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Can TTL be used with bounce flash?

 
 
Kennedy McEwen
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      02-09-2006
In article <(E-Mail Removed)-meissners.org>, Michael Meissner
<(E-Mail Removed)> writes
>
>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.
>

So what's the problem? Obviously if you use a slave that doesn't expect
a pre-flash you know it won't work. Get the right tools for the job.
--
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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Clint Kirk
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      02-09-2006
Tesco News wrote:
> 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.


Indeed!

Now there's just one thing left unanswered for me. My Fuji E550 doesn't
save the subject distance in the Exif info. I thought this was because
it didn't know the subject distance. Just because it has autofocus
doesn't mean it knows the absolute distance, it could work just by
knowing that moving the lens slightly outwards gives better contrast
than before, but now it's moved it too far so it moves it back in a
little bit... in other words, it can work by making relative
adjustments without knowing the absolute position.

Assuming it doesn't know the subject distance, it would need a
pre-flash to get the exposure right. But I don't notice a pre-flash
when I take a picture.

Could it be that the pre-flash is in undetectably close temporal
proximity to the main flash?

(Wow, I've done it! I've been wanting to use the term "close temporal
proximity" ever since I saw it in a research paper... now I can get
back to writing proper English)

Or could it be that it knows the subject distance, but the camera's
software developers were too lazy to make it write the distance in the
Exif info?

Clint

 
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Gisle Hannemyr
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      02-09-2006
"Clint Kirk" <(E-Mail Removed)> writes:
> Could it be that the pre-flash is in undetectably close temporal
> proximity to the main flash?


It could be. My E-TTL Canon G5 uses pre-flash (it will trigger an
optical slave before the shutter opens), but this happens to close to
the main event to be noticable by human perceptuion.

> (Wow, I've done it! I've been wanting to use the term "close temporal
> proximity" ever since I saw it in a research paper... now I can get
> back to writing proper English)


Congratulations!

> Or could it be that it knows the subject distance, but the camera's
> software developers were too lazy to make it write the distance in
> the Exif info?


It could be as well. My Canon G5 has manual focusing, and when turned
on, some sort of yardstick where the focus distance is marked in
meters and feet show up on the LCD - but still, there is no meaningful
distance data recorded in the EXIF.
--
- gisle hannemyr [ gisle{at}hannemyr.no - http://folk.uio.no/gisle/ ]
------------------------------------------------------------------------
SD10, Kodak DCS460, Canon Powershot G5, Olympus 2020Z
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ASAAR
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      02-09-2006
On 9 Feb 2006 08:50:06 -0800, Clint Kirk wrote:

> Could it be that the pre-flash is in undetectably close temporal
> proximity to the main flash?
>
> (Wow, I've done it! I've been wanting to use the term "close temporal
> proximity" ever since I saw it in a research paper... now I can get
> back to writing proper English)


Bull! You wrote it because you're a trekkie, Cap'n Kirk. <g>

BTW, the Canon S10/S20 in no way has "close temporal proximity"
between pre-flash and the main flash. It's impossible to avoid
noticing both flashes. But as they're fairly old digital cameras
now (even though I occasionally use the S20) they can be excused.

 
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John McWilliams
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      02-10-2006
Clint Kirk wrote:
>
> Assuming it doesn't know the subject distance, it would need a
> pre-flash to get the exposure right. But I don't notice a pre-flash
> when I take a picture.
>
> Could it be that the pre-flash is in undetectably close temporal
> proximity to the main flash?


NO! It's because the two flashes fire so close together that you can't
see it....






<s>




That *was* a nice lot of words you put together, tho.

I am perplexed as to why there's no distance in my EXIF data, either.
Often I know, but the camera should be able to record precisely what
it's focussed on....in my case a 20 D with a 50mm Canon lens, for example.

--
John McWilliams
 
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Kennedy McEwen
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      02-10-2006
In article <(E-Mail Removed)>, John
McWilliams <(E-Mail Removed)> writes

>I am perplexed as to why there's no distance in my EXIF data, either.
>Often I know, but the camera should be able to record precisely what
>it's focussed on....in my case a 20 D with a 50mm Canon lens, for
>example.
>

In that particular case, the camera certainly doesn't know what distance
it is focussed on. Only certain lenses in the Canon range return
distance data to the camera, and none of the 50mm lenses are included.

A reasonably up-to-date list of lenses with and without distance
encoders is at:
http://photonotes.org/articles/eos-flash/#distancedata
--
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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Clint Kirk
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      02-10-2006
I was thinking of a way to find out, once and for all, whether it uses
a pre-flash. I was going to take another camera, set on "B" or a very
long exposure, and in the dark I would use the E550's flash to take a
picture of a black disc with a white line across its radius, attached
to a drill rotating at 3000 rpm. If there is a pre-flash, I would get
two white lines on the disc; otherwise, only one.

Then I thought of a simpler way. I read the manual. And there it was,
on its specifications page:

Flash type: Auto flash using flash control sensor

So, no pre-flash, no distance measurement, just a dedicated flash
sensor.

Clint

 
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