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Lighting Question
Old 10-26-2007, 09:53 PM   #1 (permalink)
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I must be missing something simple or misreading Rolando's blog post.

http://rolandogomez.net/wordpress/20...-of-the-image/

Quote:
First, through the inverse square law the background of an image becomes brighter when the light modifier (light source) is moved farther away from the background while maintaining the same subject to background distance and the exposure is set for the subject. The overall contrast in the image increases and shadows become harder and more dramatic.


Second, through the same inverse square law, the background darkens when the light modifier is moved closer to the background while maintaining the same subject to background distance, with the exposure set for the subject. The overall contrast in the image is also decreased and shadows become softer and less dramatic.
In my mind, I picture a photographer; a subject; and then a background, say a wall, in a linear line. The light source is beaming toward the wall to provide light. Is that correct? If correct, wouldn't having the light source close to the wall make it brighter than if the light source were further away from the wall?
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Re: Lighting Question
Old 10-26-2007, 10:15 PM   #2 (permalink)
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Let's look at an example...

a. Light -- 2m -- Subject -- 2m -- Background

and

b. Light -- 4m -- Subject -- 2m -- Background

Do you see that in "b", the ratio of light falloff to the background from a properly exposed subject is less than in "a"? That's why the background is brighter.

If you don't...

In "a", the background is double the distance from the light as the light is to the subject.

In "b" the background is only half again the distance from the light as the light is from the subject. Aha!
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Re: Lighting Question
Old 10-26-2007, 10:27 PM   #3 (permalink)
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My assumption is that it has to do with relative brightness and the assumption that a single light source is illuminating both the model and the background and the exposure is based on the model. You are correct that the background would get lighter as the light gets closer, but so does the subject. In fact, the subject gets even more additional light than the background. So, you stop down to produce a proper exposure of the subject and the background now appears to be darker.
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Re: Lighting Question
Old 10-26-2007, 10:44 PM   #4 (permalink)
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The key is "while maintaining the same subject to background distance with the exposure set for the subject".

Assume the subject is 6 feet from the background. If the light is 2 feet from your subject (assume a straight line), then 3 stops less light (1/8th) strikes the background than your subject because it's 3 times farther away. However, if the light is 6 feet from your subject, then only 2 stops less light strikes the background than your subject because it's only twice as far away. If the light is, say, 60 feet away then there isn't much difference in the amount of light striking your subject and the amount of light striking the background (less than 1/10th. of a stop).

Assuming a white background above, meter for your subject. Three stops less light will render the background middle gray. Two stops less light will render the background about the same tonality as caucasian skin, and < 1/10th stop less light will render the background white.

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Re: Lighting Question
Old 10-26-2007, 10:47 PM   #5 (permalink)
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The only thing that changes in RG's 2 scenarios is the distance of the light from the subject and background. In Paragraph 1, the light is further from background and subject and the exposure is based on the light falling on the subject. In Paragraph 2, the lights is closer to the subject and background and again the exposure is based on the light falling on the subject. In each case the fstop is different (assuming the use of flash and a fixed shutter speed). The change in harshness of shadows is based on the distance of the light source from both subject and background. The closer the light source is to the two, the bigger it is relative to the two and the softer the light becomes. Naturally when the light is closer to the model, you are stopping down more for the exposure and that means the background will appear darker. When the light is further away, you are opening up and the background will appear lighter. Does that make sense?
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Re: Lighting Question
Old 10-27-2007, 07:38 AM   #6 (permalink)
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I thank everyone for replying. I now understand Rolando's text.

I greatly appreciate everyone's response.
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Re: Lighting Question
Old 10-27-2007, 04:46 PM   #7 (permalink)
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Quote:
Originally Posted by R_Fredrick_Smith View Post
The only thing that changes in RG's 2 scenarios is the distance of the light from the subject and background. In Paragraph 1, the light is further from background and subject and the exposure is based on the light falling on the subject. In Paragraph 2, the lights is closer to the subject and background and again the exposure is based on the light falling on the subject. In each case the fstop is different (assuming the use of flash and a fixed shutter speed). The change in harshness of shadows is based on the distance of the light source from both subject and background. The closer the light source is to the two, the bigger it is relative to the two and the softer the light becomes. Naturally when the light is closer to the model, you are stopping down more for the exposure and that means the background will appear darker. When the light is further away, you are opening up and the background will appear lighter. Does that make sense?
Cheers,
rfs
RFS,

I think you are wrong. IF you raise the flash power setting and move the light back so you keep the camera at the same f:stop OR lower the power as you move the light closer to the subject, the background will still appear darker when the light is is the main position.

This is due to the inverse square law and ratio between the flash to subject distance verses the flash to background distance. It was called Depth of Light by Bill Norman and he had published a pamphlet on it several years ago.
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Re: Lighting Question
Old 10-27-2007, 05:34 PM   #8 (permalink)
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Quote:
Originally Posted by kgphoto View Post
RFS,

I think you are wrong. IF you raise the flash power setting and move the light back so you keep the camera at the same f:stop OR lower the power as you move the light closer to the subject, the background will still appear darker when the light is is the main position.

This is due to the inverse square law and ratio between the flash to subject distance verses the flash to background distance. It was called Depth of Light by Bill Norman and he had published a pamphlet on it several years ago.
But you are not raising the flash power setting in RG's example. The only thing you are doing is moving the light source either further away or closer to the subject.
Cheers,
rfs
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Re: Lighting Question
Old 10-27-2007, 05:42 PM   #9 (permalink)
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Quote:
Originally Posted by kgphoto View Post
RFS,

I think you are wrong. IF you raise the flash power setting and move the light back so you keep the camera at the same f:stop OR lower the power as you move the light closer to the subject, the background will still appear darker when the light is is the main position.
As you move the light backward and increase the light output so that you can keep the same f-stop for the subject, the background will get brighter, not darker as you suggest.

Think about it using my above example, but let's take "b" to an extreme...

a. Light --- 2m --- Subject --- 2m --- Background

and

b. Light --- 20m --- Subject --- 2m --- Background

In the second example, you've really gotta crank up the power to the light that's now 20 meters from the subject to maintain the same f-stop you had in the "a" example. Since the background is only 10% further, it will be brighter. Inverse square law says that in "a" the background is getting only 25% of the light that the subject is, whereas in "b" the background gets 82.6% of the light reaching the subject.

The second half of your statement ... moving the light closer...will indeed darken the background.
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Re: Lighting Question
Old 10-27-2007, 05:56 PM   #10 (permalink)
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RFS,

I must have misinterpreted the "the exposure is set for the subject" part. I did that my adjusting the flash power, you interpreted that to mean adjusting the f:stop.

Either way, it is still the ratio between the two distances that determines the amount of light fall off.
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