Shutter speed and Low lighting.

On Photonotes.org, it states that : "In Av, night and Tv (shutter speed
priority) modes the camera meters for ambient (existing) light and
fills in the foreground subject using the flash. It does not assume
that the primary light source is the flash, and therefore the shutter
speed it sets is the same as it would set if you weren't using flash
at all "

My Question is that, is that really the case ? If so, how can we still
use the Aperture priority mode and tell the camera that there is an
external flash / Strobe that will kick-in to compensate for the lack of
lighting ?

Regards,
John Edwards

John Edwards wrote:

My Question is that, is that really the case ? If so, how can we still
use the Aperture priority mode and tell the camera that there is an
external flash / Strobe that will kick-in to compensate for the lack of
lighting ?

You cannot.

If you are in aperature priority mode, then the camera picks a shutter
to go with the aperature based on the light available (in order to get a
"correct" exposure.

If you want to use a flash or strobe to provide the light, then you
need to set it to manual mode and select the appropriate aperature and
shutter to acheive the desired exposure.

Bob

What camera are you referring to?


On 1/28/05 1:52 PM, in article
, "John Edwards"
wrote:

On Photonotes.org, it states that : "In Av, night and Tv (shutter speed
priority) modes the camera meters for ambient (existing) light and
fills in the foreground subject using the flash. It does not assume
that the primary light source is the flash, and therefore the shutter
speed it sets is the same as it would set if you weren't using flash
at all "

My Question is that, is that really the case ? If so, how can we still
use the Aperture priority mode and tell the camera that there is an
external flash / Strobe that will kick-in to compensate for the lack of
lighting ?

Regards,
John Edwards

On 28 Jan 2005 10:52:45 -0800, "John Edwards"
wrote:

On Photonotes.org, it states that : "In Av, night and Tv (shutter speed
priority) modes the camera meters for ambient (existing) light and
fills in the foreground subject using the flash. It does not assume
that the primary light source is the flash, and therefore the shutter
speed it sets is the same as it would set if you weren't using flash
at all "

My Question is that, is that really the case ? If so, how can we still
use the Aperture priority mode and tell the camera that there is an
external flash / Strobe that will kick-in to compensate for the lack of
lighting ?

Why do you *need* to tell the camera this ?

In Av mode, it's going to meter the scene as usual (basically, this
means metering the background, as that's more %-wise of the scene than
the foreground) and fix the shutter speed based on that meter reading.

If you take the photo now, the background will be exposed properly.
But, you want to add some flash into the foreground to illuminate your
(rather dark) subject. Shutter speed *must stay the same* as it would
had you not used the flash (presuming the shutter speed doesn't exceed
flash-sync limits, but that's a different problem), otherwise the
background will no longer be correctly exposed.

Compared to your flash, the shutter on all cameras is far too damn
slow to be effective in reducing light coming from a flash.

The real call here is how to tell the flash not to flood the
foreground subject with too much light. Use modern real-time TTL/iTTL
metering and the camera will quench the flash when needed (can't say
for sure if they can all do this in 'Av' mode however). External
flashes will either have to be manually stopped down so they don't do
this, be a proprietary flash connected to the camera so it's TTL
system can send a quench signal or have some kind of sensor built in
(in conjunction with being configured correctly) so they can
self-quench.

If you've not heard of a quench before, in these modes, the flash
strobes rapidly** (maybe 10Khz) until a meter reading from the camera,
or feedback from it's own sensor tells it "that the subject has been
illuminated enough thank you, please turn off". All of this magic
happens in the gap between opening the shutter and closing it again.

At least with digital, it costs nothing to experiment.

** Not all brands of flash actually strobe, some may have a method of
'slow burn', constant illumination. I know one of Canon's fast-flash
systems must do it this way.

--
Owamanga!

What camera are you referring to?
I have a Nikon N70 Film Camera, but I am experimenting with my Nikon
4500 Digital Camera.

The Niikon 4500 digicam, supports the Aperture/Shutter priority and
also has a manual mode. It can sync to an external speedlight but is
not fully TTL, in the sense that it controls the external speedlight
flash output, but does not actually control the zoom head/Aperture on
my speedlight (SB-28).

If you take the photo now, the background will be exposed properly.
But, you want to add some flash into the foreground to illuminate your
(rather dark) subject. snip

Would'nt a slow shutter ( calculated by metering a dark room) and an
external flash over expose the foreground subject ? I think, as you
say, it would not be a concern if I use a modern real time TTL/iTTL
camera and flash which communicate fully with each other.

-- John

John Edwards wrote:

Would'nt a slow shutter ( calculated by metering a dark room) and an
external flash over expose the foreground subject ? I think, as you
say, it would not be a concern if I use a modern real time TTL/iTTL
camera and flash which communicate fully with each other.


When you are using a flash, the exposure (the part due to the flash) is
determined by only two factors: distance to subject, and aperature. It
doesn't matter what shutter speed you use (as long as it is one the
camera and flash synch at) because the flash duration is so short. It
will always end before the shutter closes.

Bob

On Fri, 28 Jan 2005 15:14:25 -0500, bob wrote:

John Edwards wrote:

Would'nt a slow shutter ( calculated by metering a dark room) and an
external flash over expose the foreground subject ? I think, as you
say, it would not be a concern if I use a modern real time TTL/iTTL
camera and flash which communicate fully with each other.


When you are using a flash, the exposure (the part due to the flash) is
determined by only two factors: distance to subject, and aperature. It
doesn't matter what shutter speed you use (as long as it is one the
camera and flash synch at) because the flash duration is so short. It
will always end before the shutter closes.

Bob's not wrong, but he's dangerously simplistic with his "only two
factors". There are of course more. Major ones: Film speed, and flash
output power. These are however, traditionally 'static' from scene to
scene, so people often ignore them. But if you start doing math, these
will be in the equations. And with digital, ISO is just another thing
you can twiddle with.

Minor and rarer ones: The subject's reflectivity (illuminatability? I
make up words sometimes), any zoom capability the flash-head might
have and any filtration / diffuser that you introduce on the flash.

Seriously, a Klingon might need a stop more flash-power than your
typical Ferengi at the Trekky convention.

Back to your question, yes, a flash at full-power will usually be able
to completely white out close subjects on a typical portrait setup.
But as Bob has said, you shut down the aperture, and you'll cut down
the effect of the flash light (as *well* as background light of
course, but the camera in Av mode will extend the shutter speed to
compensate). Most external flashes have manual modes where you can
force them to stop down the output.

Practice, it's digital. No worries. Shoot 50 shots using different
settings and write down what you told the flash to do (do this on a PC
afterwards looking at EXIF data + your notes on any manual flash
settings).

You should also consider *when* during the exposure is the best time
for the flash. If photographing anything alive, switch to rear-curtain
flash mode otherwise the subject will react to the flash at the
beginning of the exposure and spend the rest of the time moving.
Better to hit them with the whack of light towards the end of the
exposure, then it matters not if they flinch/blink (such movement will
be caught by any ambient light). The books all have pictures of guys
on bikes and cars looking like they go backwards as a reason for
needing rear-curtain flash but the real day to day problem it solves
is living people, not car lights.

--
Owamanga!

"Owamanga" wrote in message
...
On Fri, 28 Jan 2005 15:14:25 -0500, bob wrote:

John Edwards wrote:

Would'nt a slow shutter ----------------- with each other.


When you are using a flash, the exposure (the part due to the flash) is
determined by only two factors: distance to subject, and aperature. It
doesn't matter what shutter speed you use (as long as it is one the
camera and flash synch at) because the flash duration is so short. It
will always end before the shutter closes.

Bob's not wrong, but he's dangerously simplistic with his "only two
factors". There are of course more. Major ones: Film speed, and flash
output power. These are however, traditionally 'static' from scene to
scene, so people often ignore them. But if you start doing math, these
will be in the equations. And with digital, ISO is just another thing
you can twiddle with.

Minor and rarer ones: The subject's reflectivity (illuminatability? I
make up words sometimes), any zoom capability the flash-head might
have and any filtration / diffuser that you introduce on the flash.

Seriously, a Klingon might need a stop more flash-power than your
typical Ferengi at the Trekky convention.

Back to your question, yes, a flash at full-power will usually be able
to completely white out close subjects on a typical portrait setup.
But as Bob has said, you shut down the aperture, and you'll cut down
the effect of the flash light (as *well* as background light of
course, but the camera in Av mode will extend the shutter speed to
compensate). Most external flashes have manual modes where you can
force them to stop down the output.

Practice, it's digital. No worries. Shoot 50 shots using different
settings and write down what you told the flash to do (do this on a PC
afterwards looking at EXIF data + your notes on any manual flash
settings).

You should also consider *when* during the exposure is the best time
for the flash. If photographing anything alive, switch to rear-curtain
flash mode otherwise the subject will react to the flash at the
beginning of the exposure and spend the rest of the time moving.
Better to hit them with the whack of light towards the end of the
exposure, then it matters not if they flinch/blink (such movement will
be caught by any ambient light). The books all have pictures of guys
on bikes and cars looking like they go backwards as a reason for
needing rear-curtain flash but the real day to day problem it solves
is living people, not car lights.

--
Owamanga!

Hi there, Owamanga.

You have just explained something that has been puzzling me, all day.
I have been playing with my new D70, and could not quite figure out why it
switches the Flash to "Slow" on Program and Aperture Priority, and on the
"Idiot Night" setting. Nikon must be assuming that the Flash will be
getting used as a "Fill In" on these settings, and your explanation of why
Rear Sync is much better is something that they know about. I have used
Flash a great deal over the years, but never had a Camera that had Rear
Curtain sync, or even TTL Flash, and I had never even considered this use
for it. It certainly makes a lot of sense, and I will be using Flash that
way in future.

There does seem to be a lot of misunderstanding about Flash and Exposure
Settings among the newer breed of Photographers, who seem to be able to leap
in and buy top end DSLRs as their first Camera. Whether that is a good or a
bad thing is not up to me to decide, but because these Cameras are so
Automatic and Advanced the new owners never get a chance to learn any of the
Basic Science behind our "Art". Working out Flash Exposure is dead simple -
Divide the Guide Number by the Distance to the Subject, and the answer is
the Aperture Setting. If you do not want the Subject to be fully lit by the
Flash go to one number higher, (smaller opening & less light admitted). The
situation is more complicated if the Built in Flash is TTL controlled,
because the System will just step up its output to match the new Aperture
Setting, hence the availability of Flash Exposure Compensation built into
these Cameras, but the Instruction Books are more than a little vague on its
uses. It took me quite a bit of working out "why's" to understand that bit
of my D70 instructions. I will have to do qute a bit of experimenting before
I stop relying upon my 2 elderly but trusty Vivitar 283's.

Roy

In message ,
bob wrote:

When you are using a flash, the exposure (the part due to the flash) is
determined by only two factors: distance to subject, and aperature.

That's manual flash. Auto-flash changes the length of the flash.
--


John P Sheehy

On Fri, 28 Jan 2005 20:43:45 +0000, Owamanga wrote:


Minor and rarer ones: The subject's reflectivity (illuminatability? I
make up words sometimes),

Albedo - percent of light reflected from a body.

--

Gautam Majumdar

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