How does TTL bracketing work?

Hi Alan & bruce

The book is "The New Guide to Sea & Sea" [cameras & strobes], Joe
Liburdi & Cara Sherman, 1977, ISBN 0-9621111-3-9.

(quote)

p.82 "How to bracket"

"When shooting with an external strobe in TTL mode:"

"It's impossible to bracket in the same manner as when using the
strobe in manual mode. No matter which f-stop you select, the TTL
sensor will automatically compensate for the measured light
refelecting off the film plane. Therefore, we must intentionally over
or underexpose to create different exposures.

1. Bracket by altering strobe to subject distance. Take your first
picture at the recommended f-stop & strobe to subject distance. Then
remove your strobe from the base plate and handhold it. One foot
forward or backward will efectively change your exposure by one
f-stop.

2. Bracket by changing shutter speed. Change it one speed faster and
one speed slower than the recommended exposure. Do not change f-stop."

(end quote)


I don't see how either of those two recommendations can affect the
exposure. Surely the TTL sensor will quench the strobe after 'x'
amount of light has been received by the film, regardless of strobe to
subject distance, f-stop, and shutter speed?

Hope that you can shed some light on this! Is the book just wrong,
full-stop?

TIA,
TC

Hi Alan & bruce

The book is "The New Guide to Sea & Sea" [cameras & strobes], Joe
Liburdi & Cara Sherman, 1977, ISBN 0-9621111-3-9.

(quote)

p.82 "How to bracket"

"When shooting with an external strobe in TTL mode:"

"It's impossible to bracket in the same manner as when using the
strobe in manual mode. No matter which f-stop you select, the TTL
sensor will automatically compensate for the measured light
refelecting off the film plane. Therefore, we must intentionally over
or underexpose to create different exposures.

1. Bracket by altering strobe to subject distance. Take your first
picture at the recommended f-stop & strobe to subject distance. Then
remove your strobe from the base plate and handhold it. One foot
forward or backward will efectively change your exposure by one
f-stop.

2. Bracket by changing shutter speed. Change it one speed faster and
one speed slower than the recommended exposure. Do not change f-stop."

(end quote)


I don't see how either of those two recommendations can affect the
exposure. Surely the TTL sensor will quench the strobe after 'x'
amount of light has been received by the film, regardless of strobe to
subject distance, f-stop, and shutter speed?

Hope that you can shed some light on this! Is the book just wrong,
full-stop?

TIA,
TC

TC wrote:
Hi Alan & bruce

The book is "The New Guide to Sea & Sea" [cameras & strobes], Joe
Liburdi & Cara Sherman, 1977, ISBN 0-9621111-3-9.

(quote)

p.82 "How to bracket"

"When shooting with an external strobe in TTL mode:"

"It's impossible to bracket in the same manner as when using the
strobe in manual mode. No matter which f-stop you select, the TTL
sensor will automatically compensate for the measured light
refelecting off the film plane. Therefore, we must intentionally over
or underexpose to create different exposures.

Okay up to there.
[NIT: Further, as the aperture gets smaller, you rapidly approach
the max power of the flash.]

1. Bracket by altering strobe to subject distance. Take your
first
picture at the recommended f-stop & strobe to subject distance. Then
remove your strobe from the base plate and handhold it. One foot

Not in TTL ...TTL will always adjust as they say above.

forward or backward will efectively change your exposure by one
f-stop.

Huh? if you are 10 feet away, with a manual flash:

--you would need to approach to (SQRT(2)*10)/2 = 7.07 feet to get
1 more stop of light on target (or double the amount light).

--To get 1 stop less you would need SQRT(2)*10 = 14.14 from target.

Notice now that 7.07 is half of 14.14 feet ... they are two stops
apart (4x the amount of light at 7.07 feet than at 14.14 feet.

For a given distance from subject to flash, and change in that
distance follows the inverse square law.

The only way their example would hold would be if the orig. dist
from strobe to subject was 3.4 feet away ... which for underwater
photography might well be the case in many circumstances ... is
that the context of the discussion in the book (eg, from about
3.5 feet away...)?



2. Bracket by changing shutter speed. Change it one speed faster and
one speed slower than the recommended exposure. Do not change f-stop."

Shutter speed has no effect the flash exposure (except when S is
faster than the sync speed and you get a partially exposed frame).

A flash photo is really a double exposure, one for ambient light,
one for flash light. The ambient can be controlled with both
shutter speed and aperture ... but the flash exposure can *only*
be controled by the flash power output and the aperture ...
shutter speed has nothing to do with it.


(end quote)


I don't see how either of those two recommendations can affect the
exposure. Surely the TTL sensor will quench the strobe after 'x'
amount of light has been received by the film, regardless of strobe to
subject distance, f-stop, and shutter speed?

Within its power limits, yes. The usual means of controlling TTL
flash output is either flash compensation or rating the ISO to
fool the flash (rate it higher to lower the flash power; raise
the ISO to increase the flash). In an underwater shoot, rerating
the ISO might be a bit tricky!

Do this in Aperture priority, however, becasue if you do it in
S-pri then the aperture will be determined in part by the ISO
setting.


Hope that you can shed some light on this! Is the book just wrong,
full-stop?

See above about the context of the example regarding 1 foot
change of distance...

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:
Hi Alan & bruce

The book is "The New Guide to Sea & Sea" [cameras & strobes], Joe
Liburdi & Cara Sherman, 1977, ISBN 0-9621111-3-9.

(quote)

p.82 "How to bracket"

"When shooting with an external strobe in TTL mode:"

"It's impossible to bracket in the same manner as when using the
strobe in manual mode. No matter which f-stop you select, the TTL
sensor will automatically compensate for the measured light
refelecting off the film plane. Therefore, we must intentionally over
or underexpose to create different exposures.

Okay up to there.
[NIT: Further, as the aperture gets smaller, you rapidly approach
the max power of the flash.]

1. Bracket by altering strobe to subject distance. Take your
first
picture at the recommended f-stop & strobe to subject distance. Then
remove your strobe from the base plate and handhold it. One foot

Not in TTL ...TTL will always adjust as they say above.

forward or backward will efectively change your exposure by one
f-stop.

Huh? if you are 10 feet away, with a manual flash:

--you would need to approach to (SQRT(2)*10)/2 = 7.07 feet to get
1 more stop of light on target (or double the amount light).

--To get 1 stop less you would need SQRT(2)*10 = 14.14 from target.

Notice now that 7.07 is half of 14.14 feet ... they are two stops
apart (4x the amount of light at 7.07 feet than at 14.14 feet.

For a given distance from subject to flash, and change in that
distance follows the inverse square law.

The only way their example would hold would be if the orig. dist
from strobe to subject was 3.4 feet away ... which for underwater
photography might well be the case in many circumstances ... is
that the context of the discussion in the book (eg, from about
3.5 feet away...)?



2. Bracket by changing shutter speed. Change it one speed faster and
one speed slower than the recommended exposure. Do not change f-stop."

Shutter speed has no effect the flash exposure (except when S is
faster than the sync speed and you get a partially exposed frame).

A flash photo is really a double exposure, one for ambient light,
one for flash light. The ambient can be controlled with both
shutter speed and aperture ... but the flash exposure can *only*
be controled by the flash power output and the aperture ...
shutter speed has nothing to do with it.


(end quote)


I don't see how either of those two recommendations can affect the
exposure. Surely the TTL sensor will quench the strobe after 'x'
amount of light has been received by the film, regardless of strobe to
subject distance, f-stop, and shutter speed?

Within its power limits, yes. The usual means of controlling TTL
flash output is either flash compensation or rating the ISO to
fool the flash (rate it higher to lower the flash power; raise
the ISO to increase the flash). In an underwater shoot, rerating
the ISO might be a bit tricky!

Do this in Aperture priority, however, becasue if you do it in
S-pri then the aperture will be determined in part by the ISO
setting.


Hope that you can shed some light on this! Is the book just wrong,
full-stop?

See above about the context of the example regarding 1 foot
change of distance...

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

Alan

Thank you very much for your detailed & informative answers. I've
answered nearly 6000 questions on usenet myself (about a particular
software product), & it's nice to get an answer to one of my own
questions, for a change!

It's clear that the book is incorrect. I'll follow this up with Sea &
Sea (the camera manufacturers), or the author, if I can contact him.
If I get a result, I'll take the liberty of CC'ing it to you, unless
you ask me not to do that.

As for the 3.4 foot question - yes, it is commonly recommended that
underwater pictures be taken from a max distance of about 4 feet (for
the obvious reasons). But it seems that his information is still all
wrong, in any case.

Thanks again for your answers. I'll print them off & digest them
carefully.

Yea for newsgroups :-)

Regards,
TC


Alan Browne wrote in message ...
TC wrote:
Hi Alan & bruce

The book is "The New Guide to Sea & Sea" [cameras & strobes], Joe
Liburdi & Cara Sherman, 1977, ISBN 0-9621111-3-9.

(quote)

p.82 "How to bracket"

"When shooting with an external strobe in TTL mode:"

"It's impossible to bracket in the same manner as when using the
strobe in manual mode. No matter which f-stop you select, the TTL
sensor will automatically compensate for the measured light
refelecting off the film plane. Therefore, we must intentionally over
or underexpose to create different exposures.

Okay up to there.
[NIT: Further, as the aperture gets smaller, you rapidly approach
the max power of the flash.]

1. Bracket by altering strobe to subject distance. Take your
first
picture at the recommended f-stop & strobe to subject distance. Then
remove your strobe from the base plate and handhold it. One foot

Not in TTL ...TTL will always adjust as they say above.

forward or backward will efectively change your exposure by one
f-stop.

Huh? if you are 10 feet away, with a manual flash:

--you would need to approach to (SQRT(2)*10)/2 = 7.07 feet to get
1 more stop of light on target (or double the amount light).

--To get 1 stop less you would need SQRT(2)*10 = 14.14 from target.

Notice now that 7.07 is half of 14.14 feet ... they are two stops
apart (4x the amount of light at 7.07 feet than at 14.14 feet.

For a given distance from subject to flash, and change in that
distance follows the inverse square law.

The only way their example would hold would be if the orig. dist
from strobe to subject was 3.4 feet away ... which for underwater
photography might well be the case in many circumstances ... is
that the context of the discussion in the book (eg, from about
3.5 feet away...)?



2. Bracket by changing shutter speed. Change it one speed faster and
one speed slower than the recommended exposure. Do not change f-stop."

Shutter speed has no effect the flash exposure (except when S is
faster than the sync speed and you get a partially exposed frame).

A flash photo is really a double exposure, one for ambient light,
one for flash light. The ambient can be controlled with both
shutter speed and aperture ... but the flash exposure can *only*
be controled by the flash power output and the aperture ...
shutter speed has nothing to do with it.


(end quote)


I don't see how either of those two recommendations can affect the
exposure. Surely the TTL sensor will quench the strobe after 'x'
amount of light has been received by the film, regardless of strobe to
subject distance, f-stop, and shutter speed?

Within its power limits, yes. The usual means of controlling TTL
flash output is either flash compensation or rating the ISO to
fool the flash (rate it higher to lower the flash power; raise
the ISO to increase the flash). In an underwater shoot, rerating
the ISO might be a bit tricky!

Do this in Aperture priority, however, becasue if you do it in
S-pri then the aperture will be determined in part by the ISO
setting.


Hope that you can shed some light on this! Is the book just wrong,
full-stop?

See above about the context of the example regarding 1 foot
change of distance...

Cheers,
Alan

Alan

Thank you very much for your detailed & informative answers. I've
answered nearly 6000 questions on usenet myself (about a particular
software product), & it's nice to get an answer to one of my own
questions, for a change!

It's clear that the book is incorrect. I'll follow this up with Sea &
Sea (the camera manufacturers), or the author, if I can contact him.
If I get a result, I'll take the liberty of CC'ing it to you, unless
you ask me not to do that.

As for the 3.4 foot question - yes, it is commonly recommended that
underwater pictures be taken from a max distance of about 4 feet (for
the obvious reasons). But it seems that his information is still all
wrong, in any case.

Thanks again for your answers. I'll print them off & digest them
carefully.

Yea for newsgroups :-)

Regards,
TC


Alan Browne wrote in message ...
TC wrote:
Hi Alan & bruce

The book is "The New Guide to Sea & Sea" [cameras & strobes], Joe
Liburdi & Cara Sherman, 1977, ISBN 0-9621111-3-9.

(quote)

p.82 "How to bracket"

"When shooting with an external strobe in TTL mode:"

"It's impossible to bracket in the same manner as when using the
strobe in manual mode. No matter which f-stop you select, the TTL
sensor will automatically compensate for the measured light
refelecting off the film plane. Therefore, we must intentionally over
or underexpose to create different exposures.

Okay up to there.
[NIT: Further, as the aperture gets smaller, you rapidly approach
the max power of the flash.]

1. Bracket by altering strobe to subject distance. Take your
first
picture at the recommended f-stop & strobe to subject distance. Then
remove your strobe from the base plate and handhold it. One foot

Not in TTL ...TTL will always adjust as they say above.

forward or backward will efectively change your exposure by one
f-stop.

Huh? if you are 10 feet away, with a manual flash:

--you would need to approach to (SQRT(2)*10)/2 = 7.07 feet to get
1 more stop of light on target (or double the amount light).

--To get 1 stop less you would need SQRT(2)*10 = 14.14 from target.

Notice now that 7.07 is half of 14.14 feet ... they are two stops
apart (4x the amount of light at 7.07 feet than at 14.14 feet.

For a given distance from subject to flash, and change in that
distance follows the inverse square law.

The only way their example would hold would be if the orig. dist
from strobe to subject was 3.4 feet away ... which for underwater
photography might well be the case in many circumstances ... is
that the context of the discussion in the book (eg, from about
3.5 feet away...)?



2. Bracket by changing shutter speed. Change it one speed faster and
one speed slower than the recommended exposure. Do not change f-stop."

Shutter speed has no effect the flash exposure (except when S is
faster than the sync speed and you get a partially exposed frame).

A flash photo is really a double exposure, one for ambient light,
one for flash light. The ambient can be controlled with both
shutter speed and aperture ... but the flash exposure can *only*
be controled by the flash power output and the aperture ...
shutter speed has nothing to do with it.


(end quote)


I don't see how either of those two recommendations can affect the
exposure. Surely the TTL sensor will quench the strobe after 'x'
amount of light has been received by the film, regardless of strobe to
subject distance, f-stop, and shutter speed?

Within its power limits, yes. The usual means of controlling TTL
flash output is either flash compensation or rating the ISO to
fool the flash (rate it higher to lower the flash power; raise
the ISO to increase the flash). In an underwater shoot, rerating
the ISO might be a bit tricky!

Do this in Aperture priority, however, becasue if you do it in
S-pri then the aperture will be determined in part by the ISO
setting.


Hope that you can shed some light on this! Is the book just wrong,
full-stop?

See above about the context of the example regarding 1 foot
change of distance...

Cheers,
Alan

TC wrote:

Alan

Thank you very much for your detailed & informative answers. I've
answered nearly 6000 questions on usenet myself (about a particular
software product), & it's nice to get an answer to one of my own
questions, for a change!

It's clear that the book is incorrect. I'll follow this up with Sea &
Sea (the camera manufacturers), or the author, if I can contact him.
If I get a result, I'll take the liberty of CC'ing it to you, unless
you ask me not to do that.

Sure. Nastier the better (g).


As for the 3.4 foot question - yes, it is commonly recommended that
underwater pictures be taken from a max distance of about 4 feet (for
the obvious reasons). But it seems that his information is still all
wrong, in any case.

In the context of UW photography, the 1 foot/1 stop sounds like a
rule of thumb that is likely reasonable/workable at that
distance. But if it is TTL it will have no effect at all on the
exposure. For a manual flash it is perfectly sensible.


Thanks again for your answers. I'll print them off & digest them
carefully.

My pleasure. Cheers,
Alan.

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

One thing I noticed while doing flash-exposure studies on slide film
is that you *can* bracket, but only under-bracket, as follows:

Wait many seconds for full flash recharge, take a picture.

As soon as the red "ready" light comes on, take another picture.
The batteries don't have enough time to fully recharge the capacitor,
so the second picture is a bit underexposed.

This is not precise, and probably depends on your equipment,
especially the speedlight and design of the Ready light.

One thing I noticed while doing flash-exposure studies on slide film
is that you *can* bracket, but only under-bracket, as follows:

Wait many seconds for full flash recharge, take a picture.

As soon as the red "ready" light comes on, take another picture.
The batteries don't have enough time to fully recharge the capacitor,
so the second picture is a bit underexposed.

This is not precise, and probably depends on your equipment,
especially the speedlight and design of the Ready light.

As soon as the red "ready" light comes on, take another picture.
The batteries don't have enough time to fully recharge the capacitor,
so the second picture is a bit underexposed.

This is not precise, and probably depends on your equipment,
especially the speedlight and design of the Ready light.

The old rule of thumb was: the ready lite came on @ 75-80% of full charge.
Bob Hickey