CTO Gel experiment

[kennysarmy]

Setup was:
580EXII on camera as master firing to camera left and up
580EX off camera as slave with gel firing up to bounce off ceiling with home made catchligh card in place
Ratio of lights was 1:4 in favour of the off-camera flash.
Photo's with CTO's in place needed some boost in exposure in DPP to bring up to the same light levels for shadows.
1/60th @ f5.6 (ISO 100)
Canon 40d
Shot in RAW


No CTO Gel on 580EX

1/4 CTO Gel on 580EX

1/2 CTO Gel on 580EX

Full CTO Gel on 580EX

Now I can see that as the Gel got 'warmer' it did it's job - what I do not understand is why I was able to alter each photo in DPP using the white-balance slider so they all looked the same....to me anyway.


I thought the point of the gel was to match the colour of light coming off the flashes with that generated from the lamp I was sat under.

To me the ungel'd photo looks better....

 

[kennysarmy]

Was it that my exposure did nt let enough of the ambient light in?

 

[Calzor Suzay]

Take a shot at the same settings without the flash and it'll probably be a black picture if so then yes your using the gel for no reason but have a shot with a lamp directly in the shot and it might be a different matter

 

[kennysarmy]

Will try again over the weekend...

 

[tdodd]

There are two issues here....

1. At 1/60, f/5.6, 100 ISO you will be picking up b****r all ambient light;
2. By bouncing the flash (two of them!) in what I imagine to be a confined space you have filled that space with bounced light, blending flash and ambient together throughout the room, tainting the light a little with the colours of the ceiling/walls etc., if they are not neutral..

For the test to be effective try bringing up the ambient exposure a lot - to maybe 1/60, f/2.8, 1600 ISO - and then fire one flash straight at the subject, setting WB to tungsten. Keep a good distance between the subject and the background to make sure the background is only getting lit significantly by ambient light. Then you'll see the value of CTO gel to balance flash with tungsten.

Here's an example where failure to gel the flash has led to an impossible mix of colour temperatires in the lighting, leading to a blue subject if the WB is set for ambient, or an extremely red/yellow background if the WB is set for the flash.....

A full CTO gel might have been too much, requiring the ambient warmth to be destroyed in order to achieve good flash tones, but a 1/2 CTO or 3/4 CTO would have closed the gap and improved the appearance, I think.

 

[kennysarmy]

Thanks for your post Tim