How does TTL bracketing work?
TC
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
Bruce Murphy
> 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
> 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 where:
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
Cheers,
TC
Bruce Murphy <pack...@rattus.net> wrote in message news:<m2hdqcx...@greybat.rattus.net>...
TC
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
TC
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
Alan Browne
> 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...
TC
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 <alan....@FreeLunchVideotron.ca> wrote in message news:<Uc60d.23733$sz2.3...@wagner.videotron.net>...
Alan Browne
> 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.
Bill Tuthill
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.
Bob Hickey
> 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
Alan Browne
> 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.
For static setups where a bracket is required, this could work.
But I'd hate to rely on it.
>
> This is not precise, and probably depends on your equipment,
> especially the speedlight and design of the Ready light.
and then some!