Flash output power raises a lot of questions when you are trying to decide how to spend your money.

A lot of people writing on the internet decry the versatility of portable speedlights and their use as studio-type lighting – something which is entirely wrong in my opinion; as there is nothing that can’t be done with them, as long as you have enough of them!

And you don’t have to take my word for it – just go and watch the worlds best exponent of the art, in my opinion anyway – Joe McNally. – then tell me if I’m wrong!

But with a top-of-the-line Nikon SB910 running at £340 and Canons new 600EXRT a cool £400 plus here in the UK, purchasing 10 to 15 of these puppies is a wallet-emptying proposition; though given the cash or sponsorship it’s the way I’d go all day long.

A lot of folk come to me with the same quandary – studio flash heads are a lot more cost-effective; notwithstanding their big limiting factor – lack of portability.

Leaving aside the other problems of many studio-style flash heads, namely lack of TTL and HSS/FP facility (though this can be walked-around on certain models with Pocket Wizards and the dark art of Hypersynch) they do give one big advantage – more photons for your buck.

But just how does one compare the flash output power of one unit/type with another – after all, this is what we want to know:

• Can I get more light from flash A than I can from flash B
• How many speedlights do I NOT have to buy if I get studio-style flash head C which costs 1.5x the price of one of my speedlights.

The problem is that manufacturers don’t make it easy to do direct comparisons of flash output power between brands and formats, and they tend to try and confuse the buyer with meaningless numbers and endless amounts of jargon.

Back in the days of manual-everything, we used to use flash in a very simple way using the units Guide Number.

The guide number is usually quoted as being at 100 ISO and at two values, one for metres and one for feet, and we use it with the following equation:

GUIDE No: = Distance x Aperture

So we might see a flash unit has a  guide number quoted as 40/131 at 100 ISO.  This means for example, that at 100 ISO and a flash to subject distance of 2.5 metres or 8.2 feet the correct aperture to use would be:

Guide No: divided by distance – in this case 40/2.5m or 131/8.2ft.

Either way the answer is 16, so we would set the shutter speed to the flash synch speed and the aperture to F16.

Simple!

Where things used to go a bit pear-shaped was when we introduced any form of output modifier such as a bounce board or diffuser because these spread and smooth the light and so reduce the number of photons falling on the subject by one or two stops.

But TTL flash metering soon put paid to all that grief.

Camera OEM Speedlights

Let’s compare a Nikon SB800 & SB910 – these have 100 ISO guide numbers of 38/125 & 34/112 respectively (published) – that’s right folks, the new one is weaker than the old one.

But by how much?

Well the old SB800 has a guide number that is 11.7% higher than the newer SB910, but what does this mean in terms of exposure value?

At a flash-to-subject distance of 3.4 metres, doing the maths says that our correct aperture would be 38/3.4 and 34/3.4 respectively. So the SB800 would put us at f11 (11.18 to be precise) while the SB910 would give us f10 – that’s an increase of over 1/3rd of a stop using the older unit.

When working with long lenses and wide apertures this extra 1/3rd of a stop gives me just that little bit more depth of field – and folk wonder why I don’t change mine!

Complications & Caveats

Nikon quote the two units above with guide numbers based on the head “zoom feature” being set to 35mm, which gives a fairly wide angle of lighting.  Someone said to me the other day that the new Canon 600EX was twice the power of the Nikon units I’ve already mentioned, simply because Canon quote the guide number for that device as a massive whopping 60!

The world is full of fools………..

Canon, in their infinite wisdom, quote that 60 value at a zoom head setting of 200mm.  The reality is that the guide number of this Canon unit varies between 26 with the zoom head at 20mm and 60 at 200mm – so in other words, give or take a bit, it’s pretty much in the same ball park as the Nikon units previously mentioned.

Canon speedlight naming policy tells you the units MAXIMUM guide number:

• 600EX = 60 (metres)
• 580EX = 58 (metres)
• 550EX = 55 (metres)

The 550 specs also give you zoom length variations:

• 105mm = 55 (metres)
• 50mm = 42 (metres)
• 17mm = 15 (metres)

Canon 600EX vs Nikon SB800 zoom lengths:

• 105mm = 58 vs 56 (metres)
• 50mm  = 42 vs 44 (metres)
• 14mm = 15 vs 17 (metres)

Light leaves a flash unit in a cone of sorts, and the zoom heads on speedlight style units gather this cone of light so it basically matches the angle of view of the lens you are using and results in an efficient distribution of light across the image area – that’s the theory anyway.

Making the cone “tighter” forces the photons released by the flash into a more concentrated area, thus increasing the number falling on the subject and so increasing the overall exposure value.

So when we use guide numbers to compare various flash units we must ensure that we are comparing the units on a level playing field – in other words, the values we use are for the same “cone or reflector angle”.  And if the manufacturers use different reflector angles when assessing their flash guide numbers for promotion to the public, then you guys ‘n gals run the risk of being hood-winked into buying something that ain’t strictly what you thought it was when you ordered it.

So how do speed light style flash units stack up against studio type units?

Notwithstanding the lack of FP/HSS and any TTL metering problems, studio-type flash heads have guide numbers that are usually quoted as being “with standard reflector”.  This standard reflector is something which gathers those photons and shovels them out in a 50-55 degree spread; think “standard lens” on the image diagonal.

Current top end Nikon speed lights (and Canon) have guide numbers of sub 40 at 35mm reflector angles, and those equate to roughly 64 degrees diagonal coverage.  So if we were to “tighten them up” to 50 or 55 degrees we could, as a rough guide, round the guide numbers up to 42m or 44m.

Now we are on a more even playing field.

A Bowens Gemini 500R is quoted by Bowens as having a guide number of 85 with a standard reflector, so let’s be a bit cavalier with the numbers and say that it’s double the guide number of SB800/910 or 580EX etc.

So roughly how many speed lights is this puppy going to be equivalent to in terms of real flash output power ?

Hands up those who think two………….wrong!

This is where everything you thought you knew about exposure turns to shit in front of your very eyes (but not really!), and it’s called the Inverse Square Law.

Inverse Square Law

Now listen folks, this is as simple or as complicated as you care to make it!

When we capture a scene we capture a 2 dimensional plane filled with photons travelling towards us.

When we shine any light on an object we are actually throwing a flat sheet of light at it. This sheet is expanding outwards as it travels towards the subject because the photons in that sheet of light are all diverging.

So, let’s look at something tangible as an analogy – metric paper sizes!

How many sheets of A3 paper fit on a sheet of A2 paper?

That’s right, TWO – we’ve effectively doubled the surface area of the paper.

Now exposure works in stops – and making a 1 stop change in exposure effectively doubles or halves the exposure value depending on which way we’ve made the adjustment.

So moving from A3 to A2 is like making a 1 stop change in exposure; we’ve doubled the surface area of the paper.  BUT – we’ve not doubled the papers physical dimensions.

What paper size is twice the width AND twice the height of A3 – yep, that’s right, A1.

And how many sheets of A3 fit on a sheet of A1 – right again, 4.

So we have quadrupled the papers surface area – in exposure terms that would equate to 2 stops.

Now imagine a projector throwing an image onto a big screen and the screen to projector distance is 4 metres.  We go to the screen and measure the size of the projected image and it’s 1.5 metres by 2 metres.

How big will the image be if we move the projector to 8 metres from the screen?

Answer – 3 metres x 4 metres. (and the brightness of the image will have gone down by 2 stops).

And if we move the projector to 2 metres from the screen the image will be 0.75 metres x 1 metre. (and the brightness of the image will have increased by 2 stops!).

Inverse Square Law, Lights & Distances

Let’s say we have a theoretical flash with a metres guide number of 80.

If the subject is 10 metres from the light we need an aperture of f8 because 80/10 = 8.

If we now move the light to 5 metres from the subject our aperture decrease to 80/5 = f16

Halving the light-to-subject distance means we increase the overall intensity of the light (its effective flash output power) by 2 stops, so we have to reduce our overall exposure by two stops to compensate; otherwise we’ll just end up with 2 stops of over exposure.

And of course if we move the light away to 20 metres from the subject the inverse applies and we effectively reduce the flash output power by two stops and we’ll have to open the aperture up by two stops to avoid under exposure.

But what do we have to do in order to use f16 at 10 metres AND get correct exposure?

Use a flash with a guide number of 160 is what we’d need to do – it really is that simple.

Reality

So, how many guide number 45 speed lights would we need to equal one guide number 90 studio flash head in terms of effective flash output power?

Well it isn’t two – oh that we should be so lucky!

If we have two speed lights mounted together their cumulative guide number is equal to the square root of the sum of the squares of their individual guide numbers!

Sounds scary, but the answer is 63 or thereabouts.

But here’s the thing about photo-maths – it usually ends up as something really simple and this is no exception.

If you want to double the guide number you always need 4 identical units.

Do not forget what I’ve said above about published guide numbers – you have to ensure that the values were obtained using equal criteria, and manufacturers sometimes don’t always like to furnish you with the information you need in order to do easy comparisons.

Have they got something to hide – you may think that, but I couldn’t possibly comment!

What really does piss me off the meaningless crap they do furnish you with – watt-second, w/s, watt/sec or if you like Joules values.

The only thing these values do is inform you of the “potential energy” available at the capacitor; it’s no measure of how efficiently the flash tube converts that power into photons – and the photons is ALL we’re really interested in.

Other things such as tube temperature can have dramatic effects on both light output and the colour of that light.

Conclusion

This post has been a bit of a ramble but I’ve tried as best I can to give you a rough guide on how to compare one flash source with another.

Different photographers require different things – if all you want to do is shoot portraits and still life then shutter speeds above 1/250th synch are of little importance in general terms, so access to HSS/AutoFP via speed lights isn’t needed, and normal studio lights would be a far more economical proposition.

But on the other hand 8 speed lights in one bank, and two more banks of 4 speed lights each – all HSS/AutoFP compliant – crikey, the photographic possibilities are endless, and readily achievable – if your bank balance is endless too!

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