Debunking the Myths of Flash Power

I've been trying to research exactly how to discover exactly how many stops of light any particular flash may have. After reading article after article, I've realized that there are deep misconceptions that have spread, even among very well known editorials.

As an engineer I had to figure it out myself for the sake of my own sanity, and hopefully, yours also. So here we go:

First: Guide numbers are totally useless because they can, and are manipulated by marketing. This is endemic in all industries that like to display the specifications of their products big and bright all over the box they come in. SO DON'T USE THEM!

Second: The Power rating is not alway intrinsic with the ultimate luminosity of the light.

Third: The only way that you can truly know how a particular flash works with your particular camera is to TEST IT.

... So how do you test it?

This is called Metrology (the study of weights and measures).

First: In metrological calibration, which is to verify how a something compares to an established reference, you first need to establish this reference. The one that I use (from the Beyond Photography YT channel), is (8888)

(8888) is: f8; 800 ISO; 8 ft; 1/8 power

... Why 8's? Because these are all reasonable midpoints on the system settings for both the camera and the flash, and usable distance.

Second: Setting up the calibration test:

You will need a grey-card, a camera, and a flash. Set up the grey-card exactly [8 ft] from the flash.

Next, set the flash to [1/8] power and point it directly at the card, and also make sure that its zoom setting is at its widest focal length (because this is how you are going to use it 99% of the time). This also works best with Off-Camera Flash.

Now, set your camera to [f8], and [800 ISO]. Stand close enough to the grey-card so that you can fill up the frame as much as you can and take the picture.

What you will see with a standard camera flash are relatively narrow lines near the center of your histogram. These may be partially or several stops of light from center depending on the model of flash you are using, but most of the top camera flashes are actually factory calibrated to something near this exposure (The exact center of the histogram is where [0 EV], or correct exposure is).

My Nissin Di700A is [-1 stop] from center. To find the deviation, adjust the settings of your camera and retake the shot. In my circumstance, I just changed my aperture to [f5.6].

If this interests anyone, I will make another post on exactly what the "Real Math" is in calculating the power. But I think this is enough for now.

Thanks, and I hope this helps.

Flash output is measured in watt-seconds, or joules.

It's a measure of energy.

MartinH98 said:

Flash output is measured in watt-seconds, or joules.

It's a measure of energy.

Click to expand...

I've never seen a speedlight output measured in watt-seconds and I thought there was a reason for that, but I don't remember what. Something to do with the fact that speedlights typically zoom which changes their light output for the same energy expended, whereas strobes typically do not zoom, maybe? Thus guide number is supposed to include the zoom level in its calculation.

I appreciate the technique you’ve outlined. Good to know.

However, wouldn’t it be much easier to use an exposure meter, such as a Sekonic Flashmate?

What I was trying to get at, indirectly, was that you don't set 1/8th flash power, it's not possible. You reduce the output time to 1/8th of its duration. The energy's reduced. The power's unaffected.

MartinH98 said:

Flash output is measured in watt-seconds, or joules.

It's a measure of energy.

Click to expand...

Correct! This is the KMS unit. The issue with the watt second is that you would have to discover the exact T1 time (pulse length) of the flash at every setting. This does not help most photographers.

richiebee said:

MartinH98 said:

Flash output is measured in watt-seconds, or joules.

It's a measure of energy.

Click to expand...

I've never seen a speedlight output measured in watt-seconds and I thought there was a reason for that, but I don't remember what. Something to do with the fact that speedlights typically zoom which changes their light output for the same energy expended, whereas strobes typically do not zoom, maybe? Thus guide number is supposed to include the zoom level in its calculation.

Click to expand...

You are right in saying that the manufacturers are "supposed to include". This is biggest issue I have on Guide Numbers. They never give you all of information as to HOW they derived the guide number. There always seems to be some part of the equation left out. I believe this is done on purpose, so that they can "fudge" the numbers.

MartinH98 said:

What I was trying to get at, indirectly, was that you don't set 1/8th flash power, it's not possible. You reduce the output time to 1/8th of its duration. The energy's reduced. The power's unaffected.

Click to expand...

You are correct in this statement also. But the equation also includes time which is total energy delivered. The [1/8] power, as I stated, is to establish the (arbitrary) Reference at which the light is measured. Without a reference, you cannot determine its effectiveness. Yes, you do not have to use 1/8 power, but you must use a consistent reference so that you can make easy calculations in your head when taking exposures.

This is all about the Practical Use of the flash in manual mode.

My post is about How to truly discover the Effectiveness of the flash, not its theory. When you calibrate the flash at [8888], you will know exactly how to adjust the flash and camera settings for any given exposure. You will also know the exact limits of its capabilities.

Bob said:

I appreciate the technique you’ve outlined. Good to know.

However, wouldn’t it be much easier to use an exposure meter, such as a Sekonic Flashmate?

Click to expand...

Yes and No. Flash meters are only good in Controlled settings. But if you are trying to do some candids, or want to use your flash out on the street (which is best done without using your camera's metering for the sake of speed), you will be much better off doing the calculations in your head. The other benefit is that a flash meter cannot tell you the limits of your flash's capabilities. It doesn't know that your flash is only good up to 6 feet at the aperture that you want to set it at (if it is a smaller flash). The main benefit, is that you can now calibrate all of your flashes and truly know which ones are more effective or better suited for certain situations.

I see people asking all the time "which light is more powerful", and I hear the craziest answers to the question. Like the Godox AD200, people say that "it is equal to 3 speedlights". This is the most unpractical and complete nonsense.

The Walking Man said:

I've been trying to research exactly how to discover exactly how many stops of light any particular flash may have. After reading article after article, I've realized that there are deep misconceptions that have spread, even among very well known editorials.

Click to expand...

What misconceptions? What are you talking about? Be specific.

Member said:

As an engineer I had to figure it out myself for the sake of my own sanity, and hopefully, yours also. So here we go:

First: Guide numbers are totally useless because they can, and are manipulated by marketing. This is endemic in all industries that like to display the specifications of their products big and bright all over the box they come in. SO DON'T USE THEM!

Click to expand...

Proof? Examples?

Member said:

Second: The Power rating is not alway intrinsic with the ultimate luminosity of the light.

Click to expand...

Luminosity is watts. That's a continuous light. Flash "power" is watt seconds. Right away you lost me.

Member said:

Third: The only way that you can truly know how a particular flash works with your particular camera is to TEST IT.

Click to expand...

Well, kind of, but that is true of any piece of equipment and it may not be because the manufacture is deliberately lying.

Member said:

... So how do you test it?

Next, set the flash to [1/8] power and point it directly at the card, and

Click to expand...

From how far away?

Member said:

also make sure that its zoom setting is at its widest focal length (because this is how you are going to use it 99% of the time).

Click to expand...

It bothers me when people make assumption about how the rest of us use equipment. I only use fill flash in telephoto mode, for example.

Member said:

This also works best with Off-Camera Flash.

Now, set your camera to [f8], and [800 ISO]. Stand close enough to the grey-card so that you can fill up the frame as much as you can and take the picture.

Click to expand...

You forgot a few details, like a slow enough shutter speed and making sure the flash has time to completely charge, More importantly, flash output depends on distance and "Standing close enough" doesn't cut it as a measurement tool.

Member said:

What you will see with a standard camera flash are relatively narrow lines near the center of your histogram. These may be partially or several stops of light from center depending on the model of flash you are using, but most of the top camera flashes are actually factory calibrated to something near this exposure (The exact center of the histogram is where [0 EV], or correct exposure is).

Click to expand...

So you admit they aren't lying after all ???

Member said:

My Nissin Di700A is [-1 stop] from center. To find the deviation, adjust the settings of your camera and retake the shot. In my circumstance, I just changed my aperture to [f5.6].

Click to expand...

This is what was causing you to lose sleep? Finding out the scene isn't bright enough and opening the aperture? Where's the huge misconception you mentioned at the start?

Flash power is a function of distance, which you haven't mentioned. Maybe that's why your exposures are off?

Flash, I love you! But we have only fourteen hours left to save the world...

Bob said:

I appreciate the technique you’ve outlined. Good to know.

However, wouldn’t it be much easier to use an exposure meter, such as a Sekonic Flashmate?

Click to expand...

Exactly. I use a Gossen Luna Pro F in incident mode, one I acquired in the early 80s. I set it up in the middle of my yard at night away from reflecting walls, bushes, etc, fire the flash aimed at the meter from 10 feet at full power and take notes.

I found guide numbers for reputable brands to be consistent only in relation to other flashes but a bit fudged of course as to an accurate usable guide number. Example: If the manual claimed a GN of 120, I found that number a bit optimistic for all brands claiming a GN of 120.

You're off the mark in your response. Your answers to every question you have will be revealed if you perform the test yourself.

Alex Ethridge said:

Bob said:

I appreciate the technique you’ve outlined. Good to know.

However, wouldn’t it be much easier to use an exposure meter, such as a Sekonic Flashmate?

Click to expand...

Exactly. I use a Gossen Luna Pro F in incident mode, one I acquired in the early 80s. I set it up in the middle of my yard at night away from reflecting walls, bushes, etc, fire the flash aimed at the meter from 10 feet at full power and take notes.

I found guide numbers for reputable brands to be consistent only in relation to other flashes but a bit fudged of course as to an accurate usable guide number. Example: If the manual claimed a GN of 120, I found that number a bit optimistic for all brands claiming a GN of 120.

Click to expand...

Correct! And thanks for sharing.

threw the lens said:

Flash, I love you! But we have only fourteen hours left to save the world...

Click to expand...

IKR! I know that I cannot improve the speed of my camera system, but I can improve the speed of myself through practice. This is the main benefit of knowing how to calculate flash power in your head. I don't have to worry about placing the metering guide over the subject. I don't have to worry about making sure the camera is in the right mode, because it's all in my head, and I've practiced so that I can take the shots when they come.

John Free is absolutely right when he says, "the most important way to improve your photography is to practice, just as you would anything else".

What calibration does is gives you a solid reference that you can easily determine in your head, so that when it comes time to press the shutter release, you are ready.

The Walking Man said:

I've been trying to research exactly how to discover exactly how many stops of light any particular flash may have. After reading article after article, I've realized that there are deep misconceptions that have spread, even among very well known editorials.

As an engineer I had to figure it out myself for the sake of my own sanity, and hopefully, yours also. So here we go:

First: Guide numbers are totally useless because they can, and are manipulated by marketing. This is endemic in all industries that like to display the specifications of their products big and bright all over the box they come in. SO DON'T USE THEM!

Second: The Power rating is not alway intrinsic with the ultimate luminosity of the light.

Click to expand...

I agree on the above points in that GNs are pretty much useless for calculating flash exposures or even comparing the power of different speedlights especially when they are made by different manufacturers. Most third party Manufacturers want you to think their flashes are more powerful than the camera makers. Unfortunately once you have waded through translating the basis for the GNs by each Mfr and then realised that many inflate the rating anyway then you will end up utterly confused.

Member said:

Third: The only way that you can truly know how a particular flash works with your particular camera is to TEST IT.

Click to expand...

Agree completely but the approach below is way to complicated and unnecessary with modern cameras. All you need to do is take a few test shots ‘in the field’ and then adjust the flash compensation to get the image brightness you want (sure look at the histogram if you want to). This works really well for TTL metering. With TTL that flash compensation tends to be good across a range of distances and apertures so that’s neat. Manual, you have to do it at specific distances.

More usefully, use the same technique for any bounce flash or balanced fill flash too. (Both of which are far more useful than on-camera direct flash)

Once you get the hang of your flash, it’s not hard to recognise and adapt flash compensation settings.

Multi-flash and off camera flash is just an extension of this. Though here, a basic appreciation of the inverse square rule does help.

Member said:

... So how do you test it?

This is called Metrology (the study of weights and measures).

First: In metrological calibration, which is to verify how a something compares to an established reference, you first need to establish this reference. The one that I use (from the Beyond Photography YT channel), is (8888)

(8888) is: f8; 800 ISO; 8 ft; 1/8 power

... Why 8's? Because these are all reasonable midpoints on the system settings for both the camera and the flash, and usable distance.

Second: Setting up the calibration test:

You will need a grey-card, a camera, and a flash. Set up the grey-card exactly [8 ft] from the flash.

Next, set the flash to [1/8] power and point it directly at the card, and also make sure that its zoom setting is at its widest focal length (because this is how you are going to use it 99% of the time). This also works best with Off-Camera Flash.

Now, set your camera to [f8], and [800 ISO]. Stand close enough to the grey-card so that you can fill up the frame as much as you can and take the picture.

What you will see with a standard camera flash are relatively narrow lines near the center of your histogram. These may be partially or several stops of light from center depending on the model of flash you are using, but most of the top camera flashes are actually factory calibrated to something near this exposure (The exact center of the histogram is where [0 EV], or correct exposure is).

My Nissin Di700A is [-1 stop] from center. To find the deviation, adjust the settings of your camera and retake the shot. In my circumstance, I just changed my aperture to [f5.6].

If this interests anyone, I will make another post on exactly what the "Real Math" is in calculating the power. But I think this is enough for now.

Thanks, and I hope this helps.

Click to expand...

HTH. Flash illumination with speedlights should not be complicated and doesn’t need maths to get great results. The most important lesson is to avoid using on camera direct flash as the principle illumination. (Yuck!)

I think if you perform the test you will discover that it really isn't that complicated. I find it very intuitive. It just takes a little practice, but so does everything else.

The issue with metering is that candids do not give you the time to take test shots. Light conditions are very dynamic from my experience. I want all of the precious time to be spent on composition, not adjusting the settings, or moving the meter guide over the subjects forehead. I find that even the focus and recompose technique to be inadequate when it comes to capturing the moment.

I derived this calibration by necessity. All I can say is that it works very well for me. Not only in taking the photos, but in (especially) finding exactly how my equipment works in the real world.

All I wanted to do here is just provide some content on what I've learned through experience, that works very well for me.

Thank you for your reply.

The other point I should have made is that this thread is not about using the flash, so much as it is in determining a flash's abilities.

The Walking Man said:

I think if you perform the test you will discover that it really isn't that complicated. I find it very intuitive. It just takes a little practice, but so does everything else.

Click to expand...

I’m sure what you have described will work. My point was simply that it is actually more complicated than need be. Keep it simple!

Member said:

The issue with metering is that candids do not give you the time to take test shots. Light conditions are very dynamic from my experience. I want all of the precious time to be spent on composition, not adjusting the settings, or moving the meter guide over the subjects forehead. I find that even the focus and recompose technique to be inadequate when it comes to capturing the moment.

Click to expand...

The approach I noted will work equally quickly; actually more quickly. Once the flash compensation is set once it works automatically thereafter for the same type of work. For example, I’ll use fill flash a good deal and I know that -1.7 EV on flash compensation will more or less nail it every time.

The only time you need to reset is when doing something different. For example, using bounce flash. With the difference in wall & ceiling heights and reflectance (not to mention white balance) it’s wise to check in different locations. However, as TTL flash is being used, your ‘regular’ compensation will often suffice. The ‘in the field’ only takes a few tens of seconds and maybe 2-3 test shots to dial in. And no grey cards / distances / different settings of ISO, power or aperture. Just tweak of the flash compensation.

Member said:

I derived this calibration by necessity. All I can say is that it works very well for me. Not only in taking the photos, but in (especially) finding exactly how my equipment works in the real world.

Click to expand...

Sure, no problem in that but I’d urge you to also have a go at the typical approach I describe. It really is easy.

Member said:

All I wanted to do here is just provide some content on what I've learned through experience, that works very well for me.

Click to expand...

Likewise.

Member said:

Thank you for your reply.

Click to expand...

You are most welcome. HTH.

The Walking Man said:

I've been trying to research exactly how to discover exactly how many stops of light any particular flash may have. …...

Click to expand...

Guide numbers used to be fairly useful, but it a long time ago. In the seventies and at least beginning of the eighties all manufacturers gave guide numbers for 35mm and ISO 100. Of course, that didn't say anything about light falloff at the edges, but at least it gave a pretty good indication of what to expect in the central area of the image.

Then the zoom heads started appearing and some started giving guide numbers for max zoom, and then digital and some give guide numbers for higher ISO too.

So I agree about guide numbers and it's depressing to see that even sites like Dpreview just quote manufacturers numbers without specifying zoom settings or ISO.

I've considered getting a flash for MFT and tried to find real strength info for Olympus and Nissin flashes. It seems really interesting that some of the fairly small Nissin flashes from the specs are more powerful than much bigger Olympus models - or maybe it's just guide number tricks? And what about coverage? Do Nissin (not to mention Olympus) use a head optimized for 4:3 on the mFT-models or the "standard" 3:2 aspect ratio for most other system camera sensors?

It's easy enough to test if you have a access to a lot of flashes so I'd certainly be interested to see some test results.

Here's link to one test, but he used an umbrella in the test. That's interesting enough, but introduces a extra element in the test and doesn't say much about the max range of the flashes.

http://www.shootingonabudget.com/a-power-comparison-of-popular-hotshoe-flashes/