PetaPixel Can We Please Stop Talking About Watt Seconds?


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Photography is evolving — Dramatically! I am talking about advances with smartphones, mirrorless cameras, and AI technology — this is an exciting time to be a photographer. But there must be a better way than watt-seconds to compare lights.

The change that excites me the most is the evolution of the tools that we use for lighting. Traditionally, every new photographer was taught to purchase a speedlight. Then many photographers would graduate to studio strobes or monolights. Cinematographers used big, heavy, and expensive tungsten and halogen lights.

But in the last 15 years, as cameras have become much better at shooting at higher ISOs and photographers have started shooting video with the same cameras, the evolution of LED lighting has brought us lighting gear that is smaller and lighter and now even more reasonably priced. LED lights come in all shapes and sizes and are opening the door to a hybrid approach to lighting that opens so many creative possibilities.

So here we are with cameras and photographers who are working in both stills and video and we still don’t have a universal language for talking about and comparing the power or brightness of all these different lights. If you are like me, all the physics behind light measurement is just out of the question — I gave up on math in seventh grade when they added the alphabet and squiggly lines to the equations.

Terms like guide number, watt-seconds, lumens, and lux all involve physics and those alphabet equations and when it’s all said and done, they don’t give us a useful way to compare the brightness of different types of lighting gear. So, let’s look at what those terms mean to photographers and how we might better compare light sources.

Speedlights: What is a Guide Number?




Speedlight manufacturers use a guide number system that can be misleading because they don’t all use the same distances or zoom head settings. So, guide numbers don’t translate into a useful measurement of light for continuous “always-on” LEDs.

Strobes and Monolights: What is a Watt Second?


Studio Strobe and Monolight manufacturers traditionally use “watt x second” or “watt-seconds” as part of their overall product specifications. This number is even more unreliable because watt or watt x second is not a measure of light, it is a measure of energy. It is simply how much energy is stored and made available for the capacitor to power the flash tube.

In the 1930s, Dr. Harold (“Doc”) Edgerton invented the strobe. To fire a strobe, you need a very high voltage charge which in turn ignites the gas to create a lot of heat and light, including levels of light that the human eye cannot see. This is why strobes are known to quickly overheat. They generate a lot of heat as a by-product of the gas ignition with no effective way to quickly cool them.



A capacitor, which can be described as two metal plates separated by an insulator, is able to store up a big charge and release it all at once. That “charge” is measured in watt-seconds — or put another way, the amount of power (watts) you can deliver in one second. At the time Dr. Edgerton invented the strobe, a watt-second was a measurable way to describe the energy dumped into the system from the capacitor, which in turn ignited the gas in the glass tube.

So, understand that when you look at a 200ws monolight compared to a 300ws or 600ws monolight for example, indeed, each one is more powerful than the other. But from one brand to the next, that doesn’t mean they will give you the same light output. Watt seconds is an unreliable number because different flash tubes have different energy efficiencies as do different reflectors.

What I can tell you is that the bigger speedlights on the market today have approximately 50 to 60 watt-seconds of power. Most monolights are available with power from 130 watt-seconds up to well over a thousand watt-seconds. So, monolights are considerably more powerful than speedlights.

Constant Lighting: What is a Lumen?


Now that we know a “watt” is actually a measure of power or energy use, and not light, we can talk about digital light-based LEDs, which actually use fewer watts to produce a lot more light.

We measure LED bulbs based on lumens because lumens equal brightness.



A “Lumen” is a unit created to measure the light that the human eye can see. Light energy that the human eye cannot see, such as ultraviolet and infrared, is excluded from the lumen measurement. LEDs, unlike flash tubes, have been optimized to only deliver light visible to the human eye and to not spend unnecessary energy delivering light that is not visible.

As our lighting technology is evolving, lumens are a standard measurement of light that photographers should care about and pay attention to.

This even impacts the lightbulbs that we purchase for our homes. Today, instead of the wattage of a lightbulb, labels display lumens as the brightness. To give you an idea of how they compare, an incandescent 100-watt light bulb consumes 100 watts of electrical power but most of that power is generating heat, not light. That 100 watts of energy might deliver 1,600 lumens of visible light. By comparison, a 1600 lumen LED chip can deliver that output with just under 20 watts of power.

So, we have established that lumens are the preferred measurement of light useful to the human eye.

There is another measurement that is useful for photographers and that is to measure the light that reaches the subject. Lux measures luminous flux per unit area or how much light is falling on a given surface. In simpler terms, lux will tell you how bright the illuminated subject is going to be. Since lux measures the light energy falling on a square centimeter (cm). If you measure the lux across a wall and capture every square cm where the light falls, you would be able to determine the total lumens falling on that entire wall. That is, if you are good at the kind of math that uses letters and fancy lines.

In short: lumens represent the brightness of the light leaving the source and lux is the brightness of the light as it arrives at the subject. Lux is very useful to photographers because it lets us know the amount of light reaching the subject.

While we generally use modifiers to decrease the power of a flash or strobe, with single-chip LED lights, if you want more lux on your image, you can magnify and focus the light with an optic. This delivers a lot more lux on the target, without changing the total lumens the light generates.

I could go on for hours with different examples and use case scenarios, but hopefully, you can see the challenge we have.

Especially with mirrorless cameras, photographers mix flash and LED lighting more frequently and that includes the fact that we see more LED lighting being used decoratively, especially in homes, offices, and event venues as well as outdoor settings at night.



A Possible Solution


Guide numbers won’t work for LEDs. Watt seconds are ok for speedlights and monolights that put out an incredibly short and powerful burst of light and maintain the same f-stop regardless of the shutter speed. Lumens and Lux are more practical measurements for constant LED lighting that is always on.

One way that we could begin to evaluate light and potentially deliver the most accurate measurement is “Lux-Seconds”. A Lux-Second is a unit of the quantity of illumination, or luminous exposure, in the International System of Units (SI).

As the term suggests, Lux-Seconds allows us to evaluate the total light the subject received over the period that the shutter is open. A powerful strobe is only on for a fraction of the time the shutter is open unless the strobe is operating in high-speed sync (HSS) mode. Any other light source will deliver the light for the entire time the shutter is open. Both can be measured in Lux-Seconds.

In one scenario, the lux-seconds could be the same from either source. A very short duration flash and a longer duration less powerful light can deliver the same illumination to the image depending on the shutter speed. Some of our current hand-held light meters are capable of measuring Lux-Seconds.

I have had conversations with several engineers as I collected the information for this article, and while they all offer advanced mathematical methods for being able to equate the power of one light to another, lux-seconds seems to be the logical bridge between the world of high-intensity flash, powerful continuous light and shutter speed that determines just how much light illuminates the image.



A Practical Approach


I would propose that maybe the best way to approach this conversation is to leave physics to the engineers and realize that it is time to change the way we, as photographers, think about lighting.

If raw power is what you are after, and you feel the need to over-power the sun at high noon, then flash is still the name of the game. With the advancements in camera sensors, however, most situations don’t call for all the power a xenon flash can deliver. Certainly, for portraits, weddings, and events, large 600-watt strobes are not practical and are rarely needed at full or even half power.

You can increase efficiency by using light from LEDs, which offers greater control by letting you place just the right amount of light exactly where you want it and see the results in real-time thanks to the electronic viewfinders (EVF) in mirrorless cameras.

Event photographers frequently find their venues are lit with brightly colored LEDs that are used to create an exciting fantasy-styled environment. Using a flash wipes out that color and creates dull and boring images.





Portable LED lighting allows the photographer to have a clean white light on the foreground subjects and still embrace the color-rich backgrounds without the problem of a flash being too powerful and forcing the use of very slow shutter speeds.

Even shooting colorful fashion portraits, I find myself using large screen LED TV’S as backgrounds more because of the cost savings and added creative possibilities. Working with LED lighting allows me to balance my light with the digital background and add rich colors with gels all while seeing the finished image in my EVF before I press the shutter.



I haven’t even scratched the surface of the creative possibilities LEDs offer us. There will always be scenarios where power and speed are desirable, and a flash of light may be the best solution. We have already seen some manufacturers try to combine a flash and LED light into the same unit, so it is exciting to think about what future technology holds for us.

One thing is for sure, it is an exciting time to be creative. While I don’t have a simple way to compare lights, I do hope that I have you thinking about the possibilities and if you are “math-challenged” like me — maybe you have an idea of how to easily compare all these technologies.


Header image: On the left a StellaPro CLx10, a 10,000 Lumen LED. On the right a Godox AD400, a 400ws Strobe


About the author: Joe Edelman is an award-winning Olympus Visionary photographer and photo educator. His educational mission: “To help photographers to develop a solid understanding of the HOWS and WHYS behind creating great photographs.”

In addition to being named a StellaPro Champion of Light and Tether Tools Pro, he takes great pride in sharing over 4 decades of experience to motivate and educate new and experienced photographers all over the world via his popular YouTube Channel, which boasts over 176,000 subscribers and more than 11.5 million video views. is videos are viewed by photographers in over 100 countries each week.


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