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FORUMS General Gear Talk Flash and Studio Lighting 
Thread started 10 Sep 2017 (Sunday) 10:59
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Basic gel for flash questions

 
bellbound
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Sep 10, 2017 10:59 |  #1

I'm just getting into flash photography and have a few basic questions.

When shooting photos indoors with yellowish/orange indoor lighting, should I use CTO gels on a strobe? Guessing the gels will not match the lighting. If not, how do you warm up a person's face? Is this something better done in post?

When shooting photos of someone against soft natural window light, should I use CTO gels on a strobe to warm them up?

Is there a way to look at a gel and determine its strength?

Thanks for any answers.




  
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Wilt
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Sep 10, 2017 11:17 |  #2

"It depends!" upon what you are trying to achieve!...

  • If you are wanting to make your added light (flash) to be just as warm in color temperature like the ambient incandescent illumination (which is about 2900K in color temp) you use a full CTO over your flash head.
  • If you are wanting to warm up your added light (flash) to be just a bit warmer than the cool usual flash (about 5600K) for good skin tones, you add a 1/4CTO over your flash head.
  • ...or something in between, if that is what you want

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Sep 10, 2017 11:21 |  #3

bellbound wrote in post #18448568 (external link)
When shooting photos of someone against soft natural window light, should I use CTO gels on a strobe to warm them up?

probably not.

might even use a 1/4 CTB depending on the light.

the other possibility is that you just over power the ambient so it doesn't matter


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Sep 10, 2017 12:07 |  #4

The CTO and CTS series gels have the both have the same amount of color temprature correction power (e.g. a 1/2 CTO and 1/2 CTS gel will both convert 5000K to approximately 3800K), so they will both correct for Tungsten lighting casts equally well. The only real difference between them is that the CTO has a bit more red, and CTS has a bit more yellow...

From a portrait photographer's point of view the practical usability difference is that CTO will tend to give subjects a bit of a tanned look (depending on the strength of the CTO gel), whereas a CTS will not. My personal 'rule of thumb' is to use CTO on pale subjects who I want to add a bit of 'healthy glow' to, and CTS on folks who already have a tan, and don't need any additional help in that department. :-

The Full CTS will bring your flash WB down to around 2900K, more or less neutralising the look of the Tungsten light. The 1/2 CTS will bring your flash WB to around 3800K, which will leave your backgrounds still with a touch of the warmth of Tungsten lighting.


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Alveric
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Post edited 9 months ago by Alveric.
     
Sep 10, 2017 13:45 |  #5

bellbound wrote in post #18448568 (external link)
I'm just getting into flash photography and have a few basic questions.

When shooting photos indoors with yellowish/orange indoor lighting, should I use CTO gels on a strobe? Guessing the gels will not match the lighting. If not, how do you warm up a person's face? Is this something better done in post?

When shooting photos of someone against soft natural window light, should I use CTO gels on a strobe to warm them up?

Is there a way to look at a gel and determine its strength?

Thanks for any answers.

Yes, you should use gels on the strobe if you're going for an accurate and neutral colour rendition. Strobes are manufactured to have noon daylight colour temperature (~5500K, although Canon's are more like ~5200K, which is even bluer), and flashguns will vary their temperature with their power setting. Unless you're a Photoshop maestro or a masochist, you don't want to be going through colour matching hell in post; so, better that you get it right in camera.

Quality gels tell you their strength already. I'm looking here at a Full CTO from Rogue and it says 'Full CTO (6500K to 3200K) f-stop loss = 1'. You can also run a test with different gels on the flash and chimp or import the files into your RAW processor and see if the subjects/area litten by the flash matches the areas illuminated by the ambient only.

Gels can also be used for effects. So yes, you can shoot a subject against soft natural window light and use a CTO on the strobe. Then shoot a grey card. Then, when you bring the photos into your RAW processor, you can use the pipette on the grey card to get the neutral white balance, so that the subject looks natural but the ambient looks bluer. It's done in cinema all the time.

That's kind of what I did here:

Original image. Yes, the subject looks very orange.


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Final image after neutralising the main light's WB, with the subject's skin tone rendered accurately and the background given a blue colour.


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bellbound
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Sep 10, 2017 13:45 |  #6

How does incandescent WB correction in camera or in Lightroom affect the light quality of 1/4 CTO on the flash head? OR does it not affect it?

I'm going to get some of the small booklets from both LEE and Rosco and take the booklets apart and apply some of the individual gels on the flash head with a rubber band or some other way. Should I use the included translucent flash dome head on the flash head with the gel inside the dome head, or should I just cover the head with a gel and rubber band, or does it not matter as long as the gel covers the entire head?

Thanks!




  
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Alveric
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Sep 10, 2017 13:50 |  #7

bellbound wrote in post #18448693 (external link)
How does incandescent WB correction in camera or in Lightroom affect the light quality of 1/4 CTO on the flash head? OR does it not affect it?

I'm going to get some of the small booklets from both LEE and Rosco and take the booklets apart and apply some of the individual gels on the flash head with a rubber band or some other way. Should I use the included translucent flash dome head on the flash head with the gel inside the dome head, or should I just cover the head with a gel and rubber band, or does it not matter as long as the gel covers the entire head?

Thanks!

If you use the Incandescent setting in LR it will apply a predefined value to the photo. LR won't analise the photo to see how many Kelvins of gelling you applied, it will simply set the WB value to its own preset.

It's best if you use a grey card to tell LR what the actual Kelvin value is.

I've used the gel inside the dome, but I'd advise covering the dome with the gel and a rubber band.


'The success of the second-rate is deplorable in itself; but it is more deplorable in that it very often obscures the genuine masterpiece. If the crowd runs after the false, it must neglect the true.' —Arthur Machen
Why 'The Histogram' Sux (external link)

  
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Wilt
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Post edited 9 months ago by Wilt. (3 edits in all)
     
Sep 10, 2017 15:26 |  #8

bellbound wrote in post #18448693 (external link)
How does incandescent WB correction in camera or in Lightroom affect the light quality of 1/4 CTO on the flash head? OR does it not affect it?

I'm going to get some of the small booklets from both LEE and Rosco and take the booklets apart and apply some of the individual gels on the flash head with a rubber band or some other way. Should I use the included translucent flash dome head on the flash head with the gel inside the dome head, or should I just cover the head with a gel and rubber band, or does it not matter as long as the gel covers the entire head?

Thanks!


'Correction' simply makes a photo so that anything recorded at a specific color temperature (.e.g. at 2900K) is the 'neutral WB' setting for anything within that photo. So if your added flash = ambient temperature, 'neutral gray' appears neutral in that light everywhere within the photo.

  • If you added (flash) light is cooler than ambient incandescent (e'g. using 1/4CTO), then setting color temp to 2900K makes anything in ambient light look neutral while anything illuminated by the flash looks 'cooler'
  • If you added (flash) light is same as ambient incandescent (e.g. using full CTO), then setting color temp to 2900K makes anything in ambient light look neutral while anything illuminated by the flash looks 'neutral' too
  • If you added (flash) light is cooler than ambient incandescent (e'g. using 1/4CTO), then setting color temp to 4500K makes anything in ambient light look rather warm while anything illuminated by the flash looks 'not as warm'
  • If you added (flash) light is same as ambient incandescent (e.g. using full CTO), then setting color temp to 4500K makes anything in ambient light look rather warm while anything illuminated by the flash looks 'the same as ambient'


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Basic gel for flash questions
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