How does flashlight's automatic mode work?

hi,

i know, in a traditional flashlight, there is a sensor which measure
sth. and cut off the output of light if it feels enouth. my question
is, what the sensor mesures, subject distance or amount of light?

if what it measured is distance, so i think the speedlight should be
calibrated on a 18% gray or similar. so shooting on a white wall and
taking the f-stop reading on the dial will yeids a white wall rather
than gray, am i right?

if what it measured is amount of light reflected, so when i shoot a
white wall, i should open up two stops to get it really white. am i
right?

what one above is right? thanks.

--
woody

On Mar 19, 1:57 pm, "Steven Woody" wrote:
hi,

i know, in a traditional flashlight, there is a sensor which measure
sth. and cut off the output of light if it feels enouth. my question
is, what the sensor mesures, subject distance or amount of light?

if what it measured is distance, so i think the speedlight should be
calibrated on a 18% gray or similar. so shooting on a white wall and
taking the f-stop reading on the dial will yeids a white wall rather
than gray, am i right?

if what it measured is amount of light reflected, so when i shoot a
white wall, i should open up two stops to get it really white. am i
right?

what one above is right? thanks.

--
woody

Read this Woody and read it again.

http://www.freepatentsonline.com/4301392.html


In short, open up the lens two stops. Depending on what your main
subject might be, you should have a correct exposure.

Good luck,



Draco


Getting even isn't good enough.

In article .com,
Steven Woody wrote:

i know, in a traditional flashlight, there is a sensor which measure
sth. and cut off the output of light if it feels enouth. my question
is, what the sensor mesures, subject distance or amount of light?

if what it measured is distance, so i think the speedlight should be
calibrated on a 18% gray or similar. so shooting on a white wall and
taking the f-stop reading on the dial will yeids a white wall rather
than gray, am i right?

if what it measured is amount of light reflected, so when i shoot a
white wall, i should open up two stops to get it really white. am i
right?

what one above is right? thanks.

In "traditional" automatic electronic flashes, the sensor simply
measures the light bounced back. MUCH easier than trying to measure
distance. An unusually reflective object wll indeed cause overall
underexposure. Of course, the calibration varies, as does the angle of
the sensor. Most camera/flash combinations now use sensors behind the
camera lens, eliminating the latter issue.

The Nikon flash system (and perhaps others?) also incorporate distance
readings reported by the autofocus system, making it a hybrid of both
methods.

So if you have a traditional flash and know the subject will throw it
off, opening up a bit might be wise. Or use a handheld incident flash
meter, like I do. GRIN

"Steven Woody" wrote in message
oups.com...
hi,

i know, in a traditional flashlight, there is a sensor which measure
sth. and cut off the output of light if it feels enouth. my question
is, what the sensor mesures, subject distance or amount of light?

if what it measured is distance, so i think the speedlight should be
calibrated on a 18% gray or similar. so shooting on a white wall and
taking the f-stop reading on the dial will yeids a white wall rather
than gray, am i right?

if what it measured is amount of light reflected, so when i shoot a
white wall, i should open up two stops to get it really white. am i
right?

what one above is right? thanks.

--
woody
What's the difference? It collects light
until the subject turns grey. If its further away it flashes longer; if the
subject is dark, it stays on longer. If it's white, it's stays on a shorter
time and if it's closer, a shorter time. All it does is turn things grey.
Which goes a long way to explain why most flash shots are so bad. Bob
Hickey

On 3ÔÂ20ÈÕ, ÉÏÎç8ʱ40·Ö, "bob hickey" wrote:
"Steven Woody" wrote in message

oups.com...

hi,

i know, in a traditional flashlight, there is a sensor which measure
sth. and cut off the output of light if it feels enouth. my question
is, what the sensor mesures, subject distance or amount of light?

if what it measured is distance, so i think the speedlight should be
calibrated on a 18% gray or similar. so shooting on a white wall and
taking the f-stop reading on the dial will yeids a white wall rather
than gray, am i right?

if what it measured is amount of light reflected, so when i shoot a
white wall, i should open up two stops to get it really white. am i
right?

what one above is right? thanks.

--
woody
What's the difference? It collects light

until the subject turns grey. If its further away it flashes longer; if the
subject is dark, it stays on longer. If it's white, it's stays on a shorter
time and if it's closer, a shorter time. All it does is turn things grey.
Which goes a long way to explain why most flash shots are so bad. Bob
Hickey

a lot of thanks to all your replies! i now see :-)

-
woody

Steven Woody wrote:
hi,

i know, in a traditional flashlight, there is a sensor which measure
sth. and cut off the output of light if it feels enouth. my question

Actually a "traditional" flash measures nothing. It bangs out power,
perhaps limited by a dial.

An "AUTO" flash has a sensor that reads the light bouncing back from
wherever it hits. When that equates a mid tone exposure, it interrupts
the current to the flashtube.

is, what the sensor mesures, subject distance or amount of light?

if what it measured is distance, so i think the speedlight should be
calibrated on a 18% gray or similar. so shooting on a white wall and
taking the f-stop reading on the dial will yeids a white wall rather
than gray, am i right?

Nope. No reading is taken until the shot is taken. You can incident
meter (with an incident flash-meter) at the wall for the correct setting
and it does not care at all what the color of the wall is. Simply point
the dome at the camera lens from where you want correct exposure, fire
the flash from where it will be during the shot. That reading
(aperture) can be used or the power (or distance of the flash) adjusted
to get a desired aperture setting (or a little of each).

Note that when you point your camera meter at a white wall you are
getting the reflected reading from the wall, not an indication of
aperture for the flash shot.

if what it measured is amount of light reflected, so when i shoot a
white wall, i should open up two stops to get it really white. am i
right?

See above and what the others said.

For "faultless" exposure, a flash incident reading at the subject is
best (even if likely to be boring).

With TTL-flash, if you're shooting a white subject you can either
flash-exp comp up 1.7 - 2 stops or decrease the ISO setting 2 stops to
force more power.

Some systems have lens focus integration so that the distance to the
presumed in focus subject us used to get proper exposure at that
distance. (Minolta and Sony "D" lenses and I'm sure the others have
variants on the same theme).

Cheers,
Alan

--
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-- e-meil: Remove FreeLunch.

On Mar 22, 7:23 am, Alan Browne
wrote:
Steven Woody wrote:
hi,

i know, in a traditional flashlight, there is a sensor which measure
sth. and cut off the output of light if it feels enouth. my question

Actually a "traditional" flash measures nothing. It bangs out power,
perhaps limited by a dial.

An "AUTO" flash has a sensor that reads the light bouncing back from
wherever it hits. When that equates a mid tone exposure, it interrupts
the current to the flashtube.

is, what the sensor mesures, subject distance or amount of light?

if what it measured is distance, so i think the speedlight should be
calibrated on a 18% gray or similar. so shooting on a white wall and
taking the f-stop reading on the dial will yeids a white wall rather
than gray, am i right?

Nope. No reading is taken until the shot is taken. You can incident
meter (with an incident flash-meter) at the wall for the correct setting
and it does not care at all what the color of the wall is. Simply point
the dome at the camera lens from where you want correct exposure, fire
the flash from where it will be during the shot. That reading
(aperture) can be used or the power (or distance of the flash) adjusted
to get a desired aperture setting (or a little of each).

Note that when you point your camera meter at a white wall you are
getting the reflected reading from the wall, not an indication of
aperture for the flash shot.

if what it measured is amount of light reflected, so when i shoot a
white wall, i should open up two stops to get it really white. am i
right?

See above and what the others said.

For "faultless" exposure, a flash incident reading at the subject is
best (even if likely to be boring).

With TTL-flash, if you're shooting a white subject you can either
flash-exp comp up 1.7 - 2 stops or decrease the ISO setting 2 stops to
force more power.

Some systems have lens focus integration so that the distance to the
presumed in focus subject us used to get proper exposure at that
distance. (Minolta and Sony "D" lenses and I'm sure the others have
variants on the same theme).

Cheers,
Alan

--
-- r.p.e.35mm user resource:http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems:http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz:http://www.pbase.com/shootin
-- e-meil: Remove FreeLunch.


thanks for your kindly explainations!