Gels and diffusers

[BW man]

Does using a gel force one to use a bare flash? It seems that all typical diffusers, softboxes, umbrellas etc., return the light back to white or almost white. Are gels and diffusers mutually exclusive? Thanks in advance.

 

[Nionyn]

Does using a gel force one to use a bare flash?

No. Not at all.

It seems that all typical diffusers, softboxes, umbrellas etc., return the light back to white or almost white.

What makes you think this? Try it for yourself - you'll see.
White diffusing and reflecting materials look white if they receive white light - that is, light that contains all wavelengths/colours in roughly equal quantity.
If they receive coloured light then that is what they reflect or let through.
Try shining a coloured light at a white diffuser or umbrella - or even at a piece of white paper. It does not look white any longer, but the colour of the light shining on it.

Coloured gels are colour filters. They absorb/remove light of particular wavelengths/colours.
When this filtered light passes through a white diffuser (whether it is an umbrella, on a softbox or just a plain diffuser on its own), that diffuser does not put back all the wavelengths of light that were previously removed by the colour filter. It cannot - it has no way of 'knowing' what colours were originally present. It simply lets through what is now there.

Are gels and diffusers mutually exclusive?

Not at all. I use gels with diffusers, softboxes, umbrellas, snoots and grids. They work just fine.

Thanks in advance.

You're welcome.

 

[LincolnB]

Does using a gel force one to use a bare flash? It seems that all typical diffusers, softboxes, umbrellas etc., return the light back to white or almost white. Are gels and diffusers mutually exclusive? Thanks in advance.

If you shine a green light through a white diffuser, that surface will project green light. If you bounce a blue light off of a white surface, that surface will reflect a blue light. White surfaces are passive and cannot create light; they can only reflect the light that is available in the proportion of colors that are available.

 

[BW man]

I was trying to do a "tungsten blue" scene, with a CTO gel and tungsten WB, but the flash-lit part kept coming out too cool. I guess I asked the question in frustration, it was patently silly. Something else is at play which I will have to figure out. Thank you!

 

[Nionyn]

I was trying to do a "tungsten blue" scene, with a CTO gel and tungsten WB, but the flash-lit part kept coming out too cool. I guess I asked the question in frustration, it was patently silly. Something else is at play which I will have to figure out. Thank you!

Heh, we've all asked questions that turned out to be "silly" once we knew a little more. In fact you're far from the first person to have asked your gel/diffuser question, and you can be sure you won't be the last. Don't worry about it.

In the situation you describe (WB set to tungsten, flash gelled CTO) it sounds as though what your camera describes as Tungsten WB is somewhat different from the colour of your flash with a CTO gel on it. This certainly can happen.
One possible explanation might be the power set on your flash. The colour will vary a little from full to minimum power, so changing the power on your flash could well visibly affect the final result.

Was your CTO gel full CTO?
Half and quarter CTO are also available, and if you were using one of those one would definitely expect the result you describe.
If you were in fact using full CTO but also have some half or quarter CTO gels you could add a piece of one of those to your full CTO - this would warm the flash's colour further.

Another solution would be to stick with whatever flavour of CTO you were using and set a custom white balance for that on your camera (how best to do this is likely to depend on your camera - I have often used a piece of white paper for this in the past, these days I use a neutral grey card). This would ensure that the camera 'saw' your gelled flash as 'white', and would effectively see/shift colours in the rest of the scene relative to that notional 'white'.

Hope this helps.

 

[BW man]

It surely does help.

I didn't realize that flash power affects color. Which way? The more power the more saturated or the other way around?

 

[Nionyn]

BW man wrote:

It surely does help.

Oh good!

I didn't realize that flash power affects color. Which way? The more power the more saturated or the other way around?

It's not a matter of more or less saturated, it's a matter of more or less cool/warm.

The effect is due to the duration of the flash (I'm assuming you're using hotshoe flashes here (as opposed to monolights or a pack & head system).

With hotshoe flashes a full power flash is basically a (relatively) long duration flash. As you reduce the power of the flash the duration of the flash of light is curtailed.

As I remember it (and it's quite possible that I'm remembering incorrectly) the initial part of the light output tends to be bluer, and the later part tends to be warmer in colour.
Hence, the higher the power of the flash's output (and the longer the flash's duration), the 'warmer' the light will appear to be.

It's 4am here (UK) and I've had a small gin (maybe two), so it's quite possible that I have the above the wrong way round. If so, please forgive me - but I'm sure that someone will correct my gaffe before long.

In any case, the effect I've described is relatively slight, and is not something that I've personally found to be a major problem (though it is definitely noticeable once you know about it and look for it).
It could be the source of your colour problem, but it might not be.

I'd do a couple of quick tests - then you will know from practical experience.
Set your flash to full power and your camera to a fixed white balance (maybe 'flash', 'daylight' or 'cloudy'). Point your flash at something of neutral colour such as a white wall.
Set your camera to manual, minimum ISO, and adjust the aperture so that it gives you a correct exposure at its sync speed (1/200 would probably be fine) and ensure that the ambient light is negligible and does not contribute to the exposure.
Take a photo.
Then reduce the flash's power to maybe 1/16 or 1/32 (four or five stops), open the camera's aperture by the same amount (to again give you a correct exposure) and take another photo. Again, ensure that the ambient contribution is negligible (it is likely to be greater this time).

Comparing the two photos should show you the difference between the colour of the flash's output at full power and whatever power you set it to for the second shot.
You might find that the colour difference is not particularly significant. If this is the case, you will need to look elsewhere for the unexpectedly cool flash colour in the photo you mentioned previously.

I must get to bed before I start typing complete nonsense!
I really hope the above made sense and that it is useful to you. If I'm already typing nonsense (always a possibility) I hope that someone else will chime in and correct me.

Have fun!

 

[BW man]

Thank you Nionyn, this is really fun stuff. And informative. I would never think that a few milliseconds difference in flash duration could have any noticeable effect on anything except exposure. Separately, a shot of gin sounds like a good idea too (my favorite is Bombay Sapphire).

 

[Klaus dk]

You described it very well and easily understandable.

For the record, this was much worse in the film days, because both sensitivity and colour balance of film changed with exposure time. Long exposure times need to be even longer, and short exposures were (as I recall) bluer than longer exposures.