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fall-off


amirali mohammadi
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Hello friends, I'm sorry I had a little trouble defining fall-off, the fact is that in the books on lighting, this is a bit vague; However, Herbert Zettl (a university professor) describes fall-off in his book:


We use the term falloff to mean two different yet related light/shadow relationships:
the brightness contrast between the light and shadow sides of an object, and the
rate of change from light to shadow.
Contrast

If the brightness contrast between the lighted side of an object and the attached shadow is high, the falloff is fast. This means that the illuminated side is relatively bright, and the attached shadow is dense and dark. If the brightness contrast is low, the resulting falloff is slow the brightness difference between the illuminated side and the attached-shadow side is relatively small. In extremely flat lighting, no contrast at all shows between the so-called illuminated and shadow sides. In this case, falloff no longer exists. Because most flash photography illuminates the subject directly from the front, both sides are often equally bright. Such elimination of the light/shadow contrast— and with it the falloff—results in the typically flat image of such snapshots

. Change

Calling falloff “fast” or “slow” makes more sense when applied to the rate of change between light and dark, abrupt change from light to shadow represents extremely fast falloff. 

He also wrote: Spotlights, which have a highly directional
beam, produce fast falloff and A highly diffused floodlight produces slow
falloff
Do you think this definition is correct? Thank you

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Sorry, tldr, but "fall off" is pretty simple and can be explained with a lot less words.

The closer a source is to something, the greater the contrast, and hence - a more sudden fall off.

At the extreme end, the sun, which is millions of miles away and produces a completely even light with essentially no fall off.

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I don’t think distance of source and contrast are related. An overcast sky is a distant source creating a lower-contrast effect and the sun falling through gaps in a forest canopy is a distant source creating a higher-contrast effect.

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Posted (edited)
12 hours ago, Christopher Santucci said:

Sorry, tldr, but "fall off" is pretty simple and can be explained with a lot less words.

The closer a source is to something, the greater the contrast, and hence - a more sudden fall off.

At the extreme end, the sun, which is millions of miles away and produces a completely even light with essentially no fall off.

Thank you very much for your answer, the fact is that the author (Herbert Zettl) is highly respected at our university. So my professors insisted that I use the author's definitions. However, you are quite right, it can be explained in a shorter way. Thank you Dear Mr. Santucci

Edited by amirali mohammadi
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4 hours ago, David Mullen ASC said:

I don’t think distance of source and contrast are related. An overcast sky is a distant source creating a lower-contrast effect and the sun falling through gaps in a forest canopy is a distant source creating a higher-contrast effect.

Thank you very much for your comment, dear Master Mullen. Do you think that Zettl's definitions are correct?

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I think there is no "scientific" definition for "fall-off" in terms of contrast effect, that is more or less a vague description someone gives when a room is going dark quickly around the area that is lit. You could have a person lit with a soft box and if the sides of the room are far away, or the furniture and walls are dark-toned, the perception will be that the room "falls off" despite the light being soft.

So it's a term that gets used but I would be hesitant to define it too clearly.

It is easier to talk about "fall-off rate", which is a way to avoid the more scientific term "inverse square law" since that technically only applies to point sources, though I think the rate is similar.

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23 minutes ago, David Mullen ASC said:

I think there is no "scientific" definition for "fall-off" in terms of contrast effect, that is more or less a vague description someone gives when a room is going dark quickly around the area that is lit. You could have a person lit with a soft box and if the sides of the room are far away, or the furniture and walls are dark-toned, the perception will be that the room "falls off" despite the light being soft.

So it's a term that gets used but I would be hesitant to define it too clearly.

It is easier to talk about "fall-off rate", which is a way to avoid the more scientific term "inverse square law" since that technically only applies to point sources, though I think the rate is similar.

Thank you for your complete explanation, dear master Mullen, you always answer us generously. We owe you. Thank you🌺🌺🌺

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10 hours ago, David Mullen ASC said:

I don’t think distance of source and contrast are related. An overcast sky is a distant source creating a lower-contrast effect and the sun falling through gaps in a forest canopy is a distant source creating a higher-contrast effect.

OK, let's use the same smallish fixture and place it 1' from a face, and then back it out 10' from that same face. At 10' the fall off will be almost non-existent (ENG light, I call it) on that face, at 1' there will be significant fall off.

Placing a lit 20x silk 100 yards away will throw light no unlike a single hard source with no fall off, but placing that same 20x 10' away will produce a light with more contrast, hence greater fall off.

Edited by Christopher Santucci
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7 hours ago, Christopher Santucci said:

OK, let's use the same smallish fixture and place it 1' from a face, and then back it out 10' from that same face. At 10' the fall off will be almost non-existent (ENG light, I call it) on that face, at 1' there will be significant fall off.

Placing a lit 20x silk 100 yards away will throw light no unlike a single hard source with no fall off, but placing that same 20x 10' away will produce a light with more contrast, hence greater fall off.

I don’t really agree — the contrast could be the same depending on the circumstances. I could light a face with a far away key to a 4:1 contrast ratio and I could light a face with a soft key closer to a 4:1 contrast ratio. When I light a master with farther lights and then come in with softer lights up closer, I don’t have a problem with a contrast increase.

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I had to draw pictures. What I'm saying is distance creates less contrast because (as per inverse square law), all light falling on a surface from the farther light is doing so at a lower ratio difference. The two arrows depicting light emitting from the closer light have a much higher ratio than the farther light, and hence would create more contrast.

This is why we bring lights far enough away from a flat copy shot, because that light is more even that way.

close.jpg

far.jpg

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I'm really just talking about the nature of light under any circumstance. Bringing a fixture closer (to anything) will mean greater fall off because you're going to expose for whatever, but moving that same fixture back means all that light falling on whatever is going to be more even, hence less contrast and less fall off. It's like spraying water from a hose at a wall. If you're close, part of the wall will get wetter than another part (contrast), but if you back up, more of the wall will get evenly wet.

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@amirali mohammadi

That professor is describing the same thing as two different terms. Perhaps he's using older concepts. "Fall-off", as I and every shooter/photographer I've met use it, is the fall-off rate. And it's literally the inverse square law. (Closer to the source, the steeper the drop. The further from source, the more gradual the drop.)

Contrast ratio is separate.

I would love to chat with this professor. Because contrast is dependent solely on a balance of more intense and less intense bodies of light that the subject sees. In the flash-photography example, if the camera had rotated around the subject 90°, there would be lots of contrast. But inside a white sphere, that would be truly flat. Inside a white sphere with an extremely intense hardpoint can be just as contrasty as the photography setup, if the intensity was strong enough.

His example of the spotlight/floodlight is not an accurate interpretation.

The inverse square is simple to explain. No light rays ever travel in perfect parallel, so they naturally spread from one another. And that spread is described in the inverse square law. It's a logarithmic line that has a constant rate of fall-off. You can't circumvent the law. But you can stretch or compress the line closer or further from the light, which is what we see when we spot and flood a light. An extreme example is a laser. Measured within a few meters, the fall-off rate doesn't appear to change. But over a couple kilometers, you'll see that it spreads and drops in intensity under the inverse square law. A floodlight's intensity drops so fast near the lamp that it appears to have a lower falloff. If you funnel all that light down a directional tube, the logarithmic line is stretched over a longer distance, and that same intense drop-off will appear further from the fixture.

By learning the laws of light (1: Inverse square law, 2: Law of reflection, 3: Law of refraction), and understanding them, you'll be able to identify for yourself what is anecdote, misconception, or false claims. Analyze the laws within your setups and it'll get easier.

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7 hours ago, Christopher Santucci said:

I'm really just talking about the nature of light under any circumstance. Bringing a fixture closer (to anything) will mean greater fall off because you're going to expose for whatever, but moving that same fixture back means all that light falling on whatever is going to be more even, hence less contrast and less fall off. It's like spraying water from a hose at a wall. If you're close, part of the wall will get wetter than another part (contrast), but if you back up, more of the wall will get evenly wet.

I still think you are mixing up fall-off rate with contrast. If you light a field at night from the side with an 18K HMI a block away and then light the close-up with a 12x12 soft light ten feet away, no viewer is going to say “hey, why did the contrast increase?” Same thing when you silk the overhead sun that is far away, creating a soft light only 20 feet above the actor’s head, you don’t see a huge increase in contrast (I can’t imagine telling a director that I won’t silk hard overhead sunlight because that’s only going to make the contrast problems worse.) What you do get is a more rapid fall-off rate. But the shadow side opposite the key doesn’t change much, which is what contrast generally refers to.

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1 hour ago, David Mullen ASC said:

I still think you are mixing up fall-off rate with contrast. If you light a field at night from the side with an 18K HMI a block away and then light the close-up with a 12x12 soft light ten feet away, no viewer is going to say “hey, why did the contrast increase?” Same thing when you silk the overhead sun that is far away, creating a soft light only 20 feet above the actor’s head, you don’t see a huge increase in contrast (I can’t imagine telling a director that I won’t silk hard overhead sunlight because that’s only going to make the contrast problems worse.) What you do get is a more rapid fall-off rate. But the shadow side opposite the key doesn’t change much, which is what contrast generally refers to.

Contrast and fall off are the same thing to my mind. Key someone standing 10 feet in front of another person with a fixture that's 3 feet away. Compare that to the fixture at 20 feet away. What happens is greater fall off at 3 feet away, than 20 feet away. The unlit person further back will be darker with the fixture at 3 feet away from the subject (more contrast - more fall off), and both will be more evenly lit with the fixture 20 feet away (less contrast - less fall off).

You're talking about using modifiers and different setups. I'm just talking about the nature of light on an elemental level. It's really just inverse square law at play.

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  • 2 weeks later...
On 5/2/2021 at 4:25 AM, Christopher Santucci said:

Contrast and fall off are the same thing to my mind. Key someone standing 10 feet in front of another person with a fixture that's 3 feet away. Compare that to the fixture at 20 feet away. What happens is greater fall off at 3 feet away, than 20 feet away. The unlit person further back will be darker with the fixture at 3 feet away from the subject (more contrast - more fall off), and both will be more evenly lit with the fixture 20 feet away (less contrast - less fall off).

You're talking about using modifiers and different setups. I'm just talking about the nature of light on an elemental level. It's really just inverse square law at play.

If contrast and fall off are the same in your mind might just be because they are usually interchangeable, however I think the better way to "see it in your mind is":

Contrast = The difference between A and B.

Fall off = "The rate of the contrast difference", i.e. is it going from full strength to half in 1 feet or 5 feet, 20 feet, etc... 

In practical terms it means that the sun has 0 fall off, but lots of contrast (which is also why it is such a hard task to emulate it properly with artificial lights). Basically you have to decide with lights if you gonna control the "contrast" or the "fall off", it is hard to get both right if the scene/objects being lit are very big/complex. 

The example you give is a good one, but it is also basically saying that contrast and fall off are two separate things which they are (which is why it isn't good to "combine" the two terms). 

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They are different things. If you had a 5K 20’ above an open-top set at half-spot in the center of the room, pointed straight down, and then you flew a 20’x20’ diffusion frame 10’ above the set under the light, there would be an increase in fall-off rate because the diffusion now becomes the source and it is closer — but there would not be an increase in contrast. Contrast is a ratio between the brightest and darkest thing in the frame, fall-off is the rate that the intensity drops over distance.

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On 5/2/2021 at 12:25 PM, Christopher Santucci said:

Contrast and fall off are the same thing to my mind. Key someone standing 10 feet in front of another person with a fixture that's 3 feet away. Compare that to the fixture at 20 feet away. What happens is greater fall off at 3 feet away, than 20 feet away. The unlit person further back will be darker with the fixture at 3 feet away from the subject (more contrast - more fall off), and both will be more evenly lit with the fixture 20 feet away (less contrast - less fall off).

You're talking about using modifiers and different setups. I'm just talking about the nature of light on an elemental level. It's really just inverse square law at play.

Contrast is a difference in illuminance between two points. For example, to take it out of light and/or radiology. The contrast between 'terrible' and 'wonderful' - there is no path between those two words they are just on the opposite ends of the meaning. Such as light and dark. By definition - the state of being strikingly different from something else in juxtaposition or close association.

Fall-off, in theory, is looking at plane wave propagation in free space. 'Modifiers' and different set-ups that David is using as examples does not alter the fact that light is an electro-magnetic wave. You cannot modify light into something else. If you diffuse light (scatter it) or refract it etc - which I may add your source that you most likely are using for your example, say a blondie or a Tweenie are doing said 'modifications' on a much smaller, potentially minuet to the eye, scale.

There are a dozen great practical examples in this thread - however light on an elemental (which it isn't) level is just electromagnetic radiation. That, to put it crudely, does dilute over space, that has no real relationship with a relatively artistic term. 

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On 5/10/2021 at 10:42 AM, Karl Eklund said:

If contrast and fall off are the same in your mind might just be because they are usually interchangeable, however I think the better way to "see it in your mind is":

Contrast = The difference between A and B.

Fall off = "The rate of the contrast difference", i.e. is it going from full strength to half in 1 feet or 5 feet, 20 feet, etc... 

In practical terms it means that the sun has 0 fall off, but lots of contrast (which is also why it is such a hard task to emulate it properly with artificial lights). Basically you have to decide with lights if you gonna control the "contrast" or the "fall off", it is hard to get both right if the scene/objects being lit are very big/complex. 

The example you give is a good one, but it is also basically saying that contrast and fall off are two separate things which they are (which is why it isn't good to "combine" the two terms). 

OK, so they're merely related.

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Posted (edited)

Dear friends and Masters, thank you very much for your guidance and answers. Based on your explanation, I conclude that Mr. Herbert Zettl's definitions were not very correct.
However, according to my conversation with One of the senior professors, he said that Zettl's definitions are correct  and have a scientific aspect. My problem is that more than half of the movies I analyzed were based on his definition, for example where the contrast on the character's face was high, I just analyzed that the fall-off is fast. I think I have to correct these. . Thanks again to all of you

Edited by amirali mohammadi
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On 5/1/2021 at 11:20 PM, Stephen Sanchez said:

@amirali mohammadi

That professor is describing the same thing as two different terms. Perhaps he's using older concepts. "Fall-off", as I and every shooter/photographer I've met use it, is the fall-off rate. And it's literally the inverse square law. (Closer to the source, the steeper the drop. The further from source, the more gradual the drop.)

Contrast ratio is separate.

I would love to chat with this professor. Because contrast is dependent solely on a balance of more intense and less intense bodies of light that the subject sees. In the flash-photography example, if the camera had rotated around the subject 90°, there would be lots of contrast. But inside a white sphere, that would be truly flat. Inside a white sphere with an extremely intense hardpoint can be just as contrasty as the photography setup, if the intensity was strong enough.

His example of the spotlight/floodlight is not an accurate interpretation.

The inverse square is simple to explain. No light rays ever travel in perfect parallel, so they naturally spread from one another. And that spread is described in the inverse square law. It's a logarithmic line that has a constant rate of fall-off. You can't circumvent the law. But you can stretch or compress the line closer or further from the light, which is what we see when we spot and flood a light. An extreme example is a laser. Measured within a few meters, the fall-off rate doesn't appear to change. But over a couple kilometers, you'll see that it spreads and drops in intensity under the inverse square law. A floodlight's intensity drops so fast near the lamp that it appears to have a lower falloff. If you funnel all that light down a directional tube, the logarithmic line is stretched over a longer distance, and that same intense drop-off will appear further from the fixture.

By learning the laws of light (1: Inverse square law, 2: Law of reflection, 3: Law of refraction), and understanding them, you'll be able to identify for yourself what is anecdote, misconception, or false claims. Analyze the laws within your setups and it'll get easier.

Thank you very much for your complete and comprehensive explanation, I hope people like you start writing books so that we students do not get so confused.🌺🌺

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