light modifiers: shoot-through umbrella vs bounce flash
With the contest where the light modifier had to be matched with the specific light pattern, there were clues in the images as to which modifier was used. The clues were in how the light was dispersed. Some were obvious, such as the hard side-ways shadow of direct flash with the camera held vertically. A dead give-away. But others were subtle .. but there.
For example, the image above shows the result, (used indoors in a studio situation) between the white shoot-through umbrella, and bounce flash with the Black Foamie Thing. (Click through the image to get to a larger version.)
A cursory glance at the two images will say they look the same. With more careful consideration, it will be apparent that the white shoot-through umbrella has a more contrasty image, and there is more pronounced light-fall-off across the background.
What is happening here, is that the black foamie thing acts as a flag. With all the other light-sources, the flash is still the light source, positioned where-ever you place the light-stand. But by bouncing the flash, the flash itself isn’t the light-source anymore, but rather the light source is now the area that you’re bouncing flash off.
Let’s call the area that you’re bouncing off, the “implied softbox”. This is the wall and ceiling behind you, or to the side of you. Whatever area it was that you chose to bounce the flash off. This implied softbox is much larger than the white shoot-through umbrella, and hence the light has a softer edge to it. The shadow edge transfer is less sudden with bounce flash (ie, it is softer), than it would be with either a real softbox or a white shoot-through umbrella. So the bounce flash gives less contrasty light than the white shoot-through umbrella. The umbrella is already one of the light modifiers giving soft light, but bounce flash really does open up the light falling onto your subject.
You can control it by flagging it, and even perhaps in zooming it. But it does create soft light.
Now, looking at the background, you will see that the background is more evenly lit than the image where the umbrella was used. This is easily explained again when we realize that with bounce flash, the flash itself isn’t the light source anymore. So the light source (that implied softbox), is much further from your subject than the white shoot-through umbrella. This is true even if the light-stand was in exactly the same spot in relation to the model for both of those scenarios.
Since the implied softbox (from the bounce flash), is much further from our subject, the light fall-off will be more gradual. You might even find in some situations that the background appears brighter with bounce flash than with other ways of modifying your flash. Since the distance from the light source to the subject, compared to the distance from the light source to the background, isn’t a relatively big jump as with other ways of using light closer to your subject, the background might even appear brighter.
I hope this gives some insight in how we go about analyzing the light quality in images.
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