Guide Numbers vs. Watt Seconds

A post in the photosig forum was asking about a link between Guide Numbers and Watt Seconds, and whether there was a direct conversion.

It is possible to do calculations to do this conversion, but for most real-world situations, it's unbelievably complex, with light having to be simulated or otherwise modelled as it travels via reflectors etc. before heading in its final direction.

Guide numbers don't really give information about the power of a flash though. What they give is information about how bright they make a subject at a given distance, and therefore, how wide an aperture you should have to correctly expose that subject.

It is therefore possible to do a conversion directly to an Illuminance value in Lux.

Once you have this value, you then would need to model the light to give a value in Lumens or Lumenseconds. The only one you can really calculate is an imaginary light source which evenly lights in all directions (a point light source, a bit like a star). This never applies to any strobe though, so it's purely for interest sake.

If you're allergic to maths, look away now!

First of all, go have a read of this rather interesting page on Photometry of Strobes, and in particular the section called "Photographic Light Meters, Exposure Value and Guide Number" although almost everything before this section is of use in explaining the following calculations. This page is my source for the equations to calculate all of this, so I'm making the assumption this information is correct.

Here's the initial conversion from Guide Number to Lux, with a few assumed values added in to do the sample calculation.



And the following conversion to watt seconds for an ideal point source light is purely for interest sake.



Apologies for the poor hand-writing and hopefully I don't need to make any apologies for any mistakes in the maths! If you find any though, I'd like to know about them asap!

Watt Seconds vs. Effective Watt Seconds

What is a Watt Second?

Well, it's a unit of electrical energy, also known as a Joule. It is a Power of 1 Watt for a time of 1 second.

So what is a Watt? Well it is calculated as current (measured in amperes) multiplied by voltage (measured in volts).

So 1 Watt = 1 Ampere x 1 Volt = 0.5A x 2V = 2A x 0.5V etc. etc.

So, supposing you have a 1000W strobe, it has the capability to put 1000W of electrical power into firing the flash. This has no real bearing on how bright the flash is, although everything else being equal, a 500W strobe would, in theory, be half as powerful as a 1000W strobe.

Why Watt Seconds then?

A flash fires for only a fraction of a second. Depending on the make and model, this can be anything from 1/750s to 1/2000s or shorter. Generally, the harder a flash fires, the longer the flash time is. In order to compare the wattage with that of a standard continuous light source, such as a tungsten bulb, you really need to take into account how much power is used in a given time. So, taking a 500W tungsten lamp, you can propose that running it for 1 second will take 500 Watt Seconds, which I will say as 500Ws from now on. A 500Ws flash will use a far greater power, but does so for a much shorter time. Say the 500Ws flash fires for just 1/1000s, the actual power which would be used by the flash if it lit for the whole second would be 1000 times the 500Ws value, which would be 500,000W. That would take a seriously bright light to sustain that output!

So the answer is that Watt Seconds allow the electrical power used by a continuous lamp to be compared with that of a strobe.

How is light output measured?

If you buy a scientific light meter, you'll find that when you shine a light on it, it gives a value in Lux. This is the standard accepted unit for measuring the intensity of light which falls on a surface.

The output of a light source is measured in Lumens, which is defined by the following:

1 Lumen = 1 Lux x 1 Square metre

For the same reasons of comparing with continuous lights, it is necessary to make up a value known as Lumenseconds.

How are Watt Seconds and Lumenseconds linked?

The job of a strobe is to convert electrical energy into light energy, and like any light bulb, there is an associated efficiency. Some strobes are more efficient than others, with poor models producing around 15 lumenseconds for every watt second, while more efficient models can product up to 50 lumenseconds for every watt second.

So where does the term Effective Watt Seconds come from?

According to white-lightning.com, which gives the best explanation I have found, a company called Inverse Square Systems first coined this term in relation to a product known as "Stroblox" which it released in 1985.

They had produced a strobe which was more efficient than many others on the market. Most people compared one strobe to another in terms of Watt Seconds, because the efficiency of each strobe wasn't that different at that time. So in order to market this new product, they had to show that it had a lot higher light output due to higher efficiency than the equivalent watt second strobe from a competitor.

They therefore measured the average light output for competitors strobes, compared it with their own, and came up with the statement that their strobe was effectively as powerful as a competitors strobe with a much higher watt second rating.

It was a nice bit of marketing, but unfortunately the term has stuck, and despite being pretty much meaningless, successive companies happily quote an "effective watt seconds" rating, higher than the actual watt second rating of the unit, in order to make the product seem more powerful.

Quick conclusion

So how can you compare strobes? The only reliable way is to find data for the Lumenseconds output of the units, and compare all the units that way. If you have quoted Watt Seconds, you then need to know the efficiency in terms of the number of Lumens per Watt Second the unit produces. You can then calculate the Lumenseconds value to compare the units.

As for Effective Watt Seconds? It's very useful for selling underpowered strobes.

Guide to Guide Numbers

Guide numbers are the common method of comparing the relative powers of different flashes. The value is found experimentally by firing the flash at maximum power at a subject which is a fixed distance from the flash. With no other source of light, a flash meter is used to obtain an optimum aperture for a "correct exposure", which is of course slightly subjective. Similarly a camera could be used, and photographs compared for the best exposure, although there would be significant complication introduced by having to take additional account of reflectance of the subject being photographed and the distance from the camera to the subject.

One area of common confusion is that Guide numbers can be specified as either feet or metres. Sometimes a flash will be quoted in feet, which makes it seem ever so much more powerful than if quoted in metres, but there is of course no difference.

A second common confusion is use of ISO speeds. Most Guide numbers are calculated for ISO 100, which is traditionally a very common film speed. However, quoting at ISO 200 can make a flash seem more powerful, so it's important to check what ISO speed is being quoted.

So, getting back to the experiment, say we set up our flash 10 metres from our flash meter (and subject). We then fire the flash at full power, and measure the light output. Our flash meter, set for ISO 100, gives a reading of f/8.

To calculate the guide number for our flash, we simply multiply the aperture value with the distance to our subject.

So we get:

Guide Number = Distance x Aperture
Guide Number = 10m x 8
Guide Number = 80m

So our flash has a Guide Number of 80 metres at ISO 100.

For another quick example, lets say we've bought another flash that is rated at a Guide number of 360 feet @ ISO 200. Given this incredible Guide number, it had to be a good deal, and much better than the flash we already had at 80m @ ISO 100, right?

Well let's work it out:

Since quadrupling ISO speed means we double the Guide number due to the Inverse Square law, doubling ISO speed means we multiply the the square root of 2 to get the Guide number. However, since we're wanting to half the ISO speed, we therefore divide by the square root of 2.

Converting to ISO 100, we therefore get

360 feet @ ISO 200 = 254.6 feet @ ISO 100

Converting feet into metres, we get

254.6 feet @ ISO 100 = 77.6 metres @ ISO 100

So despite the very high quoted guide number, it turns out this flash isn't actually as powerful as our first flash.

So what other trick do manufacturers have up their sleeves?

Besides using feet instead of metres, and higher ISO speeds, to make numbers sound more impressive than they are, manufacturers also have a couple of other possibilities for their marketing departments to make use of.

The major one is the hazy description of "correct exposure". In general, a manufacturer will opt for a slightly dark exposure, because this will make the Guide number of their flash higher. You, on the other hand, might feel this is too dark, and would want a brighter photo, so your Guide number wouldn't be nearly as high to make the photo the way you like it.

Another one is only an option for manufacturers of small flashguns. Most of the half decent flashguns on the market now have a zoom capability. As you zoom your lens, your camera sends signals to your flash to tell it to zoom in. What this does is modify the way the light is directed, and make the output of light more efficient for a given power of flash burst, simply because more of the light given out is directed towards the subject, rather than lighting all around the subject.

So why is this important? Well when the flash is zoomed in, it has an effectively greater guide number than when it is zoomed out. So you can guess that the manufacturers will make use of this, and quote the Guide number of the flash at its best zoom position.

The best advice therefore is to do your own tests when you get a flash, if it matters to you to know for sure. That way you know there hasn't been a marketing department involved in the Guide number you arrive at.

Cool shots

I came across a photographer from across the water who has some cool shots. Got a great studio too if you like looking behind the scenes.

There's a lot of good stuff on his pbase account at:

http://www.pbase.com/infocusinc

And his main company site too, although a lot of stuff on it is already on the pbase site.

http://www.infocusinc.net

Enjoy :)

Edge lighting glass

Sometimes, you want a pretty eye-catching graphical shot of a glass item. It might be a wine bottle, which you want to make anonymous, so you want to light it and avoid showing the label. Of course, you can still light the label, perhaps with a snoot, and just use this technique for dramatic purposes.

I didn't have a wine bottle to hand, but I did have a rather interestingly shaped bathroom perfume thingy. It has blue liquid in the shaped central area, but that won't show unless you light behind it. Same goes for a wine bottle, glass or whatever. You'll get some of the colour from the contents on the edge of the glass, but the centre will usually be pretty dark. There's some physics in there relating to reflective angles, but it's not that important.

So here's my first shot. I placed the perfume glass on a black shiny base, and used my lightbox. This is the exact same initial setup as used in the previous entry for the wristwatch.

I put a 550EX (equivalent to Canon 580EX MkII) flash on either side, lighting most of the diffusers, to create a soft light on each side. This resulted in the following image.



You'll see the top isn't particularly well lit though. On a bottle this wouldn't matter, because it has a top that isn't glass. In this photo though, it's a big deal, because the top edge isn't standing out very well.

I therefore got another flash in the form of a 580EX (equivalent to Canon 580EX MkII). I put it on a lighting stand, and used the lighting stand at an angle, balanced, to support the 580EX above the light box. It's a quick and dirty way to do it, but it works for speed. You can see in the following image, it fills in the edge at the top a bit better.



This is the setup for that shot above.



It's not bad, but the edges aren't all that sharply defined. Ideally the edges will be a thin line.

The further back you move the light source, the steeper the angle of reflection is, and therefore the thinner the edge light shows on the product. So moving the flashes to just light an area at the back of the diffuser, I get the following shot.



You can see how the setup has changed in this image.