Why does fill flash work?

I'm not a beginner but I will ask a beginner question. I have used fill flash for decades with good results. Why?

If a subject is in bright sun, the tones on the face may be Zone 6 (sunlit) and Zone 3 ( shadow). If I add 2

stops of fill, the shadows move to Zone 5 and the Zone 6 sunlit side stays where it is or close to it. Good thing

too. Otherwise, if the Zone 6 moved up two stops I would now have Zone 8 (sunlit) and Zone 5 (shadow). Printing

that for Zone 6 flesh tones would bring me back to Zone 6 sunlit flesh tones and Zone 3 shadow. Why does the fill

flash only take on the shadows and not on the sunlit?

It works because it takes much less additional light to raise a lower degree of lighting by two stops than to

raise a higher degree of lighting by two stops. For example, if the part of the subject in the shade is

receiving 20 "units" of light and the part in the sun is receiving 160 "units" (three stops more), by adding 60

additional "units" of light, you've increased the lighting in the shade by two stops, but you've increased the

lighting in the sun by less than half a stop.

Because there is less illumination to start with in the darker tones. You probably are , depending on the situation, adding to the higher values as well but just not by as much.

 

Think of it this way (but note that this is arithmetic and not logarithmic and that exposure is logarithmic in nature)

 

You have two measuring cups. A stop of exposure ( one zone) is one ounce of water and as we know each additional stop of exposure is twice as bright as the previous one.

 

So Zone 1 = 1 oz; Zone 2 = 2oz; Zone 3 = 4oz; Zone 4 = 8oz; Zone 5 = 16oz; zone 6 = 32oz; Zone 7 = 64 oz, Zone 8 = 128oz; Zone 9 = 256oz; Zone 10 = 512oz, etc.

 

So given your example, Zone 3 would be 4 ounces and Zone 6 would be 32 ounces. You add enough water, 12 ounces, to the first cup to bring it up to the Zone 5 level ( 4 + 12 = 16). You also add the same amount of water to the second cup and you get 44 ounces ( 32 + 12), so in effect while you added 2 stops to the zone 3 exposure you only added a bit less than 3/8's of a stop to the Zone 6 exposure.

Because there is less illumination to start with in the darker tones. You probably are , depending on the situation, adding to the higher values as well but just not by as much.

 

Think of it this way (but note that this is arithmetic and not logarithmic and that exposure is logarithmic in nature)

 

You have two measuring cups. A stop of exposure ( one zone) is one ounce of water and as we know each additional stop of exposure is twice as bright as the previous one.

 

So Zone 1 = 1 oz; Zone 2 = 2oz; Zone 3 = 4oz; Zone 4 = 8oz; Zone 5 = 16oz; zone 6 = 32oz; Zone 7 = 64 oz, Zone 8 = 128oz; Zone 9 = 256oz; Zone 10 = 512oz, etc.

 

So given your example, Zone 3 would be 4 ounces and Zone 6 would be 32 ounces. You add enough water, 12 ounces, to the first cup to bring it up to the Zone 5 level ( 4 + 12 = 16). You also add the same amount of water to the second cup and you get 44 ounces ( 32 + 12), so in effect while you added 2 stops to the zone 3 exposure you only added a bit less than 3/8's of a stop to the Zone 6 exposure.

This is a really good question.

 

Let's call the amount of light that it takes to go from zone 3 to zone 5 = 2X. Then X is the amount of light it takes to go from zone 3 to zone 4.

 

Ok, so far so good, now how much light does it take to go from zone 6 to zone 8. First we'll go from zone 3 to zone 8...

 

zone 3 to zone 8 is 2^5X = 32 X this is because each zone doubles the amount of light required and 2*2*2*2*2 = 2^5 = 32.

 

Now to go from zone 8 back to zone 6 we can divide by 2 twice... 32/4 = 8 so raising zone 6 to zone 8 requires 8X light output.

 

So to raise the shadow from 3 to 5 requires 2X but to raise the highlight from 6 to 8 requires 8X. Since you're only putting out 2X you don't raise zone 6 very far above its current value.

Because when you add fill flash, your shutter speed and aperture are remaining the same, if you're metering for the sunlit areas. And, your flash is nowhere near the brightness of the sun, so it's not adding much to the sunlit areas, but it's adding a lot to the shadow areas. Try metering on the shadow areas, and watch those sunlit areas get blown out! High speed shutter function is a wonderful modern innovation we have in many cameras these days, and helps our flash sync at high shutter speeds. We can now retain all the bright sky detail and fill the darker foreground with flash, producing images we could never make before.

 

By the way, I'm not sure what all that "zone" stuff is you're talking about, I'm assuming it has something to do with relative light levels in a photo?

because if the difference between light and shadow is 3 stops (8:1 ratio), then to bump the shadow by 2 stops, thats adding 4 units of light to each. you would have needed 32 units of light in order to raise the highlights by 2 stops.

 

so in reality, the flash is adding to the main but the % increase is not enough to be particularly noticeable in the case of main/highlights.

<i>Let's call the amount of light that it takes to go from zone 3 to zone 5 = 2X. Then X is the amount of light it takes to go from zone 3 to zone 4.</i><P>

This is incorrect. It takes twice as much to go from 4 to 5 as it does to go from 3 to 4 (so 4 to 5 is 1.333X and 3 to 4 is 0.667X).

Another point: it's not a good idea to equate zones with stops of illlumination. Zones refer to tonal values in a print ("zone system"); they don't correspond to light levels falling on a subject.

Zones could be called recording values. Start with a gray card, zone 5, open up one stop from the gray card correct exposure and it will record as zone 6; open up another stop and it will record as zone 7, etc. Close down one stop from the gray card correct exposure and it will record as zone 4, etc. My zone 6 and zone 3 values in the original post are what would be recorded if no fill flash was used. And I just checked with my spotmeter. In bright sun, when sunlit skin is placed on zone 6, shadowed skin falls on zone 3.

<i>Start with a gray card, zone 5, open up one stop from the gray card correct exposure and it will record as

zone 6; open up another stop and it will record as zone 7, etc. Close down one stop from the gray card correct

exposure and it will record as zone 4, etc.</i><P>

And if you push development or significantly increase paper contrast, that one-stop change in lighting will

correspond to a change of more than one "zone" on the print. If you pull development or significantly decrease

paper contrast, a one-stop change in lighting will correspond to a change of less than one "zone" on the print.

Zones and stops are fundamentally different things. The fact that they might correspond under a particular set

of circumstances doesn't mean the terms should be used interchangeably.

When metering a scene each stop is a one zone difference. If I didn't have a flash for fill, I could open up two stops and the sunlit flesh tones would now fall on zone 8 and the shadow flesh tones would fall on zone 5. Then I could give N-2 film development and the zone 8 would be brought down to close to zone 6 and the zone 5 would stay close to zone 5 on the negative.

Something not mentioned so far is that the fill flash does not come from the same direction as the sun. If it did, it

would be pointless.<BR><BR>

I think you are making a mistake in thinking that it raises the light at all levels by the same amount. It doesn't, it just

adds the same amount everywhere (subjuct to inverse square rule).<BR><BR>

A stop is a ratio (double or half). Adding two stops would be to quadruple the light. I don't think your flash is that

powerful.<BR><BR>

It doesn't take much light to raise something in the shadows up a little bit to give detail but it would take a lot to

significantly raise the highlight areas.

James, you can take the sun at, on average, 150,000 candlepower per Meter, sq. With a primary source like that, it doesn't even know your flash is there. The shadows know, but that's about it. Cheesy pun, I know, but that's the explanation.

 

Bill P.