watts vs. w/s

[cjrdc]

I am fairly new to the photography game. Can someone explain the difference between watts and watts/sec. I understand it is the energy capacity of the light, but what is the difference between say a 500w continuous light and a 500w/s strobe. Or is it the same only one is continuous and one is just a brief flash.

I have a continuous two light system with one 500w shoot through umbrella and a 300w reflector umbrella and then the flash for the camera usually mounted on bracket. I get very mixed results with these. Last week a shoot with a family came out horribly dark, and a shoot last night with a little boy came out great, almost overexposed (a few shots are). I have been considering upgrading to something else, but I want to have my facts straight so I know what I am doing when looking at different light sets.

Thanks

[jorgemonkey]

I'm still learning what all these studio light words mean, but I have a pair of 250W Tungsten lights and are pretty bright, but get very hot.

I just purchased a pair of 300 w/s strobes (the strobes are basically VERY powerful flashes) that are very bright. With the tungsten lights I had to shoot at a pretty slow shutter speed to get some decent depth of field in the photo, while with the strobes I can shoot up to 1/500th of a second and depending on how far away the subject is I can shoot around f/16 or so

[photophorous]

I don't have any practical experience with this, but I learned about energy in school. Someone with some actual experience can probably give you a more practical answer. This is my understanding of the technical stuff, but if someone disagrees with me, please say so.

First, some definitions: A Watt is a unit of power. Power is the rate at which work is done. Work is the transfer of energy. So, a Watt represents how fast energy is transferred. A Watt Second is Watts times Seconds, not Watts divided by Seconds, as is commonly shown.

A 1000 Ws strobe consumes the same amount of energy for a single flash as a continuos 1000 W light consumes over a 1 second time period. The operative word here is "consumes." These ratings of Watts and Watt Seconds are related to consumption, not output. The light output will vary depending on the efficiency of the design, which will vary with each light. This is different from "Guide Numbers" which are a measure of output.

Hypothetical example: If you have a 1000 Ws strobe and a 1000 W tungsten bulb, both with the exact same level of efficiency (which is very unlikely), a flash from the strobe and a 1 second exposure of the tungsten light would give the same exposure results, at the same fstop.

In reality, tungsten lights are much less efficient, which is where the heat comes from.

Hope this helps.

Paul

[jorgemonkey]

Well, that helped me out. Thanks!
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ps. I was wondering the exact same question when I was looking at strobes

[jdugger]

Watt/seconds is a measure of the the potention energy of the flash i.e the total amount of power that can theoretically be delivered by the flash unit. Watts is the total power consumed or used by a device usually over a set time frame. The Guide Number for a flash unit is F-stop x distance of flash to subject = guide number of flash.

The problem with watt/seconds and guide numbers from manufacterers is, are they acurate? Just because a manufacter says his strobe is rated at 100 watt/seconds does not mean that the strobe efficently delivers or converts that power to light a subject. I have several monolights that are rated anywhere from 160 w/s to 360 w/s.

In my experience if you are buying strobes or monolights get the highest rated lights you can afford and make sure that they have variable power out put control and that they can except anything from a barn door to a soft box. Different umbrellas and diffusers are going to throw more or less light at our subject anyway. Having a good light meter (when using film) that you can trust once you know your lights will be much mor helpful.

Once you get the lights your just going to have to test your lights extensively and come up with your guide number and settings.

I use a Sunpak 554 with my RB67 and I did the same thing that I did with my strobes test test test.