Believe flash meter or believe flashlight sendor?

My flashlight has a table on its back, it say, in the automatic mode, i
should use f5.6 at 3.6m. but within subject distance of 3.6m i did
some tests using the flashlight and my handheld flash meter,
1, put the meter in the subject location and measure incident flash
light
2, put the meter in the same location with the flashlight pointing to
the subject, meter is in reflective mode
in any case above, the meter show different reading ( f-stops ) other
than f5.6, some times larger sometimes smaller. my meter is a Sekonic
DulalMaster 558 and i believe it is accurate, what i can not understand
is where the differences come from and how exactly a automatic
flashlighter works
thank you.
-
woody

Steven Woody

Re: believe flash meter or believe flashlight sendor?

still can not figure it out on my L-558 :-(. meter always allows me to
select shutter speed and give the apperture reading based on the flash
strength, i then can change the shutter speed but get no control on
apperture.
-
woody


Steven Woody

Re: believe flash meter or believe flashlight sendor?

Well, like I said, look at the test images that went along with the
flash readings. They're your best indicator of how to adjust.
I can do it with the L-408, but I had to figure it out since it's not
specifically addressed in the manual for that one either. On the L-408
there's an EV/multi button & a mode button. You have to fiddle both
controls until you get the meter to where it's reading flash & keeping
the f/stop constant.


No_name

Re: believe flash meter or believe flashlight sendor?

but there are cases that ask me to use reflective reading, in which i
want to know what part of subject will fall in what zone in the
nagative and ready to shift 18% gray to include my prefered contrast.
i am so interested the Sekonic mode you mentioned. how can i set it to
read a constant aperture and give me variable shutter speeds ( in flash
measuring ) ? it seems not appeared in the manual.
-
woody


Steven Woody

Re: believe flash meter or believe flashlight sendor?

What did the test IMAGES look like. That's what I'd use to determine
what settings to use.
The table on the back of the strobe is generic. Actual conditions will
vary a whole lot. If it's got a thyristor, in automatic mode it will
shut the flash off when it registers enough light reflected from the
subject for an aperature at f/5.6 (based on whatever ISO is set in the
flash).
For flash meter readings, I'd use incident readings instead of
reflective. You want to know how much light is going onto the subject,
not how much is bouncing off.
Reflective readings are going to be influenced by the darkness of the
subject. The meter will tell you how much exposure is needed with a
given flash to make the subject 18% gray.
If it's a wedding gown, 18% gray, a gray flannel suit, 18% gray ... a
slinky, little black dress, 18% gray.
The other thing that comes to mind is the mode the Sekonic is set to
when testing. It might be set to hold the shutter speed constant and
give variable f/stops. You also should be able to set it to read a
constant aperature and give you variable shutter speeds.


No_name

Re: believe flash meter or believe flashlight sendor?

...
I'm not sure about yours, but many meters are set to meter at a certain
time, say 1/1,000 sec or slower. Many automatic flashes fire much, much
faster, giving a false reading. At the same time many manufacturers give
their lites a higher rating than they have. If you bracket a few once, then
you'll know where it works best.
Bob Hickey


Bob Hickey

Re: believe flash meter or believe flashlight sendor?

there is one to two stops difference. what i interested is not how
much the difference, but why the difference happen. if your answer
explained where the difference come from when using reflective meter,
it seemed not explain why the difference exist when when incidently
metering. actually, i found, the incident meter reading changes when
the flash put near or close to the subject ( the distance does within
the flashlighter limit ).
you said the flash sensor cut off the light if and only if it receives
enough amount of bounced light. it can be deduced that for what kind
of reflective surface, the flash light hitting on the subject will be
the same, so i should need the same exposure. if true ( i think so ),
there is obviously a conflicts with what you said "You will find that
you need more exposure when there is less reflective surface"
-
woody


Steven Woody

Re: believe flash meter or believe flashlight sendor?

...
An automatic strobe (flash) puts out a certain amount of light based on how
much is bounced back to it's photocell. In other words, the strobe lights up
and immediately measures the light it sees. When it sees a certain amount of
light, it turns off. Keep in mind, this happens _very_ fast, perhaps 1/1000
of a second or less.
How much difference did you observe in your meter readings? Just changing
where you stand when you fire the strobe can change the amount of light
reflected back to the strobe. Incident and reflected readings can differ
based on how reflective the subject is. If you reflective meter a light
color subject, you may get a higher reading than you would get reflective
metering a dark color subject.
If the reading variation is less than two stops and you are shooting print
film, don't worry about it. Do some actual tests to see what looks best on
the finished photos, and go from there. Take some pictures in a small room
with light-colored walls and ceiling at a couple different f/-stops; in a
big room with dark-colored walls and ceiling (or high ceiling), and see what
exposure gives the best results. You will find that you need more exposure
when there is less reflective surface. How much more can only be determined
by actual testing.
Ken Hart

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