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Bounce light calculations


Jonathan O'Neill
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Hi just wondering if anyone knows these workings? The reflective characteristics of the material would be required I believe, plus the size of the bounce. What are the rough calculations to work out the distance from the ultrabounce to get a reading of 67lux? (t2 , 25p , iso800)

 

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Jon

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Gosh, I'm not very math enabled to be much help. It looks like, as you correctly surmised, you'd need values for the "reflective characteristics" of the ultrabounce.  Perhaps something like the scatter and return of the light beam.

Why not just "breadboard" it and do a trial run with the actual equipment you plan to use? Most rental houses I've used wouldn't mind you doing this as long as you stayed out of the way and didn't interrupt their work flow. You could experiment with bounce placement and the effect of spot/flood into a big source.

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There isn’t really an easy formula like the inverse square rule for bounced or diffused light. There are simply too many variables going on between lamps, textiles, light angles etc.

Once you have a bit of experience you can generally get a good idea of what’s going to work. If you are nervous about your lamp choice you can alway err on the side of going bigger, and wire the lamp back.
 

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As said above there are too many variables to be able to make an accurate calculation, however, I did once go down the path of trying to find a solution of calculating light fall-off from a soft source. I asked several cinematographers and none knew an exact answer (some recommended with Ultra bounce just cut 2 1/2 stops from your original output - this isn't accurate when your measuring close to the bounce however at a distance it is somewhat a good guesstimate). 

I went ahead and asked on a physics forum and got this answer - this answer was about shooting light through a diffusion. 

"If your diffuser is good enough that the light from any portion of it close to uniform from any direction, and the light from all portions is equal in intensity, then the total illumination on your point is proportional to the solid angle from the point that intercepts the light. At large distances (small angles) this will become equal to inverse square.

At shorter distances, the falloff is slower. Very close to the source, the intensity becomes nearly uniform with distance.

The actual solid angle formula is not one that I tend to use much, but there are some formulas and references on Example Formula's"

I got this response sadly at a busy time (the original question was in relation to an upcoming job) so I never truly followed it up but plan on doing so. I do think having a somewhat accurate formula of calculating light fall-off from a large source, especially considering - "the inverse square rule is often still a useful approximation; when the size of the light source is less than one-fifth of the distance to the subject, the calculation error is less than 1%"

 

 

 

 

 

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3 hours ago, Gabriel Devereux said:

Ultra bounce just cut 2 1/2 stops from your original output - this isn't accurate when your measuring close to the bounce however at a distance it is somewhat a good guesstimate). 

It's not accurate at any distance, except the precise one that yields this result.

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17 minutes ago, Gabriel Devereux said:

Emphasise on the word 'guesstimate' - of course you're correct but I find it helps better than nothing. 

I just wonder how useful it is to post something that unknown other people told you, saying that it is possible to "somewhat guesstimate" a bounce level without specifying how far away from the bounce your subject is. Other people who are looking for information might read this and not knowing any better, spread this misinformation even further.

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On 3/1/2021 at 7:19 AM, Stuart Brereton said:

I just wonder how useful it is to post something that unknown other people told you, saying that it is possible to "somewhat guesstimate" a bounce level without specifying how far away from the bounce your subject is. Other people who are looking for information might read this and not knowing any better, spread this misinformation even further.

Miss-information is a bit harsh - now disregarding minor discrepancies of which there are MANY. And the following is a tested approximation. 

I found that a frame (6x6' frame of Ultrabounce) when 6ft away from the subject (distance is the same as source size). With the frame evenly lit by a source (again another variable, however I found it gave minor discrepancies) placed 3-4' in front of the frame (in-between the subject and the frame). The light loss - is around 2 1/3 stops - 2 2/3 stops in relation to the fixture at the combined distance from source to frame and frame to subject.

Now, as you can see - even here the light loss isn't the 'guesstimate' of the 2.5 stops above. HOWEVER, I find it to be a good guesstimate. 

I will openly disclaim that there are a sheer number of variables - in my original post above I disclaimed the sheer number of variables and I also stated a good course of action to potentially get a more accurate answer. However having tried going down that path myself, it's a long one. The above helps me, as a guesstimate - an estimate based on a mixture of guesswork and calculation. 

 

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31 minutes ago, Gabriel Devereux said:

I found that a frame (6x6' frame of Ultrabounce) when 6ft away from the subject (distance is the same as source size). With the frame evenly lit by a source (again another variable, however I found it gave minor discrepancies) placed 3-4' in front of the frame (in-between the subject and the frame). The light loss - is around 2 1/3 stops - 2 2/3 stops in relation to the fixture at the combined distance from source to frame and frame to subject

If you'd stated all of that in your original post, instead of vaguely mentioning "a distance", it might have made more sense.

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Cool thanks guys, I think it probably just comes from experience then. I'll try setting up an 8x8 and 2.5k and take various readings at different distances, and I reckon in theory adding a second 2.5k would just double those readings, another 2 x 2.5k would double again, and so on.

 

 

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9 hours ago, Jonathan O'Neill said:

Cool thanks guys, I think it probably just comes from experience then. I'll try setting up an 8x8 and 2.5k and take various readings at different distances, and I reckon in theory adding a second 2.5k would just double those readings, another 2 x 2.5k would double again, and so on.

 

 

Correct.

For the soft source measurement, there's math involved that somebody will eventually nail down, whoever has the time and ability, in a simple enough form for others to use.

But even with hard light fixtures, we don't run a calculation involving the reflector, bulb type, and lens, to determine output. We look up the photometrics for that fixture or refer to our experience. A soft source is no different. Make your own measurement cheat book if it works for you.

When first starting out, I had set a 4x4 bounce card five feet from a Joker 400 and 800, and metered five feet from the illuminated card. That helped me for a while until I grew used to the intensities by feel. A friend told me another way was to "find your f11." Knowing where f11 is with any light gimmick, you will know that half that distance is f22 and double that distance is f5.6.

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5 hours ago, Stephen Sanchez said:

Correct.

For the soft source measurement, there's math involved that somebody will eventually nail down, whoever has the time and ability, in a simple enough form for others to use.

But even with hard light fixtures, we don't run a calculation involving the reflector, bulb type, and lens, to determine output. We look up the photometrics for that fixture or refer to our experience. A soft source is no different. Make your own measurement cheat book if it works for you.

When first starting out, I had set a 4x4 bounce card five feet from a Joker 400 and 800, and metered five feet from the illuminated card. That helped me for a while until I grew used to the intensities by feel. A friend told me another way was to "find your f11." Knowing where f11 is with any light gimmick, you will know that half that distance is f22 and double that distance is f5.6.

Nice one, thanks Stephen, a cheat book sounds like a good starting point 🙂

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8 hours ago, Stephen Sanchez said:

Correct.

For the soft source measurement, there's math involved that somebody will eventually nail down, whoever has the time and ability, in a simple enough form for others to use.

But even with hard light fixtures, we don't run a calculation involving the reflector, bulb type, and lens, to determine output. We look up the photometrics for that fixture or refer to our experience. A soft source is no different. Make your own measurement cheat book if it works for you.

When first starting out, I had set a 4x4 bounce card five feet from a Joker 400 and 800, and metered five feet from the illuminated card. That helped me for a while until I grew used to the intensities by feel. A friend told me another way was to "find your f11." Knowing where f11 is with any light gimmick, you will know that half that distance is f22 and double that distance is f5.6.

I'm not sure if a true formula or calculation would ever arise and be user friendly/simple.

I keep getting stumped (admittedly no mathematician or physicist, in fact far from it). Even once understanding how to calculate light fall-off from a large source - assuming that the diffuser in this instance or bounce's intensity is uniform (which it often isn't with the nature of our lighting instruments). All that allows is calculating the fall off of light from said source (bounce, diffuser). 

Attempting to calculate the fall-off of the bounce source with only luminance information being the source lighting the bounce source adds variables upon variables and sadly not ones one can easily dismiss for an approximation, which there are many of as well.

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