What is the difference btw effective Ws and True Ws?

I have rented studio lights in the past and just used the light meter to set my lights and haven't put much thought into it beyond that. Now that I am going to purchase my own studio lights I am finding that I am quite clueless when it comes to the technicalitites of these lights. I am just about to pull the trigger on 2 Alien Bees b800 lights - but want to make sure that they are what I need. What does it mean Effective Ws and True Ws? Should I be concerned with both numbers or just the effective number?

Thanks in advance for any advice you can give me.

W/S specs, beit true or effective, are only marginally helpful when choosing flash units. The problem is they measure power usage, not light output, and manufacturers can pretty much make these numbers into whatever they want.

IMO, AlienBees' use of the "effective wattseconds" number is meant to puff up their product specs. In the case of the B800s, they're claiming that their 320W/S (true wattseconds) unit matches the light output of other 800 W/S units, made by somebody else somewhere else. It's a little vague...

What IS useful about the W/S numbers is to use them for comparison. In this case you can look at the Tech Spec page on their web site and figure out that each accending model (B400, B800, B1600) gives off roughly twice the light as the one under it.

The bottom line is the B800 is a very good medium output flash. Several folks here use them, and can probably give you some "real world" numbers. My guess is you can probably expect a meter reading (ISO 100) in the f/11 - f/16 range (medium softbox @ 10'). That would make it fine for most basic studio uses...

I use the 1600's and at times have had to run a neutral density filter, along with turning them way down and bouncing them into the umbrellas to get f/8 on a portrait.

The only really good way to choose a studio flash in my opinion is to compare Guide Numbers.

For those that are not familiar with Guide Numbers (GN) they are calculated like this:

Distance (in feet) x f/stop = Guide Number; GN differ according to the ISO of the film being used. Most strobes are rated with a GN for ISO 100.

Example: If the GN is 110 and your flash is 10 feet from the subject, then the correct f/stop to use is f/11 (110 divided by 10 (feet) = 11 (f/11).

So what power flash do you need? Let's assume that you want to shoot portraits at f/8 with ISO 100. If you want to place your lights at 5 feet from your subject your needed output would be GN 40. But if you want to use an umbrella, then you need to up that output. Minimally you would need to go to GN 80. But since your umbrella might lose more than 1 f/stop and you might want to move your lights further away than 5 feet, a more realistic GN for portraits would be GN160.

E-mail the manufacturer if they don't list the GN and ask what the GN is for each model and what reflector they used to calculate the GN. A narrow spot will yield a higher number than a wide umbrella reflector.

Calumet used to give outputs for all their studio equipment with umbrellas and with soft boxes.

Another thing to consider is does the light have a bright enough modeling light? I need a 150 watt modeling light to focus comfortably. My lights allow me to use 150 watt or 250 watt modeling lights. Some lights use household bulbs and are limited to 60 watts; some use enlarger lamps and go a bit higher.