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Fill flash math.



 
 
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  #1  
Old September 17th 08, 12:01 AM posted to rec.photo.equipment.35mm
David Farber
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Posts: 12
Default Fill flash math.

I was wondering about how fill flash affects the existing light. Of course
it will take the shadows and lighten them up a bit, but what about the parts
of the picture that are correctly exposed by the existing light? I mean the
light emanating from the flash (attached to the hot shoe) does not just go
to the shadows. Wouldn't the areas not in shadow then be overexposed by a
half stop? My fill flash photographs (from print film) have been coming out
nicely but this conundrum has me scratching my head. On second thought,
should I be using some ratio method to compensate for the existing light
exposure or is that not how it's done? I'm using a Minolta Maxxum 7 with a
3600HS flash unit.


  #2  
Old September 17th 08, 08:07 PM posted to rec.photo.equipment.35mm
Ric Trexell
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Posts: 114
Default Fill flash math.


Wouldn't the areas not in shadow then be overexposed by a
half stop?

************************************************** ***********
I don't understand why you say a half stop, but think of fill flash as only
being a fraction of the available light on the subject. The meter in the
flash (assuming you have an automatic flash) or the OTF-TTL flash camera
combo will measure the extra light and compensate for it. Doing it manually
is still sort of a hit and miss thing but generally you are not over
exposing that much. Ric in Wisconsin.


  #3  
Old September 17th 08, 10:33 PM posted to rec.photo.equipment.35mm
Alan Browne
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Posts: 12,640
Default Fill flash math.

David Farber wrote:
I was wondering about how fill flash affects the existing light. Of course
it will take the shadows and lighten them up a bit, but what about the parts
of the picture that are correctly exposed by the existing light? I mean the
light emanating from the flash (attached to the hot shoe) does not just go
to the shadows. Wouldn't the areas not in shadow then be overexposed by a
half stop? My fill flash photographs (from print film) have been coming out
nicely but this conundrum has me scratching my head. On second thought,
should I be using some ratio method to compensate for the existing light
exposure or is that not how it's done? I'm using a Minolta Maxxum 7 with a
3600HS flash unit.


For the sake of fill flash, don't worry about it. Yes, they are additive.

The Maxxum 7 has TTL-OTF metering for the flash. That is to say, light
coming off of the subject bounces off of the film into some sensors at
the bottom of the mirror box. When the flash return is adequate the
flash is stopped.

Assuming you are shooting in A-mode or P mode, the camera will determine
the shutter speed (and the aperture for P mode too) based on the
available light and the fact that the flash is on.

If the subject is that famous neutral grey of 12%, then simply set the
flash compensation wheel to -1, -1.5, -2 depending on how much fill you
need. For a lighter subject offset that setting to the positive; darker
offset further negative. (This applies for both the flash and the
available light compensations).

When you depress the shutter, the camera will set the shutter speed
(assuming A-mode) for the ambient light. Since you set a compensation
for the flash (above) that will quench the flash when that level of
flash return is received. For the balance of the exposure period, it
will be ambient based on the shutter speed. (Same for P-mode, but the
aperture is also chosen; opposite for Speed (and mind sync and other
limitations).

See the manual as well (I have a Maxxum 9 and Maxxum 7D; not the 7).
They explain this in some detail.

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  #4  
Old September 19th 08, 12:32 PM posted to rec.photo.equipment.35mm
Noons
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Posts: 3,245
Default Fill flash math.

David Farber wrote,on my timestamp of 17/09/2008 9:01 AM:
I was wondering about how fill flash affects the existing light. Of course
it will take the shadows and lighten them up a bit, but what about the parts
of the picture that are correctly exposed by the existing light? I mean the
light emanating from the flash (attached to the hot shoe) does not just go
to the shadows. Wouldn't the areas not in shadow then be overexposed by a
half stop? My fill flash photographs (from print film) have been coming out
nicely but this conundrum has me scratching my head. On second thought,
should I be using some ratio method to compensate for the existing light
exposure or is that not how it's done? I'm using a Minolta Maxxum 7 with a
3600HS flash unit.


What you forgot is why/how folks use fill flash.
Yes, it's additive. Have a look at most examples
in manuals for fill-flash: it's usually a portrait of a
person in shadow, with a light or shadow background,
AWAY from the main subject. That means if you shine the
flash at the subject, it'll light it up ok but the
background will be too far away to be seriously influenced
by the amount of flash light used.
Flash lighting is very dependent on distance from the flash,
given constant intensity.
  #5  
Old September 21st 08, 12:07 PM
Harold Gough Harold Gough is offline
Member
 
First recorded activity by PhotoBanter: Mar 2008
Posts: 31
Default

The consideration is no different from when using a reflector to fill in shadows.

In either case, the adjustment required is probably too small to visibly affect the exposure of film, and can be checked with the latter, and 1/4 stop steps are not available to film users. Using fill flash at only 1/2 stop below the main exposure seems excessive, 1/4 stop seeming more approriate.

Quote:
Originally Posted by Noons View Post
David Farber wrote,on my timestamp of 17/09/2008 9:01 AM:
I was wondering about how fill flash affects the existing light. Of course
it will take the shadows and lighten them up a bit, but what about the parts
of the picture that are correctly exposed by the existing light? I mean the
light emanating from the flash (attached to the hot shoe) does not just go
to the shadows. Wouldn't the areas not in shadow then be overexposed by a
half stop? My fill flash photographs (from print film) have been coming out
nicely but this conundrum has me scratching my head. On second thought,
should I be using some ratio method to compensate for the existing light
exposure or is that not how it's done? I'm using a Minolta Maxxum 7 with a
3600HS flash unit.


What you forgot is why/how folks use fill flash.
Yes, it's additive. Have a look at most examples
in manuals for fill-flash: it's usually a portrait of a
person in shadow, with a light or shadow background,
AWAY from the main subject. That means if you shine the
flash at the subject, it'll light it up ok but the
background will be too far away to be seriously influenced
by the amount of flash light used.
Flash lighting is very dependent on distance from the flash,
given constant intensity.
  #6  
Old September 23rd 08, 08:30 PM posted to rec.photo.equipment.35mm
David Farber
external usenet poster
 
Posts: 12
Default Fill flash math.

Noons wrote:
David Farber wrote,on my timestamp of 17/09/2008 9:01 AM:
I was wondering about how fill flash affects the existing light. Of
course it will take the shadows and lighten them up a bit, but what
about the parts of the picture that are correctly exposed by the
existing light? I mean the light emanating from the flash (attached
to the hot shoe) does not just go to the shadows. Wouldn't the areas
not in shadow then be overexposed by a half stop? My fill flash
photographs (from print film) have been coming out nicely but this
conundrum has me scratching my head. On second thought, should I be
using some ratio method to compensate for the existing light
exposure or is that not how it's done? I'm using a Minolta Maxxum 7
with a 3600HS flash unit.


What you forgot is why/how folks use fill flash.
Yes, it's additive. Have a look at most examples
in manuals for fill-flash: it's usually a portrait of a
person in shadow, with a light or shadow background,
AWAY from the main subject. That means if you shine the
flash at the subject, it'll light it up ok but the
background will be too far away to be seriously influenced
by the amount of flash light used.
Flash lighting is very dependent on distance from the flash,
given constant intensity.


Thanks for pointing this out. I use fill flash mostly outdoors where there
is a bright sun and harsh shadows. With the sun high overhead, the light is
not even in the same direction as my on camera fill flash. I think just
taking more pictures, taking good notes, and experimenting more will help me
understand this subject better. With all the options on my camera, I
sometimes pay too much attention to the numbers and start missing the good
shots.

Thanks for your reply.
--
David Farber
L.A., CA


  #7  
Old September 23rd 08, 08:48 PM posted to rec.photo.equipment.35mm
David Farber
external usenet poster
 
Posts: 12
Default Fill flash math.

Ric Trexell wrote:
Wouldn't the areas not in shadow then be overexposed by a
half stop?

************************************************** ***********
I don't understand why you say a half stop, but think of fill flash
as only being a fraction of the available light on the subject. The
meter in the flash (assuming you have an automatic flash) or the
OTF-TTL flash camera combo will measure the extra light and
compensate for it. Doing it manually is still sort of a hit and miss
thing but generally you are not over exposing that much. Ric in
Wisconsin.


Initially my logic was this:

You have a normally exposed outdoor scene. Let's call the amount of light in
this scene 100 units. Now, you add a fill flash which is set to -.5. That
would add 50 units of light. If you add them together, you get 150 units of
light for a scene which was properly exposed with 100 units of light. Of
course the real world is much more complicated than this especially since it
was pointed out to me that the fill flash is not pointing in the same
direction as the main light.

As far as manual vs. automatic, the "how to" aritcles I've read say to set
your exposure manually as if you didn't have the flash. Then, dial in the
flash compensation to whatever you like, usually between. -2 to -.5. Then,
the automatic circuits in the flash will figure it out.

Thanks for your reply.
--
David Farber
L.A., CA


  #8  
Old September 23rd 08, 09:02 PM posted to rec.photo.equipment.35mm
David Farber
external usenet poster
 
Posts: 12
Default Fill flash math.

Alan Browne wrote:
David Farber wrote:
I was wondering about how fill flash affects the existing light. Of
course it will take the shadows and lighten them up a bit, but what
about the parts of the picture that are correctly exposed by the
existing light? I mean the light emanating from the flash (attached
to the hot shoe) does not just go to the shadows. Wouldn't the areas
not in shadow then be overexposed by a half stop? My fill flash
photographs (from print film) have been coming out nicely but this
conundrum has me scratching my head. On second thought, should I be
using some ratio method to compensate for the existing light
exposure or is that not how it's done? I'm using a Minolta Maxxum 7
with a 3600HS flash unit.


For the sake of fill flash, don't worry about it. Yes, they are
additive.

The Maxxum 7 has TTL-OTF metering for the flash. That is to say,
light coming off of the subject bounces off of the film into some
sensors at the bottom of the mirror box. When the flash return is
adequate the flash is stopped.

Assuming you are shooting in A-mode or P mode, the camera will
determine the shutter speed (and the aperture for P mode too) based
on the available light and the fact that the flash is on.

If the subject is that famous neutral grey of 12%, then simply set the
flash compensation wheel to -1, -1.5, -2 depending on how much fill
you need. For a lighter subject offset that setting to the positive;
darker offset further negative. (This applies for both the flash and
the available light compensations).

When you depress the shutter, the camera will set the shutter speed
(assuming A-mode) for the ambient light. Since you set a compensation
for the flash (above) that will quench the flash when that level of
flash return is received. For the balance of the exposure period, it
will be ambient based on the shutter speed. (Same for P-mode, but the
aperture is also chosen; opposite for Speed (and mind sync and other
limitations).

See the manual as well (I have a Maxxum 9 and Maxxum 7D; not the 7).
They explain this in some detail.


Hi Alan,

I've discovered that when you're in "A" mode (and I can only speak for what
the Maxxum 7 does), the selected exposure seems to ignore the ambient light.
For example, in a scene where f./8 @ 1/30th of a second would be the proper
exposure, the camera will recognize that the flash is enabled and then might
choose an exposure of f/8 @ 1/125 of a second. This pretty much decides for
you that the flash is going to be the main light instead of the fill light.
That is why I like to meter the scene in "M" manual mode so I'll know the
exposure will be correct with the given ambient light.

Thanks for your reply.
--
David Farber
L.A., CA



  #9  
Old September 27th 08, 06:05 AM posted to rec.photo.equipment.35mm
Alan Justice
external usenet poster
 
Posts: 94
Default Fill flash math.

I've scanned the answers to this and am surprised that no one has actually
hit on the answer.

Adding a flash does not add, e.g., 1/2 stop of light to the whole frame (at
subjects equal distance from the flash): It adds the same amount of light.
If the lighted area has 100 lumens on it already (or whatever), then adding
another 100 lumens adds one stop (2x). But if the shadows are at 12 lumens
(3 stops below the lighted area), it still adds 100 lumens, but is thus 4
stops brighter than the original shadow. The result is 200 and 112 lumens
for the lighted/shadow, which is less than 1 stop apart. The original was 3
stops apart, so flash makes the lighted and shadow areas closer in
illumination.


Alan Justice

"David Farber" wrote in message
...
I was wondering about how fill flash affects the existing light. Of course
it will take the shadows and lighten them up a bit, but what about the

parts
of the picture that are correctly exposed by the existing light? I mean

the
light emanating from the flash (attached to the hot shoe) does not just go
to the shadows. Wouldn't the areas not in shadow then be overexposed by a
half stop? My fill flash photographs (from print film) have been coming

out
nicely but this conundrum has me scratching my head. On second thought,
should I be using some ratio method to compensate for the existing light
exposure or is that not how it's done? I'm using a Minolta Maxxum 7 with a
3600HS flash unit.




 




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