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#1
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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
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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
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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 |
#4
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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. -- -- 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. -- usenet posts from gmail.com and googlemail.com are filtered out. |
#5
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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 |
#6
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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. |
#7
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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:
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#8
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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 |
#9
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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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