Paul C. Buff, Inc. Technical Forum

I have two cfls are throw out 800 watts of incandescent. I cannot get enough light for my makeshift studio with them. I believe that there is some physics of light to understand, ie watts per second. However, I am not understanding all the physics behind this and would love someone to explain all this. I understand that it is about watts per second, for instance that a speedlight throws out 20,000 watts per second and therefore is much more powerful than my cfls. But in simple terms can someone explain to me about how natural light (is that for instance the measurement of 5,500 which everyone goes on about) and studio strobes compare with cfls.

I would be very grateful for a simple explanation, I get completely lost when it comes to the technical way it is measured. Regards.

800W of continuous light power is roughly the same exposure value as 800WS of flash . . . at a shutter speed of 1 second.

But at a shutter speed of 1/200, the flash exposure will be 200 times higher, requiring 160,000W of continuous light for the same exposure.

Put another way, 800W is 800WS/Second. So at 1/200 second shutter, the 800W CFL produces 4WS.

There is no such term as "Watts per Second"

You had also mentioned 5500 of "natural light". 5500 is color temperature, or a way to describe what color the light is. This value is expressed in Kelvin. The higher the number, the more blue. The lower, the more yellow/orange. Sunlight is typically around 5500K, which is considered "white" light.

Typical incandescent light bulbs produce about 2700 to 3200K, which is pretty orange compared to daylight. CFL's can be balanced to daylight, or they can not be. It depends.

Color temperature has no direct correlation to the amount of light output. You can have 1Ws of light that is 5500K or 10,000Ws of light that is 5500K. (A particular light can change color temperature with a change in power, but it is not a huge change, and is a little outside the scope of the conversation).

If you have one light source at 5500K and one at 3200K, you will have one portion of the image with the correct color, and one that is a little orange. Or you can balance to the 3200K, and you will have one area correct, and one area blue. You can play with the WB settings of your camera to see how the camera reacts to different colors.

I am a motion picture lighting technician starting to get into more still photography and in particular researching strobe lighting as an alternative to the continuous lighting I am most fluent with. I too am looking for some comparisons on how much light output I can expect from strobes as compared to some of the continuous lights I'm used to using.

The title of this post is not very specific, as "watts of constant light" is a very ambiguous phrase.



800w of what type of continuous light? Tungsten light, HMI light, Fluorescent light, LED light all operate at different efficiencies and have a different amount of light output per watt. i.e. A 400w HMI is more efficient than a 400w tungsten light and therefore has much more output than a 400w tungsten light, even though they consume the same wattage. Wattage can not be used as a unit for an amount of light; it is a unit exclusively for electrical energy consumption.



Any type of continuous light that consumes 160,000W would be insanely huge. The biggest continuous light I know of, the 100kW SoftSun plasma light, is 9 feet wide and weighs 320lbs, drawing 230A:



Is it possible that a compact strobe can put out the same amount of light, even momentarily?

I'm having trouble wrapping my mind around the Ws and Ls units for photometrics. Continuous light manufacturers use lux and footcandles at given distances, which makes a lot more sense to me personally. There doesn't seem to be an easy way to find equivalents, beyond actual exposure data.

In motion picture lighting, it is common to put 6kW HMI PARs outdoors shining in through windows to light daytime interiors. According to the ARRI photometric calculator a 6kW PAR with a wide lens, focussed at full flood, at a distance of 30ft will yield an exposure of approximately f/5.6 1/200 at 200ISO. Is there any chart for finding this type of exposure data for say the AlienBees B1600 with a particular reflector/modifier setup? Basically I want to know if I can do a similar lighting setup for stills, replacing the HMI with a strobe so that I don't need a diesel generator. According to the math above, even the 640Ws B1600 should have enough output, but for some reason (and not really having much experience with flash lighting beyond speedlights) I am skeptical. I hope to be pleasantly surprised.

Luap, apart from my husband has given the best description of my situation. It seems to me that being educated about the whole physics of lighting from our sun and our lighting system, given in laymans terms is lacking in the film and camera environment. However, what I gathered (from someone that is struggling to wrap my head around lighting physics) is that the light from natural light is extremely powerful in the 100,000 x 's category. When I use my two cfls that total 800 watts incandescent, although it does seem a lot of light either my eyes have compensated in favor of the light (and the camera doesn't see it) , or there is a speed factor of it coming from the source to the subject. I would also guess that my eyes/brain is more complex than the circular sphere of a dumb camera lense. Luap, put it in shutter speed terms which I can understand - and I thank you for that, I will use that as a measure, as crude as it may seem, my little brain can understand that!

I have one 580 ex II flash and wanted to buy some constant (cheap light) which would not get hot. I also looked into LED lighting. Both the LED lighting (unless you spend $1,000s) and cfls, are not powerful enough. In fact to get the soft light which paralells diffused sunlight, one would have to use several flashes in a huge softbox and diffuse the light with either two or three panels. Alternatively wider F stops and lower shutter speeds. However, I tried this with the cfls (800 watts) and had to go down to F2.8 shutter speed 1/60 th and distance 1ft - which was not practical. Personally tellittothevoid you are probably best, as I did, experimenting with light for a while, borrowing peoples equipment, (and not listening to the owners explanation as they often don't know what they are saying) and buying cheap types of lighting for your brain to wrap around it all. I am glad I invested $18.00 for these two cfls, because it was worth understanding and wow, I have a huge appreciation for our sun.

Things to consider, video - film is movement, is that why you can get away with lower lighting setups? Just a thought, trying to think of another element which is different from still photography. Good luck

Paul's response was in layman's terms, as many folks are unaccustomed to light measurement, and are only familiar with Watts as a way of expressing the brightness of a light. Paul was illustrating a point without getting too technical. For those with knowledge of light measurement, it does get more complex.

You are correct, different types of light will output different amount of light with the same amount of power (Watts) consumed. However, the body of the post does specify incandescent type light. Even still, measuring actual light output in Watts, or Wattseconds, is technically incorrect, as lights of the same technology can output different amounts of light with the same power consumption. Lumens/Lumenseconds would be a correct way of expressing actual light output. We do offer these values on each of our light's specs pages. Unfortunately, we are the only ones who who do, so it makes comparison with other brands a little hard.

For practical purposes, Wattseconds is accurate enough to compare one light to another, relatively. A 640Ws light will be roughly one stop brighter than a 320Ws light of another brand, all else being equal (a super cheap mystery light may not follow this as well, but quality lights should be close). However, that tells you nothing about how much light actually hits the subject.

Even if you did know exactly how much light was leaving the tube, you would then have to factor how that light was modified. A high intensity reflector will put much of that light on a small area. A wide reflector will spread it out over more area, reducing the intensity on any one area. A 7" reflector from one company, and a 7" reflector from another may have different spreads of light. Also, one 45 degree reflector may have a very sharp fall off (think, spotlight), while another may have a gradual feathering. The one with a sharp fall off will likely be more intense than the other. Different brands of softbox may have different transmissivity.

The real way to determine output would be with testing. We have a fairly extensive chart of expected output and guide numbers for our various modifiers (at ISO100). This can be found here: http://www.paulcbuff.com/output.php

The reason you get such output from the Arri light is due to a very focused beam along with a high output light source. A B1600 should be able to achieve this with the stock reflector. However, this sounds like a set up used for outdoor daylight work. ISO 200, 1/200s, f/5.6 would yield an over exposed image, before the addition of the light. You will need an ND filter, lower ISO, or smaller aperture (or combo) to not over expose the image. With this overall reduction in light, the Bee will need to be moved closer, or use a higher output reflector.

Thanks for the reply, I know it was specifically written for telltothevoid. However, I know that watts per second is not something that exists and was specified so that us mere mortals can grasp the concept. Since this is something that probably confuses at least 50% of your customers, would it be best to try to give examples on the internet. I am thinking here, of a photograph of a studio set up with specific F numbers, shutter speeds and lighting power, of your strobes and for example, a little paragraph as to what F numbers and shutter speeds you are able to get at say, 6ft. Personally, I think this could boost your sales - just a thought!

Yes, you had posted while I was typing the above response.

In a studio, your shutter speed is of little consequence. You will have a maximum shutter speed, known as x-sync or sync speed. This is typically 1/160 to 1/250, but will be specified in the user manual of the camera. You will need to stay below this speed or you will get black banding in your image.

On the low end, since interior lighting is fairly low, you can set shutter speed to 1/125, 1/60, maybe even 1/30 or lower and not see a difference (in a completely black studio, you could have a shutter speed of indefinite length and not have a difference). The limiting factor would be the amount of light you have and what the other settings are on your camera (ISO, aperture).

For simplicity, lets just say stay on 1/125 shutter speed in a studio. ISO can almost always stay at base. This will vary from camera to camera, but most things are measured at ISO100, so I will use this in my examples.

This leaves flash output and f/stop. Ideally, you would choose the aperture you wanted to shoot at, and set the flash power to match. There is an infinite number of combinations of flash power vs. aperture vs. distance. There is a tool that helps determine aperture or distance, if you know the other. This is called a Guide Number, or GN. These are calculated by multiplying distance times aperture. So if you know the guide number, and the distance, you can find your aperture; or vice versa. We list these at various power setting for various modifiers, and at ISO100. If you use a different ISO, you would change your aperture accordingly (or your distance, but that is a more complex answer).

The expected output chart can be found here: http://www.paulcbuff.com/output.php. Select your modifier, and select the power setting you will be using, and you can find the expected measurements at 5 or 10 feet, as well as the Guide Numbers.

Regarding "Any type of continuous light that consumes 160,000W would be insanely huge. The biggest continuous light I know of, the 100kW SoftSun plasma light, is 9 feet wide and weighs 320lbs, drawing 230A:

Is it possible that a compact strobe can put out the same amount of light, even momentarily?


Indeed, a lowly 160Ws Alienbee puts out on the order of 1,000,000 watts worth of peak light, but does it in about 1/2000 of a second. (Not real accurate math) - but close. So the actual intensity is higher than a 160,000W continuous light.

Great for shooting stills at high exposure values and action freezing. but of no value for movies, where the light has to be continuous.

Look at it this way: If your could turn your 160,000 continuous light on and off for 1/2000 second you would have an 80Ws flash (160,000W/2000)

Right, I follow you on this.


This is the type of data the continuous light manufacturers give. Obviously not for every scenario, but they tell you what to expect from the fixture unmodified, at different spot/flood focuses, at given distances (so you can see how quickly it falls off).



Aha! This is exactly what I was looking for, thank you. Somehow I missed it at the bottom of the product pages. According to the data, the B1600 does have a few more stops output than the 6kW HMI at the same distances. That's great news.



I'm actually talking about lighting indoor shots, but by placing the strobe outside the windows and shooting them in through the windows onto the subject. The goal is to simulate natural sunlight in a controllable, repeatable way in a wide shot, where the windows are in the shot (so you can't just place the strobe indoors in front of the window because they would be in the frame). If shot during the day, there will be some soft ambient light coming in the window from the glow of the sky and reflecting off whatever is outside but it will not be nearly as bright as being outside. The strobe will be a harder, more directional source that simulates direct sunlight (in this scenario, the actual direct sunlight is either not coming in the window because of its position in the sky, or I am intentionally blocking it from coming in the window with flags because it is not coming in at the angle I want it. If it is at the angle I want it, I just shoot the shot with that.). The strobe outside will only have to fight the little bit of soft ambience that is coming in the window. I just metered an f/1 (ISO100, 1/125) about 4ft inside my window that has no direct sunlight, whereas B1600 at full power with high output reflector will read f/22 (ISO100, 1/125) at 10ft according to the Expected Output data. That gives me some wiggle room to move the strobe farther back and/or diffuse it and still shoot at a reasonable stop, which I wouldn't have been able to do with just the ambient light from the window.

I can then add a second strobe inside, bounced and/or diffused, on the opposite side of the subject to dial in however much fill I want.

Of course if I'm shooting close-ups, I could put the strobe inside at lower power, with thicker diffusion, etc.


So that's what I want to be able to do. That's the way I'm used to working for motion picture stuff and it makes sense to me. I'm not sure if many still photo people work that way, and I'm very willing to believe that there could be a simpler way that still photographers achieve this. That being said, I did work on one big commercial photo shoot where we used HMIs outside windows in this fashion. But basically I want to know if I can do it with strobes when I'm shooting personal work without a commercial budget and full crew.

Based on the data, it looks like this will work, but ultimately I guess I should probably rent some strobes for a day and test it out before buying.