sweet spot in the inverse square law ?

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A-Marc said:

lately i am thinking a lot about the inverse square law and the light fall off with distance. I am in special thinking about a sweet spot in the light fall off, so to say, modifier to subject relation. if its too close i have a rapid fall off and i ll get dark shadow real quick, caused of the light intensity falls off so quickly. if its too far away, there will be no pronounced shadow cause the light intensity fall off is marginal.

so there must be some sort of elegant spot where the fall off is notice able but not either too abrupt nor too flat.

in my theoretical construct i struggle to incorporate the modifier size into the calculation, where i sometimes think it will not play into count as the light fall off % will be the same amount wether its a big or a small modifier.

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Depends on the distance between subject and background.

john isaacs said:

A-Marc said:

lately i am thinking a lot about the inverse square law and the light fall off with distance. I am in special thinking about a sweet spot in the light fall off, so to say, modifier to subject relation. if its too close i have a rapid fall off and i ll get dark shadow real quick, caused of the light intensity falls off so quickly. if its too far away, there will be no pronounced shadow cause the light intensity fall off is marginal.

so there must be some sort of elegant spot where the fall off is notice able but not either too abrupt nor too flat.

in my theoretical construct i struggle to incorporate the modifier size into the calculation, where i sometimes think it will not play into count as the light fall off % will be the same amount wether its a big or a small modifier.

Click to expand...

Depends on the distance between subject and background.

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A point light source is far less complex. A surface source like a soft box - the law still applies but there’s complexity compared to a point light source.

One factor I haven't seen mentioned in this discussion is the effect of falloff if you are shooting a group of people (or anything) that vary in distance from the light. In this case, the further the light is from the group, the less of a difference you will see in lighting intensity from side to side (or front to back). If you are a numbers person, you can do the math to see that if you have two subjects that are two feet apart and the light is 4 feet away, you'll have about one stop difference in intensity. at 10 feet you'll have less than half a stop.
The other thing to consider is that the softness of the light is determined by the relative size of the light source to the subject: bigger=softer. (Don't confuse softness with contrast. Often a light that is further away from the subject will result in lower contrast because light bouncing off the walls and ceiling will provide more fill.) This is why photographers use a really BIG soft box or umbrella if they are shooting a large group of people and want to keep the main light soft. The light has to be at sufficient distance to prevent falloff, but relatively large enough at that distance to produce soft-edged wrap-around shadows.

psartman said:

One factor I haven't seen mentioned in this discussion is the effect of falloff if you are shooting a group of people (or anything) that vary in distance from the light. In this case, the further the light is from the group, the less of a difference you will see in lighting intensity from side to side (or front to back). If you are a numbers person, you can do the math to see that if you have two subjects that are two feet apart and the light is 4 feet away, you'll have about one stop difference in intensity. at 10 feet you'll have less than half a stop.
The other thing to consider is that the softness of the light is determined by the relative size of the light source to the subject: bigger=softer. (Don't confuse softness with contrast. Often a light that is further away from the subject will result in lower contrast because light bouncing off the walls and ceiling will provide more fill.) This is why photographers use a really BIG soft box or umbrella if they are shooting a large group of people and want to keep the main light soft. The light has to be at sufficient distance to prevent falloff, but relatively large enough at that distance to produce soft-edged wrap-around shadows.

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yes i hear you, thanks for sharing.

my post was about theoretical concept about a "sweet spot" in the light fall off in the law.

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A-Marc said:

lately i am thinking a lot about the inverse square law and the light fall off with distance. I am in special thinking about a sweet spot in the light fall off, so to say, modifier to subject relation. if its too close i have a rapid fall off and i ll get dark shadow real quick, caused of the light intensity falls off so quickly. if its too far away, there will be no pronounced shadow cause the light intensity fall off is marginal.

so there must be some sort of elegant spot where the fall off is notice able but not either too abrupt nor too flat.

in my theoretical construct i struggle to incorporate the modifier size into the calculation, where i sometimes think it will not play into count as the light fall off % will be the same amount wether its a big or a small modifier.

Click to expand...

I generally use a 48" octabox and place it 48" from the subject. Seems like a good rule for me.

A-Marc said:

lately i am thinking a lot about the inverse square law and the light fall off with distance. I am in special thinking about a sweet spot in the light fall off, so to say, modifier to subject relation. if its too close i have a rapid fall off and i ll get dark shadow real quick, caused of the light intensity falls off so quickly. if its too far away, there will be no pronounced shadow cause the light intensity fall off is marginal.

so there must be some sort of elegant spot where the fall off is notice able but not either too abrupt nor too flat.

in my theoretical construct i struggle to incorporate the modifier size into the calculation, where i sometimes think it will not play into count as the light fall off % will be the same amount wether its a big or a small modifier.

Click to expand...

There is no one sweet spot as such but the answer by BAK about f/stops is one shorthand way of the right approach. Or you can calculate precisely if you specify your distance and lighting ratio and the limits of the acceptable falloff from the ratio.

So more versatile, if you want to calculate other choices, see the ISL calculator at my site at https://www.scantips.com/lights/flashbasics1.html#calc . In the menu 3 there, enter your desired light distance and desired light ratio (like 4 foot distance and then -1 EV falloff, for main and fill light and ratio), and then also specify your limit on what would be too much or too little falloff, like plus or minus 1/3 EV from the -1 EV rat. Any whatever numbers. But if your desired distance is numerically same number as a full f/stop (as 4 feet matches f/4), and if the limits are third stops or full stops, then the limits will also match feet numbers. Saying the ratio will be for example within plus or minus 1/3 EV fall off of the specified -1 EV ratio. You can specify anything you want, within reason. Pick the falloff limits that are acceptable to match your subject depth, like for groups for example.

But the best part is there is no need to match f/stops because that is rather limited. If your choices don't match f/stops, then the calculator gives you the precise distance limits in feet anyway. And it works the same if you use meters or feet.

However, it is important to be aware the distance from an umbrella or softbox is NOT the distance from the fabric, as that will NOT be at all accurate. Instead it is the path distance from subject to the flash tube source of the light. The path from flash tube down the reflected umbrella shaft and back to the subject, or the distance through the fabric of a softbox or shoot through umbrella, the actual path distance from flash tube to subject. The loss at the fabric is just a step function, affecting exposure, but not affecting the Inverse Square Law math of distance. There will of course be vocal nonbelievers of that, but if wise, they will do some testing before saying too much.

WFulton said:

There will of course be vocal nonbelievers of that, but if wise, they will do some testing before saying too much.

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would you mind publishing your tests and testing methodology?

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Ellis Vener said:

Ellis Vener said:

There will of course be vocal nonbelievers of that, but if wise, they will do some testing before saying too much.

Click to expand...

would you mind publishing your tests and testing methodology?

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See https://www.scantips.com/lights/flashbasics1.html#calc

Bottom of that page. Been there for years.

Just carefully try it.