Flash sync - followup question

Jeeeeeeeeze! Neil, Q.G.! Get a room or use email or something. You guys are
going round and round - and all alone!

jjs wrote:

Jeeeeeeeeze! Neil, Q.G.! Get a room or use email or something. You guys
are
going round and round - and all alone!

Not quite alone, John.
There's this "jjs" person watching us closely enough to be able to see that
we're going round and round.
;-)

"Neil Gould" wrote in message
om...
Recently, Q.G. de Bakker posted:

Neil Gould wrote:

Ok...
The shutter takes a definite time to cross the film gate, the
duration of the individual pulses is increased, but we still manage
to get the same number in that unchanged time the shutter takes to
cross the gate...
No. I need some help here. ;-)

The shutter speed is not a factor in this equation.

Who said shutter speed? ;-)
I'm talking about the definite time it takes the shutter to cross the
film gate. Which is a factor if you want the best distribution of
energy over time.

So... how is that not related to shutter speed? ;-)

In a focal plane shutter, that time is constant for shutter speeds above the
flash sync speed. The thing that changes is the width of the slit (i.e. the
distance between the first and second curtains).

David J. Littleboy
Tokyo, Japan

Recently, Q.G. de Bakker posted:

Neil Gould wrote:

Who said shutter speed? ;-)
I'm talking about the definite time it takes the shutter to cross
the film gate. Which is a factor if you want the best distribution
of energy over time.

So... how is that not related to shutter speed? ;-)

Thank you for agreeing, for changing your initial "the shutter speed
is not a factor" to "how is that not related to shutter speed"!
;-)

The shutter speed is *not* a factor related to the duty cycle of the
pulse. So, if you believe that I've changed, please accept that I have
not! ;-) I was responding to your statement, above, which suggests that
we are somehow referring to the "definite time... etc.", while at least
*I* wasn't referring to that aspect of exposure. It did get rather
confusing for me to follow that, however. ;-)

You are still going on about that duty cycle as if it posesses some
sort of saving grace.

I'm still not convinced that we're talking about the same thing. The duty
cycle of the pulse *will* determine how much light is available for
exposure. If it "possesses some sort of saving grace", it is one of simple
energy management, but as we aren't talking about power supplies, I don't
see how it's pertinent to the point at hand: how much light is available
for exposure.

I have asked it before, and will ask it again: why assume they wanted
to hold back, giving us a GN that's even more restricted than
technical thingies would allow it to be?

And, as I speculated before, it probably has to do with how many batteries
and bulk they thought the user would be willing to purchase and carry!
;-) But, we aren't discussing power supplies. ;-)

You're "lengthened pulse" can only be that if the "original" was too
short to begin with. We don't know that it is.

The existing pulse duty cycle *isn't* "too short" by design. However, it
*is* a design issue. ;-) You seem to believe that the flash tube
couldn't be steadily "on" for 1/25th of a second. Why is that?

All I'm saying is that a flash tube that is steadily on for 1/25th of a
second will provide more light than an equivalent one that is pulsed, and
that a 1/25th second burn time isn't likely to damage the flash tube,
because a typical photo flash tube can be on for a few seconds without
problems ;-) For example:

Shokai Far East Ltd.
Lamps: Flash Tube (Flashlamps & Flashtubes) Information
Manufacturer/supplier of straight, u-shape, helical, all ring xenon flash
lamps & transformers. Applications: general, photo flash display,
timing/signal/high fps, photo studio. flash rate (flashes/second): 1/7.5
seconds or less; 100 fps/more; & 1/10 seconds or less.

So, you can typically flash these tubes once for at least 7.5 seconds.
Seems safe enough for a 1/25th second burn to me. ;-)

But yes, it's easy to say they do something not quite right, and
provide an immediate solution, being they should do that which we say
they do not right right. Right?
;-)

I'm not saying that *anything* is done incorrectly w/r/t the design
parameters of the flash. What gave you that impression? I'm speaking of
the physics of flash tubes and photo flash technology. Within that
context, it is a simple statement to say that the pulse duty cycle affects
the amount of available light. I don't see it as a problem; as I said
before, I was clarifying the perspective that I'd been using to discuss
this matter.

Neil

"David J. Littleboy" wrote:

In a focal plane shutter, that time is constant for shutter speeds above the
flash sync speed. The thing that changes is the width of the slit (i.e. the
distance between the first and second curtains).

David J. Littleboy
Tokyo, Japan

Not entirely true. At higher shutter speeds there is some simultaneous travel
time of both curtains. This will reduce the overall amount of shutter travel
time down to a absolute minimum of the flash sync speed. I go over this in more
detail in my January 18th post. The bottom line is that with an "FP" sync,
either with flashbulb or with an electronic flash, the amount of time the light
has to stay on declines as the shutter speed gets faster.

Mochas Gracias John. I've been looking for this for at least 20 years. How
in the heck did you find it so easily/ Even those film experts on APUG were
of no help.

Truly, dr bob.

"jjs" john@xstafford.net wrote in message
...
"dr bob" wrote in message
...
There was an amusing photograph made in the early days of strobe. I've
forgotten the date and the photographer and the publication for that
matter.
It was a multiple exposure of "Duchamp Descending Stairway". an obvious
pun
of the famous painting. Does anyone reading here remember this? Can
someone
identify the photographer and/or the publication in which it was
originally
rendered?

Eliot Elisofon. 1952
http://elearning.winona.edu/jjs/duchamp_descending.jpg

For an enlightening site of material on Duchamp see:
http://www.understandingduchamp.com/

Neil Gould wrote:

So... how is that not related to shutter speed? ;-)

Thank you for agreeing, for changing your initial "the shutter speed
is not a factor" to "how is that not related to shutter speed"!
;-)

The shutter speed is *not* a factor related to the duty cycle of the
pulse.

That duty cycle thing again...
;-)

So, if you believe that I've changed, please accept that I have
not! ;-) I was responding to your statement, above, which suggests that
we are somehow referring to the "definite time... etc.", while at least
*I* wasn't referring to that aspect of exposure. It did get rather
confusing for me to follow that, however. ;-)

The thing we should worry about with these FP synch units is not in the
first place ho wlong each pulse lasts (though it is important) but that the
thing keeps pulsing as long as it takes the slit to traverse the film gate.
Right?
I'm still trying to explain how changing pulse length doesn't free us from
that constraint: the thing must keep pulsing as longs as it takes the [etc.]

I understand that for some reason you do want to increase the length of the
pulse, keep the number of pulses during the *definite* time it takes the
slit to [etc.] constant, and keep that *definite* time it takes [etc].
constant too.
The only way that is possible is by assuming the flash is off between each
pulse for too long (your "duty cycle" thingy). Else we would be stretching
time itself (neat trick, and not even impossible ;-))
If we assume that it isn't (and why would it be?), increasing pulse length
and [etc.] is simply not possible.

You are still going on about that duty cycle as if it posesses some
sort of saving grace.

I'm still not convinced that we're talking about the same thing. The duty
cycle of the pulse *will* determine how much light is available for
exposure. If it "possesses some sort of saving grace", it is one of simple
energy management, but as we aren't talking about power supplies, I don't
see how it's pertinent to the point at hand: how much light is available
for exposure.

Well, i believe John thinks we have been discussing exactly that for too
long already. ;-)

I have asked it before, and will ask it again: why assume they wanted
to hold back, giving us a GN that's even more restricted than
technical thingies would allow it to be?

And, as I speculated before, it probably has to do with how many batteries
and bulk they thought the user would be willing to purchase and carry!
;-) But, we aren't discussing power supplies. ;-)

No, we're not. We're discussing the output of the machine over time. Your
argument needs that to be intermittent.
And it is indeed intermittent. But to what degree? Enough to allow room for
extending the pulse length? Or so tightly packed that the pulses merge into
that wavy line?

The existing pulse duty cycle *isn't* "too short" by design. However, it
*is* a design issue. ;-) You seem to believe that the flash tube
couldn't be steadily "on" for 1/25th of a second. Why is that?

Do i? I don't think i do.

I think we're talking about units in which the output of the unit is pulsed,
i.e. many bursts following each other.
And that you now want to change the unit such that it is steadily on for the
duration, legthening the pulse until a single one covers both the beginning
and end of the exposure. (And that you want to do that by not changing the
number of pulses too, but i won't mention that now. ;-))

All I'm saying is that a flash tube that is steadily on for 1/25th of a
second will provide more light than an equivalent one that is pulsed, and
that a 1/25th second burn time isn't likely to damage the flash tube,
because a typical photo flash tube can be on for a few seconds without
problems ;-) For example:

That damage thing really doesn't bother me. It has no bearing on the matter.
I still don't see why you repeatedly mention it.

But yes, it's easy to say they do something not quite right, and
provide an immediate solution, being they should do that which we say
they do not right right. Right?
;-)

I'm not saying that *anything* is done incorrectly w/r/t the design
parameters of the flash. What gave you that impression?

You suggest that there would be some gain in lengthening the pulse,
shortening the "off"-time, saying that the pulse is too short, leading to
too little output, restricting already restricted output even more, while it
need not be like that.
Thank you, manufacturer, for imposing such unnecessary restrictions!
See?
;-)

Recently, Q.G. de Bakker posted:

Neil Gould wrote:

So, if you believe that I've changed, please accept that I have
not! ;-)

The thing we should worry about with these FP synch units is not in
the first place ho wlong each pulse lasts (though it is important)
but that the thing keeps pulsing as long as it takes the slit to
traverse the film gate. Right?

We have agreed about that point for quite some time, now. Once we got past
the matter of sync, it ceased to be an issue in this discussion.

I have asked it before, and will ask it again: why assume they
wanted
to hold back, giving us a GN that's even more restricted than
technical thingies would allow it to be?

And, as I speculated before, it probably has to do with how many
batteries and bulk they thought the user would be willing to
purchase and carry! ;-) But, we aren't discussing power supplies.
;-)

No, we're not. We're discussing the output of the machine over time.
Your argument needs that to be intermittent.

I take no position on whether the light output *needs* to be intermittent,
just that there is no technical reason why it *must* be.

And it is indeed intermittent. But to what degree? Enough to allow
room for extending the pulse length? Or so tightly packed that the
pulses merge into that wavy line?

My "argument" is a simple statement that intermittent light output will be
less than constant light output, and in that same sense, longer pulse duty
cycles will produce more light than shorter ones. ;-) How "wavy" that
line is will be determined by the physical characteristics of the tube,
e.g., how fast a cycle is possible (in the example that I presented, there
was a typical maximum rate of about 100 fps. That would suggest how many
pulses one might get during a 1/1,000 shutter speed setting). What the
manufacturer does with those parameters is a different matter (again
addressing your question, "why they wanted to hold back...").

And that you now want to change the unit such that it is steadily on
for the duration, legthening the pulse until a single one covers both
the beginning and end of the exposure. (And that you want to do that
by not changing the number of pulses too, but i won't mention that
now. ;-))

I'm not changing anything w/r/t the points that I've been making. It
puzzles me that we still miscommunicate about those points. I have the
impression that the term "duty cycle" isn't clear, or otherwise don't
understand why you bring up the notion that the duty cycle is somehow
related to the number of pulses.

All I'm saying is that a flash tube that is steadily on for 1/25th
of a second will provide more light than an equivalent one that is
pulsed, and that a 1/25th second burn time isn't likely to damage
the flash tube, because a typical photo flash tube can be on for a
few seconds without problems ;-) For example:

That damage thing really doesn't bother me. It has no bearing on the
matter. I still don't see why you repeatedly mention it.

Because it does have bearing on the only matter that I've been discussing
recently: how much light is available for exposure. How, then, can I
dismiss it? ;-)

But yes, it's easy to say they do something not quite right, and
provide an immediate solution, being they should do that which we
say they do not right right. Right?
;-)

I'm not saying that *anything* is done incorrectly w/r/t the design
parameters of the flash. What gave you that impression?

You suggest that there would be some gain in lengthening the pulse,
shortening the "off"-time, saying that the pulse is too short,
leading to too little output, restricting already restricted output
even more, while it need not be like that.

Thank you, manufacturer, for imposing such unnecessary restrictions!
See?

Sorry, but I believe that the conclusions that you are drawing -- "that
the pulse is to short... etc." -- are yours alone. I have not suggested
that different implementations of these parameters indicate a defect or
"unnecessary restriction". I've stated that these are simply design
parameters, and have tried to present the relationship to indicate a
trade-off. Perhaps, understanding them will affect a purchase decision?
;-)

Regards,

Neil