How does TTL bracketing work?

Hi folks

Hope this question is appropriate for this group.

I know how TTL strobes work. A sensor near the film plane monitors how
much light has been received. When the right amount has been received,
the sensor quenches the strobe, thus ensuring correct exposure.

Here's my question. Does the sensor measure the amount of light that
is received, or the length of time that it is received for?

My photo book says that I can bracket exposures, using my TTL strobe,
by detaching the strobe from the arm, & moving it closer to the
subject, or further away.

But if the sensor measures the amount of light, I can't see why
moving the strobe would make any difference. 'x' amount of reflected
light is 'x' amount of reflected light - regardless of whether it
comes from a short burst from a close strobe, or a long burst from a
far-away strobe.

The only way that moving the strobe can bracket the exposure, as far
as I can see, is if the sensor measures the amount of time for which
the light is received. Then, the film gets more light in 'x' amount of
time if the strobe is closer, less if it is further way.

Am I missing something obvious here?

TIA,
TC

(TC) writes:

Hi folks

Hope this question is appropriate for this group.

I know how TTL strobes work. A sensor near the film plane monitors how
much light has been received. When the right amount has been received,
the sensor quenches the strobe, thus ensuring correct exposure.

Here's my question. Does the sensor measure the amount of light that
is received, or the length of time that it is received for?

Amount.

My photo book says that I can bracket exposures, using my TTL strobe,
by detaching the strobe from the arm, & moving it closer to the
subject, or further away.

Your photo book is lying, except insofar as you might be able to use
this to /balance/ the TTL flash against ambient light (whose intensity
wouldn't change during this process)

Am I missing something obvious here?

Perhaps being a little too trusting of things in print. What photo book
was this?

B

(TC) writes:

Hi folks

Hope this question is appropriate for this group.

I know how TTL strobes work. A sensor near the film plane monitors how
much light has been received. When the right amount has been received,
the sensor quenches the strobe, thus ensuring correct exposure.

Here's my question. Does the sensor measure the amount of light that
is received, or the length of time that it is received for?

Amount.

My photo book says that I can bracket exposures, using my TTL strobe,
by detaching the strobe from the arm, & moving it closer to the
subject, or further away.

Your photo book is lying, except insofar as you might be able to use
this to /balance/ the TTL flash against ambient light (whose intensity
wouldn't change during this process)

Am I missing something obvious here?

Perhaps being a little too trusting of things in print. What photo book
was this?

B

TC wrote:
I know how TTL strobes work. A sensor near the film plane monitors how
much light has been received. When the right amount has been received,
the sensor quenches the strobe, thus ensuring correct exposure.

Here's my question. Does the sensor measure the amount of light that
is received, or the length of time that it is received for?

Since the amount of light per unit time returned to the camera
lens depends on distance to the scene and reflectivity of same,
and since TTL-suffers all the problems inherent in this
technique, it seems that it is "amount" based whe

amount = intensity_rec_d * time.

However, recently a lot of TTL flash systems have begun
pre-flashing the scene, determining the flash exposure, and then
during the actual exposure blasting off a preset amount of flash.
(Something like a studio light, except top-down not bottom-up
cap charge as in a studio light).

My photo book says that I can bracket exposures, using my TTL strobe,
by detaching the strobe from the arm, & moving it closer to the
subject, or further away.

Your book is wrong... did they really say TTL? Even for an auto
flash this would not be correct.
(What book, by the way?)

That is a technique for pre-set power-level flashes, not TTL or
Auto, (but a bit clumsy and further it changes the 'shape' of the
light on the subject.) [Say your setting is for ISO 100, 10 feet
away, f/5.6, then moving the flash further than 10 feet is a -x
and moving it closer thatn 10 ft is +x flash bracket.]

Some cameras have flash bracketing that automatically varies the
flash exposure from frame to frame... eg: you set up for -1.5,
-0.5 and +0.5 and shoot away ... you need to leave time in
between frames to recharge the flash (unless the level is very low).


But if the sensor measures the amount of light, I can't see why
moving the strobe would make any difference. 'x' amount of reflected
light is 'x' amount of reflected light - regardless of whether it
comes from a short burst from a close strobe, or a long burst from a
far-away strobe.

You're correct (...up to the limit of power of the strobe, anyway
and down to the minimum pulse it can generate).


The only way that moving the strobe can bracket the exposure, as far
as I can see, is if the sensor measures the amount of time for which
the light is received. Then, the film gets more light in 'x' amount of
time if the strobe is closer, less if it is further way.

Nope


Am I missing something obvious here?

Actually you stated it yourself, but didn't see it! See above.
What book was that?

Cheers,
Alan

--
-- rec.photo.equipment.35mm user resource:
-- http://www.aliasimages.com/rpe35mmur.htm
-- e-meil: there's no such thing as a FreeLunch.--

TC wrote:
I know how TTL strobes work. A sensor near the film plane monitors how
much light has been received. When the right amount has been received,
the sensor quenches the strobe, thus ensuring correct exposure.

Here's my question. Does the sensor measure the amount of light that
is received, or the length of time that it is received for?

Since the amount of light per unit time returned to the camera
lens depends on distance to the scene and reflectivity of same,
and since TTL-suffers all the problems inherent in this
technique, it seems that it is "amount" based whe

amount = intensity_rec_d * time.

However, recently a lot of TTL flash systems have begun
pre-flashing the scene, determining the flash exposure, and then
during the actual exposure blasting off a preset amount of flash.
(Something like a studio light, except top-down not bottom-up
cap charge as in a studio light).

My photo book says that I can bracket exposures, using my TTL strobe,
by detaching the strobe from the arm, & moving it closer to the
subject, or further away.

Your book is wrong... did they really say TTL? Even for an auto
flash this would not be correct.
(What book, by the way?)

That is a technique for pre-set power-level flashes, not TTL or
Auto, (but a bit clumsy and further it changes the 'shape' of the
light on the subject.) [Say your setting is for ISO 100, 10 feet
away, f/5.6, then moving the flash further than 10 feet is a -x
and moving it closer thatn 10 ft is +x flash bracket.]

Some cameras have flash bracketing that automatically varies the
flash exposure from frame to frame... eg: you set up for -1.5,
-0.5 and +0.5 and shoot away ... you need to leave time in
between frames to recharge the flash (unless the level is very low).


But if the sensor measures the amount of light, I can't see why
moving the strobe would make any difference. 'x' amount of reflected
light is 'x' amount of reflected light - regardless of whether it
comes from a short burst from a close strobe, or a long burst from a
far-away strobe.

You're correct (...up to the limit of power of the strobe, anyway
and down to the minimum pulse it can generate).


The only way that moving the strobe can bracket the exposure, as far
as I can see, is if the sensor measures the amount of time for which
the light is received. Then, the film gets more light in 'x' amount of
time if the strobe is closer, less if it is further way.

Nope


Am I missing something obvious here?

Actually you stated it yourself, but didn't see it! See above.
What book was that?

Cheers,
Alan

--
-- rec.photo.equipment.35mm user resource:
-- http://www.aliasimages.com/rpe35mmur.htm
-- e-meil: there's no such thing as a FreeLunch.--

Bruce, thanks for your answer. See my answer to the other respondent.

Cheers,
TC


Bruce Murphy wrote in message ...
(TC) writes:

Hi folks

Hope this question is appropriate for this group.

I know how TTL strobes work. A sensor near the film plane monitors how
much light has been received. When the right amount has been received,
the sensor quenches the strobe, thus ensuring correct exposure.

Here's my question. Does the sensor measure the amount of light that
is received, or the length of time that it is received for?

Amount.

My photo book says that I can bracket exposures, using my TTL strobe,
by detaching the strobe from the arm, & moving it closer to the
subject, or further away.

Your photo book is lying, except insofar as you might be able to use
this to /balance/ the TTL flash against ambient light (whose intensity
wouldn't change during this process)

Am I missing something obvious here?

Perhaps being a little too trusting of things in print. What photo book
was this?

B

Bruce, thanks for your answer. See my answer to the other respondent.

Cheers,
TC


Bruce Murphy wrote in message ...
(TC) writes:

Hi folks

Hope this question is appropriate for this group.

I know how TTL strobes work. A sensor near the film plane monitors how
much light has been received. When the right amount has been received,
the sensor quenches the strobe, thus ensuring correct exposure.

Here's my question. Does the sensor measure the amount of light that
is received, or the length of time that it is received for?

Amount.

My photo book says that I can bracket exposures, using my TTL strobe,
by detaching the strobe from the arm, & moving it closer to the
subject, or further away.

Your photo book is lying, except insofar as you might be able to use
this to /balance/ the TTL flash against ambient light (whose intensity
wouldn't change during this process)

Am I missing something obvious here?

Perhaps being a little too trusting of things in print. What photo book
was this?

B

Alan Browne wrote in message ...
TC wrote:
I know how TTL strobes work. A sensor near the film plane monitors how
much light has been received. When the right amount has been received,
the sensor quenches the strobe, thus ensuring correct exposure.

Here's my question. Does the sensor measure the amount of light that
is received, or the length of time that it is received for?

Since the amount of light per unit time returned to the camera
lens depends on distance to the scene and reflectivity of same,
and since TTL-suffers all the problems inherent in this
technique, it seems that it is "amount" based whe

amount = intensity_rec_d * time.

However, recently a lot of TTL flash systems have begun
pre-flashing the scene, determining the flash exposure, and then
during the actual exposure blasting off a preset amount of flash.
(Something like a studio light, except top-down not bottom-up
cap charge as in a studio light).

My photo book says that I can bracket exposures, using my TTL strobe,
by detaching the strobe from the arm, & moving it closer to the
subject, or further away.

Your book is wrong... did they really say TTL? Even for an auto
flash this would not be correct.
(What book, by the way?)

That is a technique for pre-set power-level flashes, not TTL or
Auto, (but a bit clumsy and further it changes the 'shape' of the
light on the subject.) [Say your setting is for ISO 100, 10 feet
away, f/5.6, then moving the flash further than 10 feet is a -x
and moving it closer thatn 10 ft is +x flash bracket.]

Some cameras have flash bracketing that automatically varies the
flash exposure from frame to frame... eg: you set up for -1.5,
-0.5 and +0.5 and shoot away ... you need to leave time in
between frames to recharge the flash (unless the level is very low).


But if the sensor measures the amount of light, I can't see why
moving the strobe would make any difference. 'x' amount of reflected
light is 'x' amount of reflected light - regardless of whether it
comes from a short burst from a close strobe, or a long burst from a
far-away strobe.

You're correct (...up to the limit of power of the strobe, anyway
and down to the minimum pulse it can generate).


The only way that moving the strobe can bracket the exposure, as far
as I can see, is if the sensor measures the amount of time for which
the light is received. Then, the film gets more light in 'x' amount of
time if the strobe is closer, less if it is further way.

Nope


Am I missing something obvious here?

Actually you stated it yourself, but didn't see it! See above.
What book was that?

Cheers,
Alan


Alan, thanks to you & Bruce for your answers.

I don't have the book to hand right now, so I can't quote its title
exactly. But it is issued by Sea & Sea, the manufacturer of underwater
cameras. The section in question relates to the Sea & Sea Motormarine
II EX underwater camera.

Some of simpler Sea & Sea camera/strobe combinations have the strobe
quench sensor in the strobe arm, not the camera film plane. I believe
that those are called "auto" strobes? But the II EX strobe is a
"proper" TTL strobe (afaik), with the quenching sensor inside the
camera, at the film plane.

The II EX is DX-coded & has no external adjustment for the ISO rating.
So, as far as I can see, there is no way to bracket exposures. This
seems confirmed by the latest & greatest version of the II EX - the
Motormarine III - which has a new external control, labelled
"bracketing" (or somesuch) running from -1 thru 0 to +1, in
increments.

I re-read the book (which I've had for some time) to see what it said
about bracketing with the II EX. That's when I came across the comment
about removing the strobe. I'll get an exact quote tonight, & post
back tomorrow.

Thanks for the info so far.

PS. The Motormarine cameras do not pre-flash, so that is not an issue
here.

Cheers,
TC

Alan Browne wrote in message ...
TC wrote:
I know how TTL strobes work. A sensor near the film plane monitors how
much light has been received. When the right amount has been received,
the sensor quenches the strobe, thus ensuring correct exposure.

Here's my question. Does the sensor measure the amount of light that
is received, or the length of time that it is received for?

Since the amount of light per unit time returned to the camera
lens depends on distance to the scene and reflectivity of same,
and since TTL-suffers all the problems inherent in this
technique, it seems that it is "amount" based whe

amount = intensity_rec_d * time.

However, recently a lot of TTL flash systems have begun
pre-flashing the scene, determining the flash exposure, and then
during the actual exposure blasting off a preset amount of flash.
(Something like a studio light, except top-down not bottom-up
cap charge as in a studio light).

My photo book says that I can bracket exposures, using my TTL strobe,
by detaching the strobe from the arm, & moving it closer to the
subject, or further away.

Your book is wrong... did they really say TTL? Even for an auto
flash this would not be correct.
(What book, by the way?)

That is a technique for pre-set power-level flashes, not TTL or
Auto, (but a bit clumsy and further it changes the 'shape' of the
light on the subject.) [Say your setting is for ISO 100, 10 feet
away, f/5.6, then moving the flash further than 10 feet is a -x
and moving it closer thatn 10 ft is +x flash bracket.]

Some cameras have flash bracketing that automatically varies the
flash exposure from frame to frame... eg: you set up for -1.5,
-0.5 and +0.5 and shoot away ... you need to leave time in
between frames to recharge the flash (unless the level is very low).


But if the sensor measures the amount of light, I can't see why
moving the strobe would make any difference. 'x' amount of reflected
light is 'x' amount of reflected light - regardless of whether it
comes from a short burst from a close strobe, or a long burst from a
far-away strobe.

You're correct (...up to the limit of power of the strobe, anyway
and down to the minimum pulse it can generate).


The only way that moving the strobe can bracket the exposure, as far
as I can see, is if the sensor measures the amount of time for which
the light is received. Then, the film gets more light in 'x' amount of
time if the strobe is closer, less if it is further way.

Nope


Am I missing something obvious here?

Actually you stated it yourself, but didn't see it! See above.
What book was that?

Cheers,
Alan


Alan, thanks to you & Bruce for your answers.

I don't have the book to hand right now, so I can't quote its title
exactly. But it is issued by Sea & Sea, the manufacturer of underwater
cameras. The section in question relates to the Sea & Sea Motormarine
II EX underwater camera.

Some of simpler Sea & Sea camera/strobe combinations have the strobe
quench sensor in the strobe arm, not the camera film plane. I believe
that those are called "auto" strobes? But the II EX strobe is a
"proper" TTL strobe (afaik), with the quenching sensor inside the
camera, at the film plane.

The II EX is DX-coded & has no external adjustment for the ISO rating.
So, as far as I can see, there is no way to bracket exposures. This
seems confirmed by the latest & greatest version of the II EX - the
Motormarine III - which has a new external control, labelled
"bracketing" (or somesuch) running from -1 thru 0 to +1, in
increments.

I re-read the book (which I've had for some time) to see what it said
about bracketing with the II EX. That's when I came across the comment
about removing the strobe. I'll get an exact quote tonight, & post
back tomorrow.

Thanks for the info so far.

PS. The Motormarine cameras do not pre-flash, so that is not an issue
here.

Cheers,
TC

Alan Browne wrote in message ...
TC wrote:
I know how TTL strobes work. A sensor near the film plane monitors how
much light has been received. When the right amount has been received,
the sensor quenches the strobe, thus ensuring correct exposure.

Here's my question. Does the sensor measure the amount of light that
is received, or the length of time that it is received for?

Since the amount of light per unit time returned to the camera
lens depends on distance to the scene and reflectivity of same,
and since TTL-suffers all the problems inherent in this
technique, it seems that it is "amount" based whe

amount = intensity_rec_d * time.

However, recently a lot of TTL flash systems have begun
pre-flashing the scene, determining the flash exposure, and then
during the actual exposure blasting off a preset amount of flash.
(Something like a studio light, except top-down not bottom-up
cap charge as in a studio light).

My photo book says that I can bracket exposures, using my TTL strobe,
by detaching the strobe from the arm, & moving it closer to the
subject, or further away.

Your book is wrong... did they really say TTL? Even for an auto
flash this would not be correct.
(What book, by the way?)

That is a technique for pre-set power-level flashes, not TTL or
Auto, (but a bit clumsy and further it changes the 'shape' of the
light on the subject.) [Say your setting is for ISO 100, 10 feet
away, f/5.6, then moving the flash further than 10 feet is a -x
and moving it closer thatn 10 ft is +x flash bracket.]

Some cameras have flash bracketing that automatically varies the
flash exposure from frame to frame... eg: you set up for -1.5,
-0.5 and +0.5 and shoot away ... you need to leave time in
between frames to recharge the flash (unless the level is very low).


But if the sensor measures the amount of light, I can't see why
moving the strobe would make any difference. 'x' amount of reflected
light is 'x' amount of reflected light - regardless of whether it
comes from a short burst from a close strobe, or a long burst from a
far-away strobe.

You're correct (...up to the limit of power of the strobe, anyway
and down to the minimum pulse it can generate).


The only way that moving the strobe can bracket the exposure, as far
as I can see, is if the sensor measures the amount of time for which
the light is received. Then, the film gets more light in 'x' amount of
time if the strobe is closer, less if it is further way.

Nope


Am I missing something obvious here?

Actually you stated it yourself, but didn't see it! See above.
What book was that?

Cheers,
Alan


Alan, thanks to you & Bruce for your answers.

I don't have the book to hand right now, so I can't quote its title
exactly. But it is issued by Sea & Sea, the manufacturer of underwater
cameras. The section in question relates to the Sea & Sea Motormarine
II EX underwater camera.

Some of simpler Sea & Sea camera/strobe combinations have the strobe
quench sensor in the strobe arm, not the camera film plane. I believe
that those are called "auto" strobes? But the II EX strobe is a
"proper" TTL strobe (afaik), with the quenching sensor inside the
camera, at the film plane.

The II EX is DX-coded & has no external adjustment for the ISO rating.
So, as far as I can see, there is no way to bracket exposures. This
seems confirmed by the latest & greatest version of the II EX - the
Motormarine III - which has a new external control, labelled
"bracketing" (or somesuch) running from -1 thru 0 to +1, in
increments.

I re-read the book (which I've had for some time) to see what it said
about bracketing with the II EX. That's when I came across the comment
about removing the strobe. I'll get an exact quote tonight, & post
back tomorrow.

Thanks for the info so far.

PS. The Motormarine cameras do not pre-flash, so that is not an issue
here.

Cheers,
TC