Flash power in F numbers, I don't understand

[Blasted]

Could someone explain to me the meaning of the F figures when discussing flash power please?

For example. When looking at the Elite pro 300 on the Lencarta site they mention, standard reflector at 10 feet – F16.2, 60cm softbox at ten feet – F7.1 and 150cm softbox at ten feet – F7.1.

They also mention guide no (feet) which I’m not to sure about either.

Thanks

 

[mike weeks]

Guide numbers are generally given for 100 ISO (generally we ignore shutter speed for flash power)

So if you have your meter set to 100 ISO and stand 10 feet away from the head and point the flashmeter back to the light you will get a reading of f16.2 when the flash is triggered.

A softbox will spread the light more than a standard reflector so dropping to f7.1 is totally expected

Guide No. feet or GN = distance × f number

so a guide number of 162 for the 300 at 10 feet would be

162 = 10 x f number

we can rearrange to

f number = guide number / distance

f number = 162 / 10

so f number = 16.2

 

[Blasted]

Thanks for that

 

[HoppyUK]

As Mike said, guide numbers and f/numbers are just different ways of expressing light output/power/brightness. There's another one too, that is more universal, Watt-seconds, or Ws, also known as joules. None of them is totally reliable, prone to lots of variables and exaggeration, though Ws is usually the best guide when comparing different brands.

F/numbers are quoted with a decimal after the aperture number, eg f/16.2 is 2/10ths of a stop brighter than f/16. Sometimes written as f/16d2 or variations along those lines.

 

[Blasted]

With all this in mind, If i set my speedlight up 10 feet from a subject, should i exect to get a smaller F/number? to me that would mean less control over the ambient light.

Aside from the benefit of all the usual fixed studio light set up perks such as faster recycling and stuff, what advantages does having more power bring into a studio environment?

 

[Garry Edwards]

With all this in mind, If i set my speedlight up 10 feet from a subject, should i exect to get a smaller F/number? to me that would mean less control over the ambient light.

Aside from the benefit of all the usual fixed studio light set up perks such as faster recycling and stuff, what advantages does having more power bring into a studio environment?

MUCH faster recycling, modelling lamp, the ability to use the full range of light shapers - all these are far more important than just power.

Light (+heat and sound) all follow the inverse square law, which means that every time you double the distance, 3/4 of the power is lost. So, if you increase the distance you need a larger aperture to compensate and you're right, that does mean less control over the ambient light. You use the shutter speed to control the effect of the ambient light, subject to the limitations of the camera shutter, which usually caps the shutter speed at either 1/200th or 1/250th. This can be a real problem using speedlights outdoors, it isn't normally a problem in the studio, where ambient light levels are very low.

 

[HoppyUK]

Edit: Garry got there first

With all this in mind, If i set my speedlight up 10 feet from a subject, should i exect to get a smaller F/number? to me that would mean less control over the ambient light.

All artificial light follows the Inverse Square Law pretty closely. Guide numbers are based exactly on this principle. ISL says double the distance equals one quarter of the brightness. Eg, if your speedlite is 10ft from the subject and correct exposure is f/8, then if you move it closer to 5ft, correct exposure will be two stops higher, ie f/16. Move it back to 20ft and brightness will drop two stops to f/4.

Aside from the benefit of all the usual fixed studio light set up perks such as faster recycling and stuff, what advantages does having more power bring into a studio environment?

You don't need much flash power for normal studio portraits. A big speedlite has around 100Ws equivalent power, and that's usually enough. For a studio head, 200Ws is a good starting point to cope with most things like solo portraits, couples and small groups. If it's bigger groups, you'll need more power. Remember that in the studio, where there is no significant ambient light, you can use ISO to effectively increase flash brightness, eg changing from ISO100 to ISO200 effectively doubles the flash power in exposure terms, ISO400 doubles it again, and so on.

 

[Blasted]

Thanks guys.

One of the reaons I'm interested in flash power is because I'm also thinking about flash fall off, seperating the different flashes into their respective lighting roles, controllng background seperation, shadows and the relative size of light source. All stuff I've read about,all important and all very confusing.