position, that the full open position would be slowed, even though the opening mechanism was at its fullest extension.
But even if this is so, it would only affect exposure at the widest aperture. For typical, stopped-down shots, the effect on the exposure would be none, none...more black.
And herewith I officially enter myself as a belligerent in this thread.
I have to agree. That quote sounds like a precise description of M-sync flash. If you look at some old shutters like this Ilex pictured on S. K. Grimes' website:
you'll see there are numbers for different flash sync settings. These refer to the number of milliseconds in advance of the full opening of the shutter required for synchronizing different types of flashbulbs. For X-sync, one sets the shutter on the "0" setting, so that the strobe fires precisely when the shutter is wide open, not before, because the peak is virtually instantaneous. The only relevant concern here is for studio flash units, which might actually have a longer duration at full power than the shutter speed.