Doing The Math with Flash

flyinglentris

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First, let me apologize in advance if it turns out to be too early to post this thread. I have recently received my CyberCommander for my Einsteins, but still want to get a Sekonic LiteMaster Pro L-478D-U Light Flash Meter as not everything I was looking for in a flash meter resided in the CyberCommander.

I have in addition to my new Einsteins, a 580EX II, a 430EX II and a MT-24EX Macro Flash.

I'm kind of sticking in my brain pan on how much automation has stolen away from clearly understanding flash exposure problems. G numbers? I only find them mentioned in my 580EX II manual with regard to the LCD panel's range scale as stops. Inverse Square Law of fade-off? Automated away to oblivion. What happens when I have two or more Einsteins and their flash energies overlap on the subject? Is it additive? Should I take the main reading and then the fill and subtract one from the other to round things out? Or does that happen automatically with no control over it by me. Will the Sekonic meter help to resolve these issues when I get it? Or is the grey card my only key to salvation? Am I an ignoramus for asking these questions?

I'm new to big gun studio lighting and would appreciate knowing how others have applied themselves to what appears to be evolving into an automated mystery.

I want to have control and do the math.

--
flyinglentris in LLOMA
 
Last edited:
First, let me apologize in advance if it turns out to be too early to post this thread. I have recently received my CyberCommander for my Einsteins, but still want to get a Sekonic LiteMaster Pro L-478D-U Light Flash Meter as not everything I was looking for in a flash meter resided in the CyberCommander.

I have in addition to my new Einsteins, a 580EX II, a 430EX II and a MT-24EX Macro Flash.

I'm kind of sticking in my brain pan on how much automation has stolen away from clearly understanding flash exposure problems. G numbers? I only find them mentioned in my 580EX II manual with regard to the LCD panel's range scale as stops. Inverse Square Law of fade-off? Automated away to oblivion. What happens when I have two or more Einsteins and their flash energies overlap on the subject? Is it additive? Should I take the main reading and then the fill and subtract one from the other to round things out? Or does that happen automatically with no control over it by me. Will the Sekonic meter help to resolve these issues when I get it? Or is the grey card my only key to salvation? Am I an ignoramus for asking these questions?

I'm new to big gun studio lighting and would appreciate knowing how others have applied themselves to what appears to be evolving into an automated mystery.

I want to have control and do the math.
Guide numbers are nearly useless for speedlights and always useless for studio strobes. Because speedlights nearly always zoom and the Guide Number depends on the amount of zoom and the manufacturer nearly always quotes it at only one zoom setting you can't really use it to calculate the camera settings. Because studio strobes are used in modifiers and the modifiers have a very large influence on the amount of light received by the subject the Guide Number of the light is of no practical use.

Photographers use the inverse square law all the time. If you want to increase or decrease the amount of light falling on the subject a small amount you will very often just move the light or the subject. You would generally not move either very much because you are also changing the softness of the light (which changes to look of the shadows).

18% grey cards are used to help get the right colour temperature setting in you camera or in post, 12% grey cards are used to calibrate exposure meters. Neither will help you do what you want to do, If I understand what you want to do (and I'm not sure I do).

A light meter measures the intensity of the light incident on or reflected from a subject it doesn't teach you elementary classical optics. You need a textbook for that.

Actually, elementary classical optics doesn't help much either. Controlling the amount of light falling on the subject is trivial. Placing the lights and controlling the kind of light you're generating is the hard part. The canonical source of information on photographic lighting is Light Science and Magic by Hunter, Biver and Fuqu. It will tech you all the optics (the science part) you need and introduce you to some magic.
 
Guide numbers are nearly useless for speedlights and always useless for studio strobes. Because speedlights nearly always zoom and the Guide Number depends on the amount of zoom and the manufacturer nearly always quotes it at only one zoom setting you can't really use it to calculate the camera settings. Because studio strobes are used in modifiers and the modifiers have a very large influence on the amount of light received by the subject the Guide Number of the light is of no practical use.
Yes. I can conceive of that as the last time I used guide numbers was with an old Vivitar flash that I used on my Minolta XE-7 SLR. Not so oddly, I have been sitting around today reviewing the speedlite manuals and reading up on the CyberCommander and Einstein manuals. It is better to know what one is doing than to be spoon fed by either hand holding or by automatic systems. As a former pilot, I would call it situational awareness and readiness to adapt to whatever situation one finds themselves dealing with.
Photographers use the inverse square law all the time. If you want to increase or decrease the amount of light falling on the subject a small amount you will very often just move the light or the subject. You would generally not move either very much because you are also changing the softness of the light (which changes to look of the shadows).

18% grey cards are used to help get the right colour temperature setting in you camera or in post, 12% grey cards are used to calibrate exposure meters. Neither will help you do what you want to do, If I understand what you want to do (and I'm not sure I do).
What I am trying to do is come to terms with the uncomfortable feeling that I don't know all I should know about using studio lighting. I'm OK with admitting my ignorance, so long as it is understood that I am determined to wipe out that ignorance.

It does not surprise me that Flash usage is vague, especially when referencing history which demonstrated that flash was often used all the time going from magnesium powder to flash bulbs, strips and cubes. There was a time that folks didn't take pictures without flash and did it arbitrarily of position, intensity or hardness. Today, one can often shoot without flash and be confident. It's when one shoots with flash that the head scratching comes to play.

Mark it, I understand what is going on with fill and light modifiers, HSS, second curtain, bounce and so forth. It's the regulation of these techniques that intrigues me.
A light meter measures the intensity of the light incident on or reflected from a subject it doesn't teach you elementary classical optics. You need a textbook for that.

Actually, elementary classical optics doesn't help much either. Controlling the amount of light falling on the subject is trivial. Placing the lights and controlling the kind of light you're generating is the hard part. The canonical source of information on photographic lighting is Light Science and Magic by Hunter, Biver and Fuqu. It will tech you all the optics (the science part) you need and introduce you to some magic.
I found the book as a PDF and now have it to read. Thanks for the reference. I felt like what I needed to fill the knowledge hole was a good text reference and you have been so good to provide it. Thanks.

--
flyinglentris in LLOMA
 
Last edited:
First, let me apologize in advance if it turns out to be too early to post this thread. I have recently received my CyberCommander for my Einsteins, but still want to get a Sekonic LiteMaster Pro L-478D-U Light Flash Meter as not everything I was looking for in a flash meter resided in the CyberCommander.

I have in addition to my new Einsteins, a 580EX II, a 430EX II and a MT-24EX Macro Flash.

I'm kind of sticking in my brain pan on how much automation has stolen away from clearly understanding flash exposure problems. G numbers? I only find them mentioned in my 580EX II manual with regard to the LCD panel's range scale as stops. Inverse Square Law of fade-off? Automated away to oblivion. What happens when I have two or more Einsteins and their flash energies overlap on the subject? Is it additive? Should I take the main reading and then the fill and subtract one from the other to round things out? Or does that happen automatically with no control over it by me. Will the Sekonic meter help to resolve these issues when I get it? Or is the grey card my only key to salvation? Am I an ignoramus for asking these questions?

I'm new to big gun studio lighting and would appreciate knowing how others have applied themselves to what appears to be evolving into an automated mystery.

I want to have control and do the math.
Here's a simplified math based solution for your example:

The light from both units falls on your subject so the light itself is additive, obviously, but accounting for that relationship using photographic F numbers is not so obvious. The key to this is the fact that the intensity of the light measured is proportional to the square of the F number reported by the meter.

Assume one flash meters f/5.6 alone and the other meters f/4 alone. We immediately know one light is twice as bright as the other. Squaring both f number (using the more precise 5.6569 value for f/5.6) gives 32 and 16 respectively. Sum these to get 48, then get f/6.9 by taking the square root. This is the "square root of the sum of the squares" process.

This approach is based upon my math process for computing the % flash contribution when using fill flash with meters which don't report that value. It ignores any directional aspects of the two lights and obviously breaks down when the lights are 180 degrees apart. Key and fill lights are usually on opposite sides of the subject and that is the purpose of using the dome out on meters like my Sekonic L-358. There are several schools of thought on how to meter portraits and where to point the meter which I will not go into.
  • John
 
I'm new to big gun studio lighting and would appreciate knowing how others have applied themselves to what appears to be evolving into an automated mystery.

I want to have control and do the math.
Skip the automation and just look at it. Used my CC yesterday with Einsteins and a Digibee but only to turn things on and off and change power. It was portraits so I used constant color vs action mode that I use with dancers.

Before I turned anything on, I had a lighting plan. The first setup was with soft light so I used big modifiers close to subject for both key and fill. The background was lit by the spill. Conventional 2:1 ratio, key to fill or a medium key. My exposure was far enough below ambient to kill it. Set a custom white balance with the white side of a Lastolite EzyBalance.

Second setup was soft light with a grid on axis which killed back ground spill and eliminated need for shadow fill on faces but I added a rim/hair light from behind for separation. Dark background and lower key, more dramatic. Later I added a light on the background with a gobo. So the background exposure was completely separate from foreground exposure. My goal was to not have the background overwhelm the foreground, just add interest. If I run out of monolights, I'll add in speed lights.

FInal setup was switch to tungsten Fresnel lights so flood to spot adjustment, barn doors, snoot and scrims for control. This is wysiwyg. Set a custom white balance with the white side of a Lastolite EzyBalance. Used one on a boom with snoot for key, put a cto gel on the Digibee and adjusted it's LED modelling light for fill and one more tungsten light with barndoors for a narrow diagonal splash on light on background.

Test shot for instant review and histogram for exposure for all. What I really care about is what it looks like so I move lights and adjust power to taste. I move then so much, re-metering would be endless.

I do use my flash meter to setup large group shots so I have a reasonable exposure before the crowd shows up and all I have to do is build a pose. Or if its static and I have a lot of time. Think of the histogram as the 21st century light meter.
 
First, let me apologize in advance if it turns out to be too early to post this thread. I have recently received my CyberCommander for my Einsteins, but still want to get a Sekonic LiteMaster Pro L-478D-U Light Flash Meter as not everything I was looking for in a flash meter resided in the CyberCommander.

I have in addition to my new Einsteins, a 580EX II, a 430EX II and a MT-24EX Macro Flash.

I'm kind of sticking in my brain pan on how much automation has stolen away from clearly understanding flash exposure problems. G numbers? I only find them mentioned in my 580EX II manual with regard to the LCD panel's range scale as stops. Inverse Square Law of fade-off? Automated away to oblivion. What happens when I have two or more Einsteins and their flash energies overlap on the subject? Is it additive? Should I take the main reading and then the fill and subtract one from the other to round things out? Or does that happen automatically with no control over it by me. Will the Sekonic meter help to resolve these issues when I get it? Or is the grey card my only key to salvation? Am I an ignoramus for asking these questions?

I'm new to big gun studio lighting and would appreciate knowing how others have applied themselves to what appears to be evolving into an automated mystery.

I want to have control and do the math.
Here's a simplified math based solution for your example:

The light from both units falls on your subject so the light itself is additive, obviously, but accounting for that relationship using photographic F numbers is not so obvious. The key to this is the fact that the intensity of the light measured is proportional to the square of the F number reported by the meter.

Assume one flash meters f/5.6 alone and the other meters f/4 alone. We immediately know one light is twice as bright as the other. Squaring both f number (using the more precise 5.6569 value for f/5.6) gives 32 and 16 respectively. Sum these to get 48, then get f/6.9 by taking the square root. This is the "square root of the sum of the squares" process.

This approach is based upon my math process for computing the % flash contribution when using fill flash with meters which don't report that value. It ignores any directional aspects of the two lights and obviously breaks down when the lights are 180 degrees apart. Key and fill lights are usually on opposite sides of the subject and that is the purpose of using the dome out on meters like my Sekonic L-358. There are several schools of thought on how to meter portraits and where to point the meter which I will not go into.
  • John
Bingo!

Thanks for the good example.
 
I'm new to big gun studio lighting and would appreciate knowing how others have applied themselves to what appears to be evolving into an automated mystery.

I want to have control and do the math.
Skip the automation and just look at it. Used my CC yesterday with Einsteins and a Digibee but only to turn things on and off and change power. It was portraits so I used constant color vs action mode that I use with dancers.

Before I turned anything on, I had a lighting plan. The first setup was with soft light so I used big modifiers close to subject for both key and fill. The background was lit by the spill. Conventional 2:1 ratio, key to fill or a medium key. My exposure was far enough below ambient to kill it. Set a custom white balance with the white side of a Lastolite EzyBalance.

Second setup was soft light with a grid on axis which killed back ground spill and eliminated need for shadow fill on faces but I added a rim/hair light from behind for separation. Dark background and lower key, more dramatic. Later I added a light on the background with a gobo. So the background exposure was completely separate from foreground exposure. My goal was to not have the background overwhelm the foreground, just add interest. If I run out of monolights, I'll add in speed lights.

FInal setup was switch to tungsten Fresnel lights so flood to spot adjustment, barn doors, snoot and scrims for control. This is wysiwyg. Set a custom white balance with the white side of a Lastolite EzyBalance. Used one on a boom with snoot for key, put a cto gel on the Digibee and adjusted it's LED modelling light for fill and one more tungsten light with barndoors for a narrow diagonal splash on light on background.

Test shot for instant review and histogram for exposure for all. What I really care about is what it looks like so I move lights and adjust power to taste. I move then so much, re-metering would be endless.

I do use my flash meter to setup large group shots so I have a reasonable exposure before the crowd shows up and all I have to do is build a pose. Or if its static and I have a lot of time. Think of the histogram as the21st century light meter.
That's a lot going on and clearly, your experience gives you great confidence in how you assemble things to produce desired results.

Reading your post presents some new stuff for me too. I do plan on some DB800s for those high watt modelling lights to use as continuous lighting, but not much more than that for now, except the ModLite mainframe, a snoot and barndoors to with the DB800s. I figure by the time I acquire those DB800s I'll have waded in as far as I dare for this year.
 
I found the book as a PDF
You do understand that the PDF is a pirated copy of the book?

Photographers and authors get very exercised by what they (incorrectly) call the theft of intellectual property (copyright violation is not theft). I'm not a professional photographer and have only written a handfull of technical articles but I can see their point.

The photography community is a welcoming community but there are some buttons it's not wise to push.

Buy the bloody book!
 
Reading your post presents some new stuff for me too. I do plan on some DB800s for those high watt modelling lights to use as continuous lighting, but not much more than that for now, except the ModLite mainframe, a snoot and barndoors to with the DB800s. I figure by the time I acquire those DB800s I'll have waded in as far as I dare for this year.
My confidence also resulted in a lot of bad images as I was trying things.

I'm assuming you have the standard reflectors. You can use bull dog clips to attach gels. Next thing I'd recommend is a set of grids.

The 64" soft silver PLMs and diffusers have been bread and butter modifiers for years. But my most used modifier now is the 18" Omni with diffuser sock and very often, the grid.

Also have the LiteMod mainframes, makes it a lot easier to mount barn doors than wrestling them on to standard reflectors. And makes the snoot usable.
 
I found the book as a PDF
You do understand that the PDF is a pirated copy of the book?

Photographers and authors get very exercised by what they (incorrectly) call the theft of intellectual property (copyright violation is not theft). I'm not a professional photographer and have only written a handfull of technical articles but I can see their point.

The photography community is a welcoming community but there are some buttons it's not wise to push.

Buy the bloody book!
I can buy the book. I don't know whether the PDF is pirated or not, but I did see it for sale on e-Bay. I hadn't even considered it. The book is only about $16.00 as a PDF. Hard Copy is $109.00.

--
flyinglentris in LLOMA
 
Last edited:
Reading your post presents some new stuff for me too. I do plan on some DB800s for those high watt modelling lights to use as continuous lighting, but not much more than that for now, except the ModLite mainframe, a snoot and barndoors to with the DB800s. I figure by the time I acquire those DB800s I'll have waded in as far as I dare for this year.
My confidence also resulted in a lot of bad images as I was trying things.

I'm assuming you have the standard reflectors. You can use bull dog clips to attach gels. Next thing I'd recommend is a set of grids.

The 64" soft silver PLMs and diffusers have been bread and butter modifiers for years. But my most used modifier now is the 18" Omni with diffuser sock and very often, the grid.

Also have the LiteMod mainframes, makes it a lot easier to mount barn doors than wrestling them on to standard reflectors. And makes the snoot usable.
I looked for Fresnels and wondered whether the LiteMode mainframe would mount a Fresnel, but it appears that Buff's lights don't support Fresnel lenses.
 
I found the book as a PDF
You do understand that the PDF is a pirated copy of the book?

Photographers and authors get very exercised by what they (incorrectly) call the theft of intellectual property (copyright violation is not theft). I'm not a professional photographer and have only written a handfull of technical articles but I can see their point.

The photography community is a welcoming community but there are some buttons it's not wise to push.

Buy the bloody book!
I can buy the book. I don't know whether the PDF is pirated or not, but I did see it for sale on e-Bay. I hadn't even considered it. The book is only about $16.00 as a PDF. Hard Copy is $109.00.
$34.41 https://www.amazon.com/Light-Science-Magic-Introduction-Photographic/dp/0415719402
 
I found the book as a PDF
You do understand that the PDF is a pirated copy of the book?

Photographers and authors get very exercised by what they (incorrectly) call the theft of intellectual property (copyright violation is not theft). I'm not a professional photographer and have only written a handfull of technical articles but I can see their point.

The photography community is a welcoming community but there are some buttons it's not wise to push.

Buy the bloody book!
I can buy the book. I don't know whether the PDF is pirated or not, but I did see it for sale on e-Bay. I hadn't even considered it. The book is only about $16.00 as a PDF. Hard Copy is $109.00.
$34.41 https://www.amazon.com/Light-Science-Magic-Introduction-Photographic/dp/0415719402
I don't use Amazon. They seem to want to force the Prime card on me and I'm not game for that. The 5th edition PDF I found is $12.99 at some outfit called SetMalls.

--
flyinglentris in LLOMA
 
Last edited:
Others have explained the math quite well. Allow me to address how to learn to use "big" flashes with a light meter. First off, don't use any of the automation for the time being. Use the manual settings because won't have enough experience with the lights to know what the automatic modes are doing. Also, forget everything you know.

To use strobe efficiently you start by deciding what ISO and aperture you want to shoot with. Studio photographers generally use the camera's lowest ISO and an f/stop somewhere between f/4 and f/11 depending on what's being shot. The shutter speed should be your camera's highest sync speed so that you don't get light pollution from ambient sources.

I know that deciding on the f/stop first is completely backwards from the conventional internet wisdom, but it's the way professional photographers work and will save you many hours of frustration.

Here's a really good basic overview of how to set your lights—he's using an L358 but it applies to other Sekonics as well.

So let's say you decide you want to shoot at f/8. Bring in the main light, set it at 1/2 power and hook it up to your light meter; stand where your subject is going to be, point the light meter at the strobe and hit the flash button. You'll get a reading; adjust your light until the meter reads f/8.

Turn the main light off and then set the exposure of your fill; let's say you want it half as bright, you would use your flash meter to measure the strobe output and adjust the power till the meter reads f/5.6.

Continue metering and adjusting whatever other lights you choose, remembering to only fire one light at a time.

As a last step turn on all your lights, put your meter in the subject position and fire everything at once. You should still get a reading of f/8 but depending on how the light bounces around your shooting space you may have to go up or down on either your power or your aperture to get a solid f/8.

Voila! Perfect exposure and your sanity remains intact.
 
Last edited:
Others have explained the math quite well. Allow me to address how to learn to use "big" flashes with a light meter. First off, don't use any of the automation for the time being. Use the manual settings because won't have enough experience with the lights to know what the automatic modes are doing. Also, forget everything you know.

To use strobe efficiently you start by deciding what ISO and aperture you want to shoot with. Studio photographers generally use the camera's lowest ISO and an f/stop somewhere between f/4 and f/11 depending on what's being shot. The shutter speed should be your camera's highest sync speed so that you don't get light pollution from ambient sources.

I know that deciding on the f/stop first is completely backwards from the conventional internet wisdom, but it's the way professional photographers work and will save you many hours of frustration.

Here's a really good basic overview of how to set your lights—he's using an L358 but it applies to other Sekonics as well.

So let's say you decide you want to shoot at f/8. Bring in the main light, set it at 1/2 power and hook it up to your light meter; stand where your subject is going to be, point the light meter at the strobe and hit the flash button. You'll get a reading; adjust your light until the meter reads f/8.

Turn the main light off and then set the exposure of your fill; let's say you want it half as bright, you would use your flash meter to measure the strobe output and adjust the power till the meter reads f/5.6.

Continue metering and adjusting whatever other lights you choose, remembering to only fire one light at a time.

As a last step turn on all your lights, put your meter in the subject position and fire everything at once. You should still get a reading of f/8 but depending on how the light bounces around your shooting space you may have to go up or down on either your power or your aperture to get a solid f/8.

Voila! Perfect exposure and your sanity remains intact.
I won't have the Sekonic until about 3 weeks, maybe 4. I'll try things out then.

Your post hints that the Sekonic can be attached to the flash unit to fire it. Is that correct?
 
then I'm going to mute you.

This is the last time we will interact.
I'm sorry to hear that from you. I don't know what your complaint is. I bought the PDF from a company that represents itself as reputable and I won't have Amazon forced on me.
 
The question is: >> Am I an ignoramus for asking these questions? <<

Your meter will be very helpful.

Thinking will be very helpful.

Your spare hand will be very useful. You'll use it to block light from reaching the meter.

BAK
 
First, let me apologize in advance if it turns out to be too early to post this thread. I have recently received my CyberCommander for my Einsteins, but still want to get a Sekonic LiteMaster Pro L-478D-U Light Flash Meter as not everything I was looking for in a flash meter resided in the CyberCommander.

I have in addition to my new Einsteins, a 580EX II, a 430EX II and a MT-24EX Macro Flash.

I'm kind of sticking in my brain pan on how much automation has stolen away from clearly understanding flash exposure problems. G numbers? I only find them mentioned in my 580EX II manual with regard to the LCD panel's range scale as stops. Inverse Square Law of fade-off? Automated away to oblivion. What happens when I have two or more Einsteins and their flash energies overlap on the subject? Is it additive? Should I take the main reading and then the fill and subtract one from the other to round things out? Or does that happen automatically with no control over it by me. Will the Sekonic meter help to resolve these issues when I get it? Or is the grey card my only key to salvation? Am I an ignoramus for asking these questions?

I'm new to big gun studio lighting and would appreciate knowing how others have applied themselves to what appears to be evolving into an automated mystery.

I want to have control and do the math.
Guide numbers are nearly useless for speedlights and always useless for studio strobes. Because speedlights nearly always zoom and the Guide Number depends on the amount of zoom and the manufacturer nearly always quotes it at only one zoom setting you can't really use it to calculate the camera settings. Because studio strobes are used in modifiers and the modifiers have a very large influence on the amount of light received by the subject the Guide Number of the light is of no practical use.

Photographers use the inverse square law all the time. If you want to increase or decrease the amount of light falling on the subject a small amount you will very often just move the light or the subject. You would generally not move either very much because you are also changing the softness of the light (which changes to look of the shadows).

18% grey cards are used to help get the right colour temperature setting in you camera or in post, 12% grey cards are used to calibrate exposure meters. Neither will help you do what you want to do, If I understand what you want to do (and I'm not sure I do).

A light meter measures the intensity of the light incident on or reflected from a subject it doesn't teach you elementary classical optics. You need a textbook for that.

Actually, elementary classical optics doesn't help much either. Controlling the amount of light falling on the subject is trivial. Placing the lights and controlling the kind of light you're generating is the hard part. The canonical source of information on photographic lighting is Light Science and Magic by Hunter, Biver and Fuqu. It will tech you all the optics (the science part) you need and introduce you to some magic.
18% gray cards are for printing. Not recommended for white balance OR exposure.
 
I found the book as a PDF
You do understand that the PDF is a pirated copy of the book?

Photographers and authors get very exercised by what they (incorrectly) call the theft of intellectual property (copyright violation is not theft). I'm not a professional photographer and have only written a handfull of technical articles but I can see their point.

The photography community is a welcoming community but there are some buttons it's not wise to push.

Buy the bloody book!
I can buy the book. I don't know whether the PDF is pirated or not, but I did see it for sale on e-Bay. I hadn't even considered it. The book is only about $16.00 as a PDF. Hard Copy is $109.00.
$34.41 https://www.amazon.com/Light-Science-Magic-Introduction-Photographic/dp/0415719402
I don't use Amazon. They seem to want to force the Prime card on me and I'm not game for that. The 5th edition PDF I found is $12.99 at some outfit called SetMalls.
Did you actually get the PDF? Lots of complaints against that company and a number of others that are all at the same address.
 

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