Off Camera Flash for dramatic skies/portraits

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steve
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#1
I am very new to the concept of OFF Camera Flash, and would like some advice on how to set up for a portrait with a dramatic sky.

I get the bit about taking an ambient reading of the background so in this case the cloudy sky, so lets say ISO 200, F16 @ 1/160TH.

I then place OCF to the left of the subject to light it.

Then should I take a light meter reading from the face towards the added light source? based on the above what should I be aiming to achieve on the light meter. ( I understand shutter sync speed)

thanks in advance
 
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Steven
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#2
First, I probably wouldn't use f/16 unless I had to. Secondly, you may very well want to use a higher ISO in order to make your flash "more powerful" (countered w/ higher x-sync). And thirdly, you do not want the exposure to change, so there is no point in re-metering.

If you use TTL sync then the flash will set it's power based on it's own/secondary metering (mine tend to meter a bit strong typically, so I start w/ -1 FEC usually).
If you know your lights and the inverse square law, and you are using manual flash/exposure; then you can use a secondary metering of the subject to give you a starting point for flash power. I.e. if you know your light adds 2stops from 10ft at 1/2 power, then you know that from 15ft it would need to be at full power for 2stops, you know it will need to be at 1/8 power for 2stops from 5 ft, and you know it needs to be at 1/4 power for 1stop at 10ft.

Edit: if you do not know your lights and you are using manual, then you can use a hand held incident flash meter to give you the starting point for flash power... I don't use them...

(And yes, I know it would actually be more like 14ft for 2 stops @ full power)
 
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Phil
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#3
First things first.
To create a 'moody' sky means under exposing it.
So at midday you're looking at f22 at flash sync speeds (ish). That takes a lot of lighting power. 5 or 6 flash guns, a couple of AD 360's or a 600Ws portable studio light.

Move to late on the day and the light you need is more manageable. You can balance a nice sunset with a decent speed light in a softbox.

You can use TTL for the flash, you can use a flash meter, or you can set half power and check your image and adjust to taste.
 
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riu
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steve
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#4
First things first.
To create a 'moody' sky means under exposing it.
So at midday you're looking at f22 at flash sync speeds (ish). That takes a lot of lighting power. 5 or 6 flash guns, a couple of AD 360's or a 600Ws portable studio light.

Move to late on the day and the light you need is more manageable. You can balance a nice sunset with a decent speed light in a softbox.

You can use TTL for the flash, you can use a flash meter, or you can set half power and check your image and adjust to taste.
thanks for the info, will be using manual flash, but will try the latter two methods. If I want the ambient to remain as metered, but want more light on the subject, are my options, more power, flash closer to subject or open Aperature up?
regards
Steve
 
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Garry Edwards
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#5
Steve, if you want the ambient to remain as metered but want more light on the subject, then your subject will be overexposed by the amount of the difference between the 'extra' light on the subject and the ambient light on the background...

Changing the aperture WON'Tt help, as it will affect the ambient and the flash equally.
Changing the shutter speed WILL help as, within reason, a faster shutter speed won't affect the flash but will affect the contribution made by the ambient. But unless you use a tail end sync trigger, you will be limited to the native sync speed of your camera, typically 1/200th - 1/250th.
Having the flash closer to the subject WILL help, because the amount of light from the flash follows the Inverse square law, which means that if you get it twice as close you'll have 4x the effective power. In theory that applies to the ambient light too, but in practice if the light is coming from 93 million miles away, another couple of feet makes no difference.
Having more power WILL help, because in bright sunlight, hotshoe flashes are overwhelmed by the ambient light.
 
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Richard
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#6
thanks for the info, will be using manual flash, but will try the latter two methods. If I want the ambient to remain as metered, but want more light on the subject, are my options, more power, flash closer to subject or open Aperature up?
regards
Steve
For more light on the subject, the options are: raise the shutter speed as high as possible up to max x-sync, or increase flash power by either turning the flash up, moving it closer, or fitting a more efficient modifier. Changing aperture or ISO effects both ambient and flash exposure equally, ie it doesn't affect the flash/daylight ratio.

The main problem with flash outdoors, when you want to over-power and darken the ambient, is that bright sun is very bright. You need something above 400Ws-ish bare minimum (say 4x speedlites) and preferably a couple of thousand Ws. You could then find yourself in the slightly contradictory situation of needing an ND filter to pull the shutter speed down to x-sync level. As Phil says, avoid bright sun - it makes things 100x easier.

BTW, I wouldn't bother with a light meter. The right flash/daylight ratio is both variable and very subjective and the best way to get that is to eyeball it off the camera's LCD. Just make sure you can see that properly (ie a shady spot) and also enable blinkies and use the histogram to check overall exposure.

Edit: crossed post with Garry.
 
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riu
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steve
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#7
Thanks everyone, I think the "flash" penny as dropped. Now out to play and test the theory . Once again thanks.
 
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Peter
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#8
Don't put the sun in the shot if you don't have the power, hide it with a tree / building to dull its power, or wait for a cloud...lots you can do to make simple speed lights work, but you might find yourself removing them in post :)
 
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