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Can TTL be used with bounce flash?

 
 
David Arnstein
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      02-08-2006
I have access to two flashes that have a TTL mode: the Metz Mecablitz
54 MZ-4 and the Sony HVLF32X. I am trying to understand what TTL is,
and in particular if I can use it when bouncing the flash light off a
ceiling.

I *think* that TTL means that the camera determines how much light is
needed, and then signals the flash unit to emit that much light.

If my understanding is correct, then TTL cannot be used when performing
"bounce flash." This, because the camera does not know how reflective
the ceiling is. Therefore, its determination of needed light is not
possible.

Am I understanding this correctly? Both of these flash units will do
bounce flash in TTL mode, without complaining. So I suspect that my
understanding is faulty.

These two flash units also offer another automatic mode, wherein the
camera signals the flash gun what aperture, shutter speed, and ISO
sensitivity it is using. The flash unit then measures the output of its
own flash light hitting a sensor on the flash unit itself. The flash
unit cuts power to its flash tube when its sensor has absorbed the proper
amount of light for the signalled aperture, shutter speed, and ISO
sensitivity.

Right?

To me, this second mode seems suitable for use with bounce flash.
Neither the camera nor the flash unit need to know about the optical
properties of the ceiling. The flash unit uses the good old empirical
method, measuring light returning from one, two, or more reflective
surfaces.

I'd appreciate any corrections to my (very weak) understanding.
--
David Arnstein | Have fun with your spams:
(E-Mail Removed) | http://www.bluesecurity.com
 
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Kennedy McEwen
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      02-08-2006
In article <dsbemq$nsd$(E-Mail Removed)>, David Arnstein
<(E-Mail Removed)> writes
>I have access to two flashes that have a TTL mode: the Metz Mecablitz
>54 MZ-4 and the Sony HVLF32X. I am trying to understand what TTL is,
>and in particular if I can use it when bouncing the flash light off a
>ceiling.
>
>I *think* that TTL means that the camera determines how much light is
>needed, and then signals the flash unit to emit that much light.


Exactly how TTL flash metering works depends as much on the camera as it
does on the flash, but usually it means it meters the light coming back
from the subject through the lens, so it will take the reflectivity of
the bounce screen into account. Depending on the camera, this metering
can be achieved during the actual exposure itself or based on a lower
power test flash emitted either when the shutter release is half pressed
or immediately before the shutter opens.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
 
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Bruce Hoult
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      02-08-2006
In article <dsbemq$nsd$(E-Mail Removed)>,
http://www.velocityreviews.com/forums/(E-Mail Removed) (David Arnstein) wrote:

> I have access to two flashes that have a TTL mode: the Metz Mecablitz
> 54 MZ-4 and the Sony HVLF32X. I am trying to understand what TTL is,
> and in particular if I can use it when bouncing the flash light off a
> ceiling.
>
> I *think* that TTL means that the camera determines how much light is
> needed, and then signals the flash unit to emit that much light.
>
> If my understanding is correct, then TTL cannot be used when performing
> "bounce flash." This, because the camera does not know how reflective
> the ceiling is. Therefore, its determination of needed light is not
> possible.
>
> Am I understanding this correctly? Both of these flash units will do
> bounce flash in TTL mode, without complaining. So I suspect that my
> understanding is faulty.


There are (at least) two quite different ways that this works, depending
on whether you're using film or digital.

Film cameras will often measure the light being reflected back off the
film as it is being exposed, and use that to kill the flash at the
appropriate time.

Digital SLRs (at least the Nikon ones) tell the flash to fire twice.
The first time is used to measure the necessary exposure (with the
shutter closed) and then the second tiem the camera tells the flash how
much more or less light is needed. That's why the D50, D70 etc need the
new SB600/SB8800 series of flashes for automatic exposures.

--
Bruce | 41.1670S | \ spoken | -+-
Hoult | 174.8263E | /\ here. | ----------O----------
 
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secheese
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      02-08-2006
TTL flash works great with bounced light.

 
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Dennis Pogson
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      02-08-2006
David Arnstein wrote:
> I have access to two flashes that have a TTL mode: the Metz Mecablitz
> 54 MZ-4 and the Sony HVLF32X. I am trying to understand what TTL is,
> and in particular if I can use it when bouncing the flash light off a
> ceiling.
>
> I *think* that TTL means that the camera determines how much light is
> needed, and then signals the flash unit to emit that much light.
>
> If my understanding is correct, then TTL cannot be used when
> performing "bounce flash." This, because the camera does not know how
> reflective the ceiling is. Therefore, its determination of needed
> light is not possible.
>
> Am I understanding this correctly? Both of these flash units will do
> bounce flash in TTL mode, without complaining. So I suspect that my
> understanding is faulty.
>
> These two flash units also offer another automatic mode, wherein the
> camera signals the flash gun what aperture, shutter speed, and ISO
> sensitivity it is using. The flash unit then measures the output of
> its own flash light hitting a sensor on the flash unit itself. The
> flash unit cuts power to its flash tube when its sensor has absorbed
> the proper amount of light for the signalled aperture, shutter speed,
> and ISO sensitivity.
>
> Right?
>
> To me, this second mode seems suitable for use with bounce flash.
> Neither the camera nor the flash unit need to know about the optical
> properties of the ceiling. The flash unit uses the good old empirical
> method, measuring light returning from one, two, or more reflective
> surfaces.
>
> I'd appreciate any corrections to my (very weak) understanding.


TTL = "thru the lens". The expression came into common parlance when
light-metering was changed from external, hand-held light meters to built-in
meters at or near the film plane. Judging by the current demand for old
Weston Master light meters on E-Bay, it would seem the photographic world is
going into reverse.

I can imagine a whole gaggle of digital photographers waving their old Westo
n Masters around, not believing the aperture/shutter info on the digital
screen, and going to full manual set-up for each shot.

Progress? Don't make me laugh!

Dennis.


 
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Clint Kirk
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      02-08-2006
As a related point of interest, how do automatic compact digital
cameras get the exposure right when the built-in flash is used?

 
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Ronnie Sellar
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      02-08-2006
Clint Kirk wrote:

> As a related point of interest, how do automatic compact digital
> cameras get the exposure right when the built-in flash is used?


My Olympus C310ZOOM uses preflashes to measure the exposure in the same way
the iTTL and dTTL modes do on Nikon DSLRs.

Ronnie
 
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Tesco News
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      02-08-2006
"Clint Kirk" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) ps.com...
> As a related point of interest, how do automatic compact digital
> cameras get the exposure right when the built-in flash is used?



Hi.

Some of them do it the old fashioned way. I read it in a manual for one,
but cannot remember which.

The Autofocus gives a distance setting, and the camera then sets the
aperture. Guide Number divided by distance equals aperture is the formula
which we all used to use before Auto Flash, Flash Meters and TTL became
common. It works remarkably well.

We should all have these figures in the back of our minds when using Flash,
so that we will know what to set when the Automation lets us down, as it
does.

Roy G


 
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Kennedy McEwen
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      02-08-2006
In article <dsd0rq$mof$1$(E-Mail Removed)>, Ronnie Sellar
<(E-Mail Removed)> writes
>Clint Kirk wrote:
>
>> As a related point of interest, how do automatic compact digital
>> cameras get the exposure right when the built-in flash is used?

>
>My Olympus C310ZOOM uses preflashes to measure the exposure in the same way
>the iTTL and dTTL modes do on Nikon DSLRs.
>

Not surprising really. Olympus pioneered TTL flash metering 30 years
ago when they introduced the OTF metering OM-2 and the QA-310 flash
unit.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
 
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Michael Meissner
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      02-09-2006
Kennedy McEwen <(E-Mail Removed)> writes:

> In article <dsd0rq$mof$1$(E-Mail Removed)>, Ronnie Sellar
> <(E-Mail Removed)> writes
> >Clint Kirk wrote:
> >
> >> As a related point of interest, how do automatic compact digital
> >> cameras get the exposure right when the built-in flash is used?

> >
> >My Olympus C310ZOOM uses preflashes to measure the exposure in the same way
> >the iTTL and dTTL modes do on Nikon DSLRs.
> >

> Not surprising really. Olympus pioneered TTL flash metering 30 years ago when
> they introduced the OTF metering OM-2 and the QA-310 flash unit.


However on film cameras, TTL flash measures the reflections off of the film, so
it didn't need a preflash. Digital cameras typically will emit a preflash,
measure the strength of the pre-flash, and then issue the flash for real.
Unfortunately if you have slave flashes (or studio strobes), these will get
confused by the pre-flash. There are slave triggers and slave flashes that
know about the pre-flash.

--
Michael Meissner
email: (E-Mail Removed)
http://www.the-meissners.org
 
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