Colour Gel, how?

[jingphotography]

What light ratio you will go with colour gel and key light?
I have had a quick go tonight:
white background (sorry no grey available)
Straight front keylight through 1.5m Octagon box
model to background : key light to model = 2.5m : 1.8m
it didn't work, I think there is light spill to background.
Will try lighting from sides and clamshell light tomorrow.

Would Beauty Dish work? with honeycomb?

What is the 'correct' f-stop for a colour background? Thanks

 

[Garry Edwards]

A white background will work but will only produce pastel colours, not saturated ones, unless you have a lot of distance between subject and background.

No one light shaper will work any better or worse than any other, or at least not to any appreciable extent. What really makes a difference is how far the light has to travel to reach the subject and how much further it has to travel to reach the background - if I'm reading this correctly, your key light is travelling 1.8m before reaching the subject and a further 2.5m before reaching the background. So, if you are using say f/11 to expose the model correctly the fall of of light means that the background is still getting roughly f/4 - not bad but if you were to move the keylight to half that distance then the background would be getting far less light, which would completely solve your problem of light spill onto the background.

 

[jingphotography]

Garry
based on this

Ekm(key light to model) : Ekb(key light to background) =1:6

How did you work out F4?

I used a snoot fitted wit colour gel to create the coloured background. Shall I aim to get the same light reading as the key light will case on the background?

 

[Garry Edwards]

My rough estimate is based on:
Ekm = 1.8m
Ekb = 4.7m

Following the formulae of the Inverse Square Law (which in fact doesn't apply in strictly linear terms but is a good starting point) once the light has travelled twice the light to front subject distance of 1.8 m it has lost 2 stops of power, so at a distance of 3.6m the f/11 exposure on the front subject has become f/5.6. It then has to travel a further 1.1m, making a total extra loss of light energy of about another stop, making it about f/4

It's all 'about and approx' simply because the falloff of light from any type of light used in the studio doesn't follow the ISL in linear terms as I mentioned; the ISQ only refers to a point source of energy and your 1.5m octa softbox is a long way removed from a point source...

But forget about the theory, or at least don't let it dominate. Simply move your key light closer to the front subject and the problem falls away just as quickly as the light reaching the back subject