Flash sync - followup question

Fred Leif wrote:

Sort of like 'reinventing' the flash bulb ??

Yes. But now using long-burn electronic flash.
For the first part of this thread see the one called "New strobes for focal
plane shutters".

Fred Leif wrote:

Sort of like 'reinventing' the flash bulb ??

Yes. But now using long-burn electronic flash.
For the first part of this thread see the one called "New strobes for focal
plane shutters".

"Q.G. de Bakker" wrote:

wrote:

[...] If you had a flash unit
that could deliver say 1/25 sec of light, then the best time to fire the
flash would be at the start of the first shutter curtain's travel. Am I
correct?

Yes. The flash should be ignited the moment the first curtain starts to
travel.

It should have reached full level the moment the first bit of film is
uncovered. Which was a problem using flash bulbs, but should not be a
problem with electronic flash units.
It will have to keep output at a constant level during the entire burn
time/travel time to get even exposure.
And of course using shorter times/narrower slits will result in less
exposure.

Yes, but the faster the shutter speed the more narrow the slit, and the less
time the flash unit has to provide light. The maximum amount of time the
strobe will have to provide light is the "X"-sync shutter speed - assuming you
were using that speed - which no one will - and therefore you're actually
going to need less time since the second curtain will be trailing right behind
the first curtain. I'm thinking the best way of approaching this would be to
obtain an older body with an "FP" sync and try somehow to eliminate the built
in delay. I would think it would be more difficult to make an "X" sync body
fire at the beginning of the first curtain's travel.
Thanks for the answers

"Q.G. de Bakker" wrote:

wrote:

[...] If you had a flash unit
that could deliver say 1/25 sec of light, then the best time to fire the
flash would be at the start of the first shutter curtain's travel. Am I
correct?

Yes. The flash should be ignited the moment the first curtain starts to
travel.

It should have reached full level the moment the first bit of film is
uncovered. Which was a problem using flash bulbs, but should not be a
problem with electronic flash units.
It will have to keep output at a constant level during the entire burn
time/travel time to get even exposure.
And of course using shorter times/narrower slits will result in less
exposure.

Yes, but the faster the shutter speed the more narrow the slit, and the less
time the flash unit has to provide light. The maximum amount of time the
strobe will have to provide light is the "X"-sync shutter speed - assuming you
were using that speed - which no one will - and therefore you're actually
going to need less time since the second curtain will be trailing right behind
the first curtain. I'm thinking the best way of approaching this would be to
obtain an older body with an "FP" sync and try somehow to eliminate the built
in delay. I would think it would be more difficult to make an "X" sync body
fire at the beginning of the first curtain's travel.
Thanks for the answers

In message , writes
Yes, but the faster the shutter speed the more narrow the slit, and the less
time the flash unit has to provide light.


With a focal plane shutter, the curtains can move quite slowly: it's the
gap between them that gives the shorter exposure time.
Take the K60 for example, with an X-sync speed of 1/30. This means that
the first curtain probably takes about 1/30 of a second to traverse the
film, followed by the second one taking 1/30 of a second (providing it
sets off as soon as the first one gets to the end).
The electronic flash fires at the point where both curtains are out of
the way. The flash duration is short enough for this to work.

For faster shutter speeds, the curtains don't move any faster: all that
changes is the gap between them. The second curtain sets off before the
first one has completely traversed the film.

This means that your long-duration flash needs to burn for the time it
takes the second curtain to cross the film, probably 1/30. Providing it
can be triggered just as the second curtain starts moving and burns
until it stops, then you can ignore the first curtain. This would let
you use your long-burn flash at all of the higher speeds.

Different cameras will have different shutters, so the speed with which
the curtains traverse the film will be different. And vertically-run
shutters have a shorter distance to cross than horizontally-run ones, so
can X-sync at higher speeds.

Pity the poor camera designer trying to get a focal plane shutter to
wizz evenly across the long dimension of a 6x9 film gate.
--
----------------------------
Paul Friday

"Paul Friday" wrote in message
...

[... snip the good stuff...]
; Pity the poor camera designer trying to get a focal plane shutter to
wizz evenly across the long dimension of a 6x9 film gate.

Ah, or 120mm?

--
Mr. Flashbulb

"Paul Friday" wrote in message
...

[... snip the good stuff...]
; Pity the poor camera designer trying to get a focal plane shutter to
wizz evenly across the long dimension of a 6x9 film gate.

Ah, or 120mm?

--
Mr. Flashbulb

Mr. Friday, I believe your math is wrong - see below.

Paul Friday wrote:

In message , writes
Yes, but the faster the shutter speed the more narrow the slit, and the less
time the flash unit has to provide light.

With a focal plane shutter, the curtains can move quite slowly: it's the
gap between them that gives the shorter exposure time.
Take the K60 for example, with an X-sync speed of 1/30. This means that
the first curtain probably takes about 1/30 of a second to traverse the
film, followed by the second one taking 1/30 of a second (providing it
sets off as soon as the first one gets to the end).

1/30 + 1/30 = 1/15
If there's a 1/30 flash sync the second curtain cannot be in front of the film
until after the first curtain clears. Otherwise you wouldn't be able to expose
the complete film area.

The electronic flash fires at the point where both curtains are out of
the way. The flash duration is short enough for this to work.

For faster shutter speeds, the curtains don't move any faster: all that
changes is the gap between them. The second curtain sets off before the
first one has completely traversed the film.

In other words the distance between the first and second curtains gets smaller.
Therefore the amount of time the second curtain takes relative to the first
curtain's movement will get shorter and shorter. Example: 1/30 flash sync.
First curtain takes 1/60 to "open up" - second curtain takes another 1/60 to
close. 1/60 + 1/60 = 1/30. Now you up the shutter speed to 1/60. The gap
between the curtains is halved. The first curtain again takes 1/60 to complete
it's travel, then the second curtain, trailing behind it takes 1/120 to "close
up". Why - because the first curtain only traveled 1/120 sec before the second
curtain started. Then the second curtain requires 1/120 sec to complete it's
travel after the first curtain has cleared. Therefore at 1/60, you would only
require the flash, if it was started at the same time as the first curtain, to
stay on for 1/60 + 1/120 = 1/40 sec. You're always going to need as a minimum
1/60 sec of coverage, but the additional amount of time will be dependent on the
shutter speed. To look at this from another point of view, let's just assume you
had a shutter speed of 1/infinity on the camera. In other words, the curtains
move but the film isn't exposed. The second curtain would be butted up against
the first curtain. The amount of time both curtains would take to transverse the
film would be 1/60sec if the camera had a 1/30 sec "X" sync.

This means that your long-duration flash needs to burn for the time it
takes the second curtain to cross the film, probably 1/30.

No, it needs to burn for the entire length of the first curtain's movement and
continue until the second curtain has come to a rest.

Providing it
can be triggered just as the second curtain starts moving and burns
until it stops, then you can ignore the first curtain. This would let
you use your long-burn flash at all of the higher speeds.

Yes, but you'd loose some light by doing this. The GN will go down as the
shutter speed goes up. By reducing the amount of time the flash is activated
you'll just press the GN down even further.

Different cameras will have different shutters, so the speed with which
the curtains traverse the film will be different. And vertically-run
shutters have a shorter distance to cross than horizontally-run ones, so
can X-sync at higher speeds.

Pity the poor camera designer trying to get a focal plane shutter to
wizz evenly across the long dimension of a 6x9 film gate.

No, pity me the poor amateur trying to get something like this to work.

Mr. Friday, I believe your math is wrong - see below.

Paul Friday wrote:

In message , writes
Yes, but the faster the shutter speed the more narrow the slit, and the less
time the flash unit has to provide light.

With a focal plane shutter, the curtains can move quite slowly: it's the
gap between them that gives the shorter exposure time.
Take the K60 for example, with an X-sync speed of 1/30. This means that
the first curtain probably takes about 1/30 of a second to traverse the
film, followed by the second one taking 1/30 of a second (providing it
sets off as soon as the first one gets to the end).

1/30 + 1/30 = 1/15
If there's a 1/30 flash sync the second curtain cannot be in front of the film
until after the first curtain clears. Otherwise you wouldn't be able to expose
the complete film area.

The electronic flash fires at the point where both curtains are out of
the way. The flash duration is short enough for this to work.

For faster shutter speeds, the curtains don't move any faster: all that
changes is the gap between them. The second curtain sets off before the
first one has completely traversed the film.

In other words the distance between the first and second curtains gets smaller.
Therefore the amount of time the second curtain takes relative to the first
curtain's movement will get shorter and shorter. Example: 1/30 flash sync.
First curtain takes 1/60 to "open up" - second curtain takes another 1/60 to
close. 1/60 + 1/60 = 1/30. Now you up the shutter speed to 1/60. The gap
between the curtains is halved. The first curtain again takes 1/60 to complete
it's travel, then the second curtain, trailing behind it takes 1/120 to "close
up". Why - because the first curtain only traveled 1/120 sec before the second
curtain started. Then the second curtain requires 1/120 sec to complete it's
travel after the first curtain has cleared. Therefore at 1/60, you would only
require the flash, if it was started at the same time as the first curtain, to
stay on for 1/60 + 1/120 = 1/40 sec. You're always going to need as a minimum
1/60 sec of coverage, but the additional amount of time will be dependent on the
shutter speed. To look at this from another point of view, let's just assume you
had a shutter speed of 1/infinity on the camera. In other words, the curtains
move but the film isn't exposed. The second curtain would be butted up against
the first curtain. The amount of time both curtains would take to transverse the
film would be 1/60sec if the camera had a 1/30 sec "X" sync.

This means that your long-duration flash needs to burn for the time it
takes the second curtain to cross the film, probably 1/30.

No, it needs to burn for the entire length of the first curtain's movement and
continue until the second curtain has come to a rest.

Providing it
can be triggered just as the second curtain starts moving and burns
until it stops, then you can ignore the first curtain. This would let
you use your long-burn flash at all of the higher speeds.

Yes, but you'd loose some light by doing this. The GN will go down as the
shutter speed goes up. By reducing the amount of time the flash is activated
you'll just press the GN down even further.

Different cameras will have different shutters, so the speed with which
the curtains traverse the film will be different. And vertically-run
shutters have a shorter distance to cross than horizontally-run ones, so
can X-sync at higher speeds.

Pity the poor camera designer trying to get a focal plane shutter to
wizz evenly across the long dimension of a 6x9 film gate.

No, pity me the poor amateur trying to get something like this to work.

Sorry, I realized that your math is correct: 1/30 + 1/30 is = to 1/30 for a focal
plane shutter. But the rest of my argument should be correct if you just double all
of the values. The bottom line is that if you synchronize the flash with the second
curtain, you are going to lose a whole lot of light.

wrote:

Mr. Friday, I believe your math is wrong - see below.

Paul Friday wrote:

In message , writes
Yes, but the faster the shutter speed the more narrow the slit, and the less
time the flash unit has to provide light.

With a focal plane shutter, the curtains can move quite slowly: it's the
gap between them that gives the shorter exposure time.
Take the K60 for example, with an X-sync speed of 1/30. This means that
the first curtain probably takes about 1/30 of a second to traverse the
film, followed by the second one taking 1/30 of a second (providing it
sets off as soon as the first one gets to the end).

1/30 + 1/30 = 1/15
If there's a 1/30 flash sync the second curtain cannot be in front of the film
until after the first curtain clears. Otherwise you wouldn't be able to expose
the complete film area.

The electronic flash fires at the point where both curtains are out of
the way. The flash duration is short enough for this to work.

For faster shutter speeds, the curtains don't move any faster: all that
changes is the gap between them. The second curtain sets off before the
first one has completely traversed the film.

In other words the distance between the first and second curtains gets smaller.
Therefore the amount of time the second curtain takes relative to the first
curtain's movement will get shorter and shorter. Example: 1/30 flash sync.
First curtain takes 1/60 to "open up" - second curtain takes another 1/60 to
close. 1/60 + 1/60 = 1/30. Now you up the shutter speed to 1/60. The gap
between the curtains is halved. The first curtain again takes 1/60 to complete
it's travel, then the second curtain, trailing behind it takes 1/120 to "close
up". Why - because the first curtain only traveled 1/120 sec before the second
curtain started. Then the second curtain requires 1/120 sec to complete it's
travel after the first curtain has cleared. Therefore at 1/60, you would only
require the flash, if it was started at the same time as the first curtain, to
stay on for 1/60 + 1/120 = 1/40 sec. You're always going to need as a minimum
1/60 sec of coverage, but the additional amount of time will be dependent on the
shutter speed. To look at this from another point of view, let's just assume you
had a shutter speed of 1/infinity on the camera. In other words, the curtains
move but the film isn't exposed. The second curtain would be butted up against
the first curtain. The amount of time both curtains would take to transverse the
film would be 1/60sec if the camera had a 1/30 sec "X" sync.

This means that your long-duration flash needs to burn for the time it
takes the second curtain to cross the film, probably 1/30.

No, it needs to burn for the entire length of the first curtain's movement and
continue until the second curtain has come to a rest.

Providing it
can be triggered just as the second curtain starts moving and burns
until it stops, then you can ignore the first curtain. This would let
you use your long-burn flash at all of the higher speeds.

Yes, but you'd loose some light by doing this. The GN will go down as the
shutter speed goes up. By reducing the amount of time the flash is activated
you'll just press the GN down even further.

Different cameras will have different shutters, so the speed with which
the curtains traverse the film will be different. And vertically-run
shutters have a shorter distance to cross than horizontally-run ones, so
can X-sync at higher speeds.

Pity the poor camera designer trying to get a focal plane shutter to
wizz evenly across the long dimension of a 6x9 film gate.

No, pity me the poor amateur trying to get something like this to work.