How do you know how much power to add to a flash in manual mode?

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jonneymendoza

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Hi.

as the title states. How do you?

Do yo simply take a few test shots and guess or do you know a genuine way to calculate teh correct flash power to apply to a given scene to make sure the flash does not overexpose the subject or underexpose?

Thanks
 
Manual flash?

You can calculate it (that's what Guide No's are for) you can take a guess (from experience) and then adjust (using the screen to assess) you can use a flash meter.

TTL flash

Set the camera to Manual, set the flash to TTL, dial in expected flash compensation, check adjust FEC as required.
 
Is it fully manual?

My favourite flash is the Vivitar 285. IT has several semi-automatic modes where as long as you are not too far away, it will output a set amount of light as judged by its built in* sensor. These settings are primarily so you can use it at your desired aperture setting as the main light but if you fool it, you can make it put out a stop or to less light in order to use it as a fill flash.

* Actually, it's not built in. You can take it off of the front of the flash and attach it to a lead so the flash can be remote but the sensor is still on the camera.

And a fourth option to add to Phil's list - You can use the exposure table usually on the back of the flash (or dial, etc.)


Steve.
 
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So the ideal way is this:

1. do test shots and look at the histogram NOT THE LCD

2. Use a light meter?

I have not used a light meter before but here is how i assume it works. you set the shutter speed and ISO on the light meter, connect all your strobes and flashes together wirelessly and take a meter reading off teh subject using the light meter which triggers your strobes and flashes to fire, giving you the end result of what apparture is needed for correct exposure for this shot when the strobes and flashes have been triggered?
 
That's pretty much it, though it depends on how your lights are positioned and what effect you want. Personally, I'd take individual light readings from each light first to get the ratios how you want.
 
jonneymendoza said:
Where does the output of the flash come into play in that equation?
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In the guide number. The number is usually stated for ISO 100 and is now in metres but could be in feet for an older flash

So at ISO 100 with a distance of 8 metres for example, look up the guide number and divide that by the distance.

So if the guide number is 60, aperture = 60/8 = 7.5 - f8 is close enough.

JonathanRyan said:
GN / metres = aperture
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tiler65 said:
I thought it was GN = distance × aperture
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Both are equal. They are just arranged differently.


Steve.
 
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Steve Smith said:
In the guide number. The number is usually stated for ISO 100 and is now in metres but could be in feet for an older flash

So at ISO 100 with a distance of 8 metres for example, look up the guide number and divide that by the distance.

So if the guide number is 60, aperture = 60/8 = 7.5 - f8 is close enough.

Steve.
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So if Johnny wants to shoot at f4 from 8m (flash) away - how does he work out what MANUAL POWER setting to use on his flashgun?

Is it f4 x 8m = GN 32 so therefore Johnny needs to dial in 1/2 power (as it is nearest setting for a GN60 flashgun) to obtain a reasonable exposure?


here is a little canon calculator but I am sure it works for any gn .... LINKY
 
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If you have f8 as a setting but you now want to use f4, you know that f4 lets in two stops more light than f8 so the flash must be made to put out two stops less. i.e. quarter of its full power.

Inverse square law doesn't come into it as the distance stays the same.


Steve.
 
wuyanxu said:
May I ask how to calculate required flash power based on guide number?
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I'd have linked to a chart - but you can see it from the above. That's what guide numbers are for.

The caveat (there's always one) being the zoom setting. You'll notice that advertised GN's are always measured at full zoom, the narrower the light pattern the brighter. Different flashguns will vary a little in power with zoom based on the efficiency of the reflector.

The 580ex has a GN of 58(metres) at 105mm but only 42(metres) at 50mm.
 
jonneymendoza said:
Is this a maths thread? :) hehe
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You asked a question to which an appropriate answer is 'do the maths':D

If the maths are too much and trial and error is a bit too random, you're left with buying a flashmeter;)
 
treeman said:
As Phil has said above.

Personally I use a meter as I find it more reliable than the screen on the back of the camera.
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I also totally rely on the meter specially during the early mornings. Sometimes your natural eyes cannot really judge the light.
 
Many thanks all for the explanation on how to use the flash guide number. I have always wondered how they are used.

TP forum at its finest. :)
 
Phil V said:
I'd have linked to a chart - but you can see it from the above. That's what guide numbers are for.

The caveat (there's always one) being the zoom setting. You'll notice that advertised GN's are always measured at full zoom, the narrower the light pattern the brighter. Different flashguns will vary a little in power with zoom based on the efficiency of the reflector.

The 580ex has a GN of 58(metres) at 105mm but only 42(metres) at 50mm.
Click to expand...

There's another caveat too, and that's guide numbers assume the shot is being taken in a 'normal' room (whatever that is, never specified) and a lot of light is reflected back from surroundings, boosting the exposure. Used outdoors, rule of thumb is add an extra half to one stop. Also, some manufacturers' standard room appears to be a small white-walled toilet, as they often over-estimate.

Basically guide numbers are not good for accurate exposure calcs. Try using them to estimate bounce-flash exposures, or for fill-in flash :thinking: But back in the day, that's all we had :eek:
 
HoppyUK said:
There's another caveat too, and that's guide numbers assume the shot is being taken in a 'normal' room (whatever that is, never specified) and a lot of light is reflected back from surroundings, boosting the exposure. Used outdoors, rule of thumb is add an extra half to one stop. Also, some manufacturers' standard room appears to be a small white-walled toilet, as they often over-estimate.

Basically guide numbers are not good for accurate exposure calcs. Try using them to estimate bounce-flash exposures, or for fill-in flash :thinking: But back in the day, that's all we had :eek:
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I always had auto guns, with multiple power levels and a leaf shutter, balancing flash with ambient was easier than I find it using ettl.
 
dave_bass5 said:
Can i ask if anyone can recommend a cheap light meter, say £50-ish, or is it worth paying more for something like the Sekonics?

For me it would only be for occasional use, more something to try out really.

Cheers.
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I just bought the sekonic L308s for £90 second hand from ebay, but you can get the earlier models l308 l308B for a around your budget. I believe there was a couple went last week on here for around that

Works well and so easy to use
 
SpikeK6 said:
I just bought the sekonic L308s for £90 second hand from ebay, but you can get the earlier models l308 l308B for a around your budget. I believe there was a couple went last week on here for around that

Works well and so easy to use
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Thanks for that. I hadn't thougt about a secondhand one, ill keep my eyes peeled for one.
 
JonathanRyan said:
Did they teach you nothing at school? :D

a = bc
=> a/b = c
=> a/c = b



Flash to subject. Camera distance has no relevance here.
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Still trying to get my head around this. On my 430ex2, the power settings goes from 1/64, 1/32 .....1/1 being the highest.

So my question is, how do i get the relivent figure about with your formulae?

say i know wroughly the distance between flash and subject, say 10m to keep it simple and apparture is f10. what is the formulae?

10/10(distance / apparture) = 1? and 1 being 1/1 to dial in my flash?

say if its f 10/5 = 0.5 im guessing 1/50 on my flash settings?

Also: just found out that some flashes have a build in light meter!

[YOUTUBE]qNkrFkclnC8[/YOUTUBE]
 
Your 430ex flashgun has a GN of 43 at 105mm, I'll bet though that you're using it at 50mm (off camera) so that's 34m (100 iso) (according to the chart in the manual)

GN = distance × f-number

So 34 = 10 x 3.4 (f 3.4 at full power at 10m)

You're more likely to be at 2 metres though :)

So 34 = 2 x 17 (f16 at full power at 2m)

1/2 power would be f8

1/4 power f5.6

1/8 power f4

1/16 power f2.8

1/32 power f2

1/64 power f1.4

Take on board the light loss from the room, and also any modifiers (assume a stop at least) but the above should get you 'in the ball park' from there you can adjust to taste.
Also remember that upping the ISO a stop or 2 helps greatly with recycle times.
 
Your 430ex has no 'Auto' setting (as per that video) it's either ETTL or Manual (as far as I can work out)

With the flash set on Manual mode it'll tell you the flash power distance on the LCD panel if it's camera mounted, not sure how this works off camera without ETTL triggers though, but you could certainly have a play with it camera mounted and see how the distance changes with changing the aperture / ISO.
 
Phil V said:
Your 430ex flashgun has a GN of 43 at 105mm, I'll bet though that you're using it at 50mm (off camera) so that's 34m (100 iso) (according to the chart in the manual)

GN = distance × f-number

So 34 = 10 x 3.4 (f 3.4 at full power at 10m)

You're more likely to be at 2 metres though :)

So 34 = 2 x 17 (f16 at full power at 2m)

1/2 power would be f8

1/4 power f5.6

1/8 power f4

1/16 power f2.8

1/32 power f2

1/64 power f1.4

Take on board the light loss from the room, and also any modifiers (assume a stop at least) but the above should get you 'in the ball park' from there you can adjust to taste.
Also remember that upping the ISO a stop or 2 helps greatly with recycle times.
Click to expand...

Fantastic advice Phil, thanks Mandy
 
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