How much flash power do you really need for your home studio?

There’s a lot of different lighting equipment on the market, all sold using sometimes confusing and even misleading jargon,  and it’s reasonable to assume that more = better and that the more power a bit of equipment has, the better the results it will produce – but is that actually true?

Old-fashioned electric bar radiator

Old fashioned electric fire – uses a lot of power, puts out very little light

Let’s deal with the jargon first.
A lot of sellers, and especially people selling on Ebay, talk about “Watts”. But watts is just an expression of consumed energy, not an expression of delivered energy. For example a 200 watt continuous lamp uses 200 watts of power. Some of that power ends up as light, some ends up as heat and some is lost in the system and doesn’t end up anywhere except on your electricity bill. As an extreme example of how little this means in terms of light, you don’t get a lot of light out of one of the old-fashioned electric heaters because the heater is designed to convert the energy it uses into heat, but even in a powerful halogen  light, designed for photographic lighting, about 2/3rds of the power ends up as heat.

With flash, the correct term is joules or  Watt-seconds (Ws) (the same thing as joules), not watts. Some of the energy ends up as heat with a flash head too, but nearly all of it ends up as light.

Ws quantifies the energy content or storage capacity of the flash capacitors. If a flash unit has a storage capacity of 300 Ws, it has enough power to operate a 300-watt lamp for 1 second or a 300,000-watt lamp for 1/1000 second. The number of watt-seconds gives no real information about the actual amount of light that the flash head produces, and sometimes you can get say a 400Ws flash that actually produces less power than a 200Ws flash of another make.

So, Ws doesn’t actually mean a lot – but what does?
Mostly, reputable flash manufacturers test their lights under real-world conditions and publish the results. The result of the test is called a guide number.

The guide number tests are (or should be) carried out to a benchmark standard.
1. The flash head is fitted with a standard reflector
2. The flash head is placed with the flash tube 2m from the flash meter used to measure the light output
3. The tests are carried out with the camera set at 100 ISO
Using these test conditions should ensure that if you buy that flash head you should be able to get the same results as the tester, although results will vary a bit, depending on the size and decoration of your studio – a small studio with a low white ceiling and white walls will produce higher readings than a large studio with a high black ceiling and black walls.

Guide numbers are expressed in 2 different ways, metres and feet, but the results are the same. For example, the SmartFlash has a guide number of 100 (ft) or 32.8 in metres. Dividing the number of feet into 100 or the number of metres into 32.8 gives the same result.
Let’s assume that the guide number is expressed in feet, and that the guide number is 100. What does that actually mean?

Well, if you divide the distance in feet into the guide number, the answer is the lens aperture that you will get with the flash head set at full power at that distance from the subject. For example, if the flash head is 9′ from the subject then 100 divided by 9 is 11 (near enough) so a lens aperture of f/11 will give the correct exposure.
It follows that if the flash head is only 4 1/2 ‘ from the subject the correct aperture will be f/22 and if the flash head is 25′ from the subject then the correct aperture will be f/4
So, if the flash head is 25’ away from the subject (say a very large group of people) then either you need more power or you need to find a way of working around the lack of power, and we’ll get back to that.

What I’ve just said is really another way of explaining how the Inverse Square Law works. The inverse square law is very important because it affects the amount of light that reaches the subject as the distance from the flash head to the subject changes. Every time you double the distance between the light source and the subject, you lose 3/4 of the power.


And if the flash head is just 4 1/2 ‘ away from the subject, which is pretty normal for a portrait of a single person, then you’ll need to reduce the power of the flash quite a lot, unless you really do want to use a tiny lens aperture.
Reducing it to half power will give you f/11
Reducing it to quarter power will give you f/8
Reducing it to one eighth power will give you f/5.6
And reducing it to one sixteen power will give you f/4.

Now, in theory at least, this means that even our lowest powered flash head, the SmartFlash, will give you all the power you’re likely to need, in most situations, because you should be able to turn the power down enough to allow the use of a large aperture if you want to, and you should also have enough power for when the flash head needs to be a long way away from the subject.

But the amount of power is also affected by two other factors, the ISO speed available on your camera and the type of modifier fitted to the flash head.
ISO speed is very important. Years ago when everyone was shooting on film, there was nothing much we could do about ISO speed – we could always change to a ‘faster’ film with a higher ISO speed, but doing this always produced a lot of grain, so we were pretty much limited to films of around 100 ISO.

And we had similar problems with the early digital cameras too. For example the Canon D30, the Nikon D100 and the Kodak DCS14 cameras were all capable of fine results at 100 ISO but the image quality fell apart at higher ISO settings. But now, with modern DSLR cameras, the results are pretty good at reasonably high (but not very high) ISO settings and pretty much every DSLR camera is capable of producing acceptable results at 400 ISO. So, if you move the ISO setting from 100 to 200 ISO you will effectively double the power of your flash head, and if you move it from 100 to 400 ISO you will effectively quadruple the power!

Light modifiers
The light modifier fitted to your flash head can make an enormous difference. Modifiers that spread the light out over a wide area, such as softboxes and umbrellas, appear to ‘eat’ about 3/4 of their light at a distance of 10′. And modifiers such as our high intensity reflector concentrate the light into a smaller area and so produce far more effective power.

You can see from this that you don’t need a lot of power for most home studio photography, and certainly not for portrait photography, mainly because the light is normally very close to the subject. But a shot like the one on the left (where all the light, including the light from the ‘window’) is from flash heads that are sometimes a very long way away from the subject, much more power can sometimes be needed.

So does it make sense to have powerful flash heads?
Sometimes, yes.  But choosing a flash head of say 400Ws instead of a 300Ws one doesn’t make any sense because the difference is almost too slight to notice. An increase from 200Ws (say the Lencarta SmartFlash to the ElitePro 300 or the SuperFast 300 will only give you a half-stop more of flash power but will give you even faster recyling, because the flash will normally be used at a lower power setting. And choosing say the ElitePro 600 or the SuperFast 600 will give you another full stop of power, which can be handy with large groups and for use with light modifiers that ‘eat’ power, as well as giving you more features and more adjustments.

What effect does the shutter speed have on the exposure?
In most cases, none at all. The sole function of the shutter is to be open when the flash fires and, within reason, it doesn’t matter whether the shutter speed is 1/60th or 1/125th second. If the shutter speed is very long you may sometimes get some light contamination from whatever ambient light is present in your studio, and if it’s too short then you’ll get a black line at the bottom of your photo, caused by the second shutter curtain blocking the light – which is why most of us use 1/125th second with flash. You can check the effect of any ambient light simply by setting the shot up for use with flash and then taking the shot without flash. If your digital camera sensor is black or nearly black then you know that, at that shutter speed, the ambient light doesn’t matter.

Outdoor photography
Although you don’t need much power for most indoor photography, If you’re shooting outdoors in sunlight and you want the flash to be more powerful than the sun, then it’s very different and you need a lot more power, and you also need to use the fastest shutter speed that will work with flash on your camera.