CTO gel and WB calculation

Discussion in 'Lighting Forum' started by spacecreature, Nov 28, 2015.

  1. spacecreature

    spacecreature Mu-43 Regular

    52
    Dec 7, 2012
    Hey guys,
    I have a question regarding White Balance calculations. I am using a Yongnuo 560 speedlight with Selens correction gels.

    On the notes provided with the gels, it reads:
    Full CTO (6500K to 3200K)
    1/2 CTO (6500K to 3800K)
    1/4 CTO (6500K to 4600K)

    Since my flash's output is at 5600K and not 6500K, I am wondering how to do the conversion. Do I simply deduct by the same amount, or does it not work that way? I mean would I be right to assume the following?
    Full CTO (5600K to 2300K)
    1/2 CTO (5600K to 2900K)
    1/4 CTO (5600K to 3700K)

    In practice, it seems off though. So would like to make sure regarding the conversion method. Thanks in advance:)
     
  2. barry13

    barry13 Super Moderator; Photon Wrangler

    Mar 7, 2014
    Southern California
    Barry
    Hi, are you using the gels to match the ambient lighting?
    If so, the camera WB should be set to match the lighting as well.

    Barry
     
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  3. MichaelSewell

    MichaelSewell Mu-43 Top Veteran

    511
    Sep 1, 2015
    Burnley, UK
    Michael A. Sewell
    The rule of thumb is to have your WB set to the dominant light source within the scene.
    If your scene is mostly lit by ambient, with a touch of flash (an example here would be a strong sunlit day with additional flash to lift the shadow), then the white balance would be set to the ambient.
    If, on the other hand, you are overcoming the ambient with flash, then set your WB to flash.
    If you are using gels to match the Current lighting of the scene, then match your gels and WB accordingly. Don't worry about the Math, just see if it sits right on the screen. This is what @barry13@barry13 was referring to above.

    The time to disregard this is when you are creatively using your white balance to influence the image.
    An example would be setting your WB to fluorescent to photograph a bride and groom during the sunset. You would also gel your lights "window green" to match your WB setting. This would cause the colours of the sunset to shift a little in the spectrum, and reveal that beauitiful magenta between the blues and oranges which are now a little darker in tone. I have examples if you want.
     
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  4. Clint

    Clint Mu-43 All-Pro

    Apr 22, 2013
    San Diego area, CA
    Clint
    Not all gels are created equal so the light they block will vary -
    For Roscoe Cinegels approximately
    CTO - 1.1
    1/2 - .5
    1/4 - .3
    you can also increase or decrees the gel effect by increasing or decreasing flash power. I use these values as a starting point.
     
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  5. MichaelSewell

    MichaelSewell Mu-43 Top Veteran

    511
    Sep 1, 2015
    Burnley, UK
    Michael A. Sewell
    And by purchasing good quality gels such as Roscoe, as Clint has done, consistency is much better.

    Cheap gels are a good entry point for getting your feet wet, but when you want to get predictable results and consistency, the better known brands deliver.
     
  6. spacecreature

    spacecreature Mu-43 Regular

    52
    Dec 7, 2012
    Thanks for your replies, but I shoot on all manual and I like to set the WB myself and not let the camera decide and go crazy with each different shot giving a different value. Let's say I am shooting at an event with a speedlight, most of the time the available light is a mix but leaning towards incandescent yellow (most hotels and halls have a huge mix, it's just crazy! ). I want a bit of ambient in the photos (for the location to show through but also maybe to rimlight my subjects) but my subjects will be lit with my flash mostly. If I use full CTO and set my WB to incandescent I never get a descent result, it looks weird so I know that's a wrong setting. Since most gels are not all created equal like Clint said, I want to use the info provided in the gels package to achieve a better result. The info is as stated above.

    I would just like to know, if when it is stated that a CTO gel takes you from 6500K to 3200K, would that also mean it would takes you say from 5600K to 2300K??? Or is that not how it works? I could do a custom white balance on grey card, but I prefer to have the actual K value so that I could go slightly warmer or colder if I wanted instead of getting a spot on WB . I tried doing a custom white balance and then checking the K value in camera raw, but the sliders there are split into temperature and tint so not sure if the temperature alone would give the related k value??
     
  7. Klorenzo

    Klorenzo Mu-43 All-Pro

    Mar 10, 2014
    Lorenzo
    I'm quite sure the light temperature scale is anything but linear so no, you can not do simple arithmetic on those values.
    Then I suspect that even finding a tool for the correct calculation (I found none online) the tint could still be slightly wrong.

    Maybe you can find a good enough value by trial: shoot a white balance calibration sheet with the flash light only with different WB K values until you find one that gives you a pure neutral colour in the final image. Does it make sense?
     
  8. spacecreature

    spacecreature Mu-43 Regular

    52
    Dec 7, 2012
    I think you are right regarding temperature calculation , I tried googling for answers but it was hard formulating the search query :) that's why I am asking here. I think I will just do a custom white balance using like half CTO, then gel my speedlights with a full CTO or maybe just put 1/4 extra over the 1/2 to get a warmer light on my subjects. Ideally I would need a Sekonic color meter to test effect of each gel on the speedlight but those are quite expensive :(
     
  9. MichaelSewell

    MichaelSewell Mu-43 Top Veteran

    511
    Sep 1, 2015
    Burnley, UK
    Michael A. Sewell
    TBH, I'm not aware of anyone suggesting anything other than manually manipulating the whitebalnce.
    You're quite right in the assumption that the camera can only assess whatn's seen through the lens prior to the exposure, and unless you are talking iTTL or eTTL etc, the camera has no idea the scene is about to be flash lit.
     
  10. JBoot

    JBoot Mu-43 Regular

    106
    Dec 4, 2012
    Scotch Plains, NJ
    Jerry
    I use an expo disk to custom white balance and shoot a test shot or two with each setting in each room with a colorchecker passport pro. This gets me close and allows me to recustom my white balance in Lightroom very quickly across groups of pics if I feel it's necessary.

    For most events I'm in two or three rooms and often the main room might have lower lights for party time and the house lights up a bit for ceremonies. So I save these before the event and test the flash in each room with the expected lighting. For party lighting flash is dominant.
     
  11. JBoot

    JBoot Mu-43 Regular

    106
    Dec 4, 2012
    Scotch Plains, NJ
    Jerry
    ... Apologies... And I gel accordingly. I buy the rosco cinegel sheets.
     
  12. Clint

    Clint Mu-43 All-Pro

    Apr 22, 2013
    San Diego area, CA
    Clint
    I believe you would be correct about a CTO taking values from 6500K to 3200K and 5600K to 2300K. Which would be great IF, you actually new what the light temperatures are.

    Which brings up two other points, the first which you mentioned is the crazy light. The second is that tungsten and fluorescents vary across such a wide range of the temperature spectrum that I don't know how one can go by the numbers except as beginning point. Here is how I approach it -

    Depending on the event and the lighting it is not uncommon for me to shoot at 1/30 to get the ambient and keep my ISO down. I'll start with a 1/4 cut Straw (I don't like CTO) and move up to a full cut of Straw to something that produces a decent result. Seldom do I use a full cut. Same scenario where florescent is dominant.

    One hotel/convention center I've been shooting in for two years their 25 foot ceilings have a mixture of mercury vapor, florescent, and tungsten. I don't even bother with any gels and have the camera on auto white balance - it does better than I can. I think trying to match color temperature in uncontrolled environments is more of an art than science.

    However having consistent color temperatures in my gels is necessity cause I need a stable/known factor to start with, and that is the gel material.
     
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  13. spacecreature

    spacecreature Mu-43 Regular

    52
    Dec 7, 2012
    I think it's probably the CTO rendering itself I am not liking even when WB seems correct; I should get me some CTS to experiment. I think I only once heard of that one but never gave it another thought. I only have CTO. Thank you Clint for reminding me about that , need to check it out!

    Regarding shutter speed, yes I do go down that much even slower if I have to but sometimes its not possible when there is dancing or movement and the light is not all coming from the flash :( And not sure if it's just my OMD or if they are all like that but by 800 I am already playing with fire!!! Hope to see better m43 cameras soon as I have been delaying my backup camera purchase for too long now.


    Thanks again everyone for taking the time to reply!