Bi-WEEKLY LIGHTING THEME: Exposing for Color Gels

Discussion in 'Lighting Equipment' started by brooks short, Nov 28, 2004.

  1. Recently in this Lighting Forum there was a post about exposing for color gels. There were some suggestions about using an incident or reflected light meter to expose for correct color and density. The idea being that if you use the indicated meter exposure when measuring just the light from the gel, what you see will be what you get. That's not often the case. Depending on the density and color of the gel, the exposure often has to be adjusted and sometimes the light from the gel actually changes color as the exposure is increased or decreased. For Example: If you want to create a dark blue glow or background color, even if you start with a dark blue gel, and you use the indicated meter exposure the blue will be too light. The light meter will place the blue as a mid-tone when you want a darker, richer blue that's really more of a shadow tone. Or perhaps you want a bright warm glow or background color so you use a light yellow or orange gel. Again, the indicated meter exposure will place that gold color as a mid-tone which is not light enough to create the bright warm effect you or the client wants. In fact the metered mid-tone exposure setting for both the blue and yellow gels will be different camera settings but they will produce the exact same density and tone. One will be blue and the other will be yellow but they will both be the same mid-tone density. Take a look at this chart of gel exposures (see image below). In the studio I lit a black seamless paper with no gel, deep red, medium purple, deep green and a deep blue gel and bracketed expposures in one stop increments. Along the bottom of the chart columns you'll see an exposure value for the exposure brackets. N is the indicated exposure from an incident meter held at the black paper and pointed back towards the lens. N-1 is one stop darker than indicated. N+1 is one stop lighter than indicated and so on. All brackets were done by f-stops and the brackets were exposed by strobe and captured digitally. The top row of brackets was shot without any color gels to have a neutral reference and show exposure changes only. The Red gel is best represented by the N-1 exposure. The Purple gel looks like the N exposure. The Green gel really looks like the N-1 exposure. The Blue gel is quite dark and looks most like the N-2 exposure. There's another effect that's visible here. These shots were done with a digital camera but I've noticed this very same effect when shooting transparency film. As the exposure increases, the color of some gels also changes. The red gel turns to orange at the N+2 exposure and becomes even more straw-like at N+3. The purple gel shifts to magenta at N+1 as it's exposure becomes brighter. Surprisingly the green gel stays consistently green though it does become noticable yellow at N+3 exposure. And the blue gel has a mind of it's own. At N+1 there's a shift to purple and then at N+2 another shift, this time to cyan. This change in color as exposure changes is a good reason to run a few tests if you find yourself using color gels in your work. If you have a means of proofing exposure and color while you are shooting with gels these changes will become apparent as you set up your lighting. If you're shooting film Polaroid helps to some extent though Polaroid color and especially contrast are not very accurate. Some gels are a completely different color on color Polaroid film, yet they are accurate on transparency film. Shooting digitally with captures appearing on a computer screen rather than the lcd screen on the camera is a good way of testing if the final photography is also digital with the same camera. If you don't have access to Polaroid or digital capture then an exposure test such as the one I've shown here is very helpful. If you're shooting transparency film, bracket from N-3 to N+3 in 1/2 or 1/3 stop brackets. You'll see some very subtle and useful changes by using 1/3 stop brackets. Sleeve the processed film, label the exposures and you can refer to the color and exposures changes for future reference. You can do the same for print film if you first shoot a neutral shot on each roll without a gel for color reference when printing the film. If you're shooting digitally then do the same but carry it through to your preferred method of print output whether you make your own prints or send them to the lab. Check out the chart of exposures and colors below and you can see the color shift and exposure changes that I've mentioned. Then we'll take a look at some examples of color gels in use.
    00AG8z-20659184.jpg
     
  2. Sorry, my chart was a little too big. Here it is again......
    00AG99-20659284.jpg
     
  3. So, how do you determine the right exposure to place your gel color at one stop above or below or even at the normal mid-tone value in your scene? The value (N, N+1, N-1, etc) of the color from a gel is related to the exposure or f-stop of the rest of your scene. If your subject is receiving f-11 worth of light, you set your camera to f-11. An N-1 exposure for your darker background gel color would then require a meter reading of f-8. An N+1 exposure for your lighter background gel color would require a meter reading of f-16. Sounds confusing because it is. Remember that your camera and subject are set on f-11. The color gel's exposure moves up or down relative to the camera and subject's f-11 exposure. More power to the gel light (a lighter color) results in a higher f-stop on the light meter. Less power (a darker color) means a lower f-stop from the light meter. Change the power on the gel light or move that light closer or farther away to change the amount of exposure for the color gel. You coud also apply diffusion or ND to lower the exposure. Or you could double up the gel or even mix more than oone color of gel. One of the great things about using gels for color backgrounds is that you can start with a black, grey or white, smooth or textured background and create any color that you want. For complete control and the purest color from a gel, the background must be shielded from any of the white light used on the set and lit only by the color gel light. Usually that means placing the background as far away from the subject as possible so that it can be lit seperately from the subject. If you are using a textured background and want to see some of the texture you might want the background closer so the texture shows with whatever depth of field that you have available. In that case you'll have to shade the background with gobos to keep it free from all light except for the color gel light. Or at least make an effort in that direction. Here are some sample photos using gels on the background and in one case on part of the set. The first shot is of the mighty Express Mail Eagle roosting in his nest. The background was a grey textured painted canvas lit separately with a straw color gel. Exposure was an N+1 relative to the rest of the scene. I used an 11" reflector with a tight grid to create a smaller round area on the background for the color and the area under the tree branch was shaded with a gobo. The second shot of the Arizona Tea bottle used the same grey background and the same blue gel as in the exposure chart, exposed at N-1. The third shot for the Gift Fruit Catalog uses the same red gel as in our chart in the fake fireplace opening to simulate the glow from the fire. Exposure was N+2 to raise the red to an orange value. The fourth shot of "Steady Eddie", a local DJ, uses a mix of blue gel on the left and a magenta gel on the right along with Eddie's own optical spots projecting a cyan flower doo-dad. Blue on the left was N-2, magenta on the right was N+1 and the optical spots were N relative to the exposure on Eddie himself. The last shot of motorcycle gloves uses no gell, just a neutral glow on a dark grey textured art board at N+1. We got through this entire theme talking about placing exposures at N-1, N+1 or N+2 without ever once mentioning the Zone System of exposure placement. But there it is! Pretty smooth, don't you think? #8^) You can buy color gels at any theatrical supply house or places online like The Set Shop, B&H and Calumet. Two manufacturers of lighting gels are Lee and Rosco. Last time I bought them they were about $8 apiece for a 20"x24" sheet. You can also use color plastic or cellophane. Be careful of the heat from tungsten lights and even from strobe modeling lights.
    00AGAN-20659584.jpg
     
  4. Excellent!

    U.K. Readers might like to know that a good U.K. source for sheets and half-sheets is http://www/srbfilm.co.uk. For 25' x 4' rolls, go to http://www.leefilters.com/home.asp and follow the links to their customers who supply these sizes (they don't sell direct)
     
  5. 'N is the indicated exposure from an incident meter held at the black paper and
    pointed back towards the lens.'

    You mean a reflected reading?

    With black paper, an incident reading should be giving you results which are close to
    black, whereas you are getting mid-tones.
     
  6. Elliot,

    Remember, the black paper is being lit either with a color gel or white light as shown in the exposure color chart. It is no longer black. You could use a black, grey or white background. Using the indicated exposure from the reading of this light gives a mid-tone value. You could use a black, grey or even a white background, it wouldn't matter the results would be the same.
     
  7. Thanks for this well chosen (bi-)weekly theme.

    Just to add on the information Gary posted for UK photographers, I buy all my Lee lighting filters from these guys: http://www.lxstore.com, they've always been quick and have a good stock.

    Stay away from these other guys though www.bluearan.co.uk.

    I will try to dig a few shots later to illustrate your theme.
     
  8. Bravo, Brooks! Also a reminder that the type of lamps (tungsten, halogen, HMI, CSI, strobe flash), their age, and the voltage driving them and thus their colour temperature, or the use of dimmers, have an effect on the aparent colour produced by gels......
     
  9. Vincent,

    Thanks for the info for our UK readers. Sample photos would be GREATLY appreciated!

    Graham,

    Good point. All the more reason to run some exposure and color tests. Also a good reason to use strobe light which is more consistent than changing shutter speeds and dealing with reciprocity when using continuous sorces.
     
  10. I took this shot using three different Lee lighting filters - Blue for the rim light on the left, Golden Amber (134) for the fire effect on the face and Moonlight blue (183) for the fill.

    The gradient in the amber filter illustrates the change of colour of the amber filter. Her cheek gets more red as the light on her face falls off.

    Also the moonlight blue and the standard blue were used at different power which makes the moonlight blue (neck and behind her ear) dark and dirty while the blue on her neck and back of her head much brighter.

    [​IMG].
     
  11. 'You could use a black, grey or even a white background, it wouldn't matter the
    results would be the same.'

    Sure, if you were taking reflected readings. But you said incident. Hmm, maybe I'm
    missing something here.
     
  12. Vincent,

    Rim lighting, 3 color gels and a cukaloris in one shot! Sweeeet!
     
  13. I do have to agree with Eliot and ask the question again: You are talking about REFLECTED light meter readings?!? If it's INCIDENT then only an 18% GREY background should render the tones in your chart and the N for white light should be close to black. Another interesting issue to consider is a meters response to color. Most handheld meters are calibrated to read white light (Tungsten and daylight) and some of the colors throw the meters off, especially primary red and green. I still carry an analog Minolta spot meter in my kit for greenscreen work because my digital spot meter gets thrown off by about a 1/3 stop from chroma green spot meter readings. Another issue to consider is focus, if the forground element is lit with colored light only and shot on negative film. Take the extreme red for example, the layer sitting upfront on negative stock. If you set your focus normally, it will be slightly off in your final image since you're only exposing the red layer. Many people in my field (cinematography) will use a magenta filter on the lights and correct to red in color correction to deal with this problem when for example shooting a scene in a photo darkroom. Finally I'd like to contribute a still I took from a music video shot on a tiny white cyc. My REFLECTED light reading on the background was N-1.5. The bright spot is the key light falling on the BG, caused by too small of a cyc & the necessity to move the camera during the shot.
    00AGhN-20671884.jpg
     
  14. For people unfamiliar with the Zone system that Brooks mentions, here's the link to a thread I starded in the Nikon forum, where I tested the D-70's dynamic range. The Zone system is IMHO really the best way to predict your results from lighting a subject. http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00ADfX
    00AGiD-20671984.jpg
     
  15. Elliot, Florian,

    Man, you're right. I don't know what I was thinking. A REFLECTED Reading should be much more accurate. When I did the exposure/color chart I used an incident meter but the background was black seamless.

    When I do gel backgrounds in the studio the backgrounds are usually dark, either a medium grey, black or dark blue/grey. I never use a white background because it's too hard to light seperately from the subject.

    I suppose if you were consistent in the background tone ie: always used a white bkgd. or a grey bkgd. or a black bkgs. then your choice of meter would be irrelevant.

    But, I really should shoot some tests with a color gel, white background and incident meter just to see what happens. I'll use a spot meter and post the results...unless one of you would like to shoot that test? Any volunteers?
     
  16. Brooks,

    you're probably too experienced to even use a meter ;)

    "Learn it really really well, then throw it away and act like you never knew it"
     
  17. Florian,

    I must confess that since I've been shooting primarily digitally for the last 3 years or so, I seldom use a meter in the studio. It's so easy to shoot a capture and check it out on the computer screen.

    My bad.
     
  18. I was curious to see how some of my lighting filters respond to variation of exposure so I selected a few of them and carried out the experiment to build a chart similar to the one Brooks produced.

    The following was done using Lee lighting filters, on white background and a Sekonic L-508 set to spot reading (reflected).

    To my surprise I don't find the (N) exposures consistent, for some of the reds (135 for example) feel brighter than mid-tone and some of the greys darker than mid-tone.

    For each filter, I have calibrated the flash output to read f8 in reflected mode then took the shots from f22 (N-3) to f2.8 (N+3).

    Is this normal or something I would have overseen?

    [​IMG]
    Click on the picture to get a full view with info
     
  19. I forgot to mention I used a Fuji S2 Pro, colour balance set to daylight, colour setting and contrast set to Standard.
     
  20. Vincent,

    I may be talking total nonsense or I may be some way towards an explanation of the inconsistencies that occur with + - exposures, especially on the blues, reds and greys.

    If you look at the Lee swatch book, which has detailed info on the colour mixes, and if you bear in mind that meters don't read all wavelengths equally, it might all start to make sense.

    This is nothing more than a theory - not even that, it's just a thought, but I'll put it to the test when I get a moment, perhaps measuring and photographing both pure and bastard blues.

    If my thinking is correct then greens will read more consistently and photography without significant colour changes because mostly they are a mix of pure blue and pure yellow and don't include any red.
     
  21. Vincent,

    These variations might be exagerated because we're using digital capture rather than film. No real evidence for that, it's just a thought. On my screen I don't see as much change in density below N-1 in your test, at least not nearly as much as I saw on my test.

    Perhaps we need a test done on actual transparency film? Not that I'm about to go out and buy film or anything. #8^)

    Garry,

    That would be an interesting test indeed.

    In any event, whether these tests are off in some slight way or not, the important thing is to have some method of testing and proofing when you are using color gels. Even if those tests just get you in the ballpark so that every shoot with color gels isn't a major surprise.

    Anyone have some photos using gels that they'd like to share?
     
  22. Here is a photo with gels and a pic of the set-up. Maybe a little weak but blue gel on
    model right and red gel on model left background.

    http://www.photo.net/photodb/folder?folder_id=420941
     
  23. Last night I found a willing subject and went to town. Two stobes with blue, one with red, another with green, standard short lighting, a fill card, a gobo, and a maybeling eye shadow kit to brighten the name. Comments and suggestions welcome.
    00AN1V-20814184.jpg
     
  24. Mike,

    Very nice image. I like the subtle red and blue hits of color.
     

Share This Page