What is a gel?:
Lighting gels were originally made from gelatin, hence their name, now they are made from plastic materials such as polycarbonate or polyester. They come in sheets (20"x24"), half-sheets, "paper sized" (8.5x11"), 25’ x 4’ rolls and various other odd shapes and sizes depending on your needs The predominant manufacturers of gels for use in both theatrical lighting and photography are Rosco, Norman & Lee. Lighting gels are used to modify the color of light, not the lens. Colored filters (such as Cokin filters) are optically flat and can be placed over the lens. We will be talking about light color modification using gels, not colored filters in this post though.
First, the details:
The size of gels you buy depends on your budget and storing capabilities. For photography, we need a lot less gel "real estate" than our theatre counterparts. I recommend 8.5x11" sheets of gels for storing ease. However, you can get better prices when you buy larger. For the typical photographer, a 2x3" gel can last more than a few years. I will only show you three different gels in this tutorial. This is so you don't get overwhelmed with different variables. These first tutorials are designed to ONLY show you how the gear works. I want you to go out and try this stuff. If you just sit here and read this blog and others, you will not become a better photographer. You need to do this stuff to make the magic happen.
Not all gels are created equal. When looking into purchasing gels, you want to make sure a gel can handle high temperatures. Some gels can dissolve on the face of a flash with one full power pop (flash heads can get very hot). A good place to get a starter pack is here. You can get them at better prices if you shop around though. The Rosco sample pack has become infamous for photographers. You can find it through Amazon here. This sample pack is perfect for speedlights and gives an immense array of colors to work with.
Note: All the small square images in this tutorial showing colors and density's of gels are representations of gels created through Illustrator, for consistency.
Different gel types and other variables:
There are hundreds of different shades of gels on the market. One thing to know is that even though there is a common naming convention, all gels are not created equal. For example, the Lee "bastard red" gel is not the same color as the Rosco "bastard red" gel. While they are similar, I recommend finding a single manufacturer of your choice and sticking with them to maintain continuity. I personally love Rosco and use them for all my gels. This is just personal preference though.
There are stops also associated with gels as well. Can't escape those pesky stops! ;) The most common stops in gels are 1/8, 1/4 & 1/2. Simply put, the number that corresponds with the gel you are using is the amount of light that your gel is "eating". Let's say you have your camera and flash set up so it is "perfectly" exposed and you put a 1/2 gel on your flash, the part of the exposure that is lit by the flash is now going to now be underexposed by 1/2 a stop. Same scenario but you use a 1/8 gel, then your shot will be underexposed by 1/8 of a stop. This rarely makes a deal breaking difference. But you should understand that every time you modify light in any way, you are losing light. Using a 3/4 gel will also give you a "stronger" color than a 1/8 gel since the gel has more color. The following images should help this make more sense.
1/8 CTO gel:
3/4 CTO gel:
Another thing to pay attention to is flash "ttl" functions. This automatic metering does not know that you are using a gel and will only meter what it thinks is right. So if you double up two 1/2 gels on a flash and fire a shot, your flash could under-expose a shot by a full stop. This doesn't have to be the case. You just want to pay attention to your shots and make sure you compensate for this if your camera produces these types of results. Shooting manual is recommended, but if you need to shoot in another mode (any setting other than manual) your exposure compensation may need to be adjusted. For this reason, full auto is not recommended.
Common color temperatures and how to compensate for them:
The Kelvin temperature for the average speedlight is 5400° K, direct sunlight is 5200° K, a cloudy day is 6200° K, shade is 8000° K, a new 100-Watt bulb is 2800° K, quartz lighting is 3200° K, fluorescent bulbs vary from 3000–6000° K, and daylight settings vary according to the time of day. Light banks/umbrellas/light modifiers/softboxes will vary in Kelvin temperature according to the color temperature of the flash tube and the fabric material used to manufacture these products.
For quick reference I'll tell you the most common ambient lighting conditions and what color gel you should use:
PLEASE UNDERSTAND THIS IS JUST FOR QUICK REFERENCE! If you really want to take your lighting techniques to the next level, you will want to refine this rough list to fit your needs. I have also just used a close example of color in the following images
Daylight is fairly simple. It's already around 5500K. Use standard film or set digital cameras on daylight. Use flash/strobes without filtration. If you find the flash too "white" use a CTO gel. You can also cover only a portion of your flash if your CTO is casting too much color. This will mix the white and CTO colored light to give a satisfactory result. (In case you haven't noticed, I'm all about fine tuning...)
CTO gel 3/4:
Daylight in the shade
Color temperatures can vary widely. Basically, it's all shades of blue. Blue is considered a "cool" light temperature (even though on the Kelvin scale it considered hot). It's a fine background color and actually helps lift the subject off the background if the subject is lit with standard flash or strobe (5000K-5500K). It allows the subject to reflect "warmer" tones and appear more alive than the background. So I recommend not using a gel for this. However, if you did want to match color temperatures, I recommend a 3/4 blue gel.
Blue gel 3/4:
Normally I don't use strobes in an environment like this since the sky is the biggest softbox out there and I like how it looks. Sometimes you need to add extra light though, so if you are using lighting, you will need to color balance ambient to around 8000K. Use a blue gel for this. The amount of cloud cover can dictate Kelvin color temperatures. So you will need to adjust accordingly when selecting gels. Just make sure it's in the blue family. If your just shooting w/o lighting gear, just adjust WB for shade. I personally don't make this adjustment and leave my camera on manual WB and just adjust my images globally in post.
Blue gel 1/8:
In all cases where tungsten light (includes indoor lamplight, gymnasium and parking light bulbs, among others) is the prominent light source, it's use a 3/4 CTO gel. It converts 5500K daylight sources to 3200K to somewhat match ambient light.
CTO gel 3/4:
There really isn't a sure-fire correction for fluorescent light. Each tube is different and unpredictable since it's a pulsating light source. It's regulated by alternating current (60 cycles per second in the U.S.). This means any shutter exposure faster than 1/50th is still unpredictable. A window green gel will get you in the right area though. You should experiment and see what works for you though.
Window Green gel 3/4:
Refining which gels to use:
This is the part your not going to like to hear. Unless your shooting in tethered mode or have the luxury of being able to ingest your images right away. There is no accurate way of telling how close the you are balancing your color temp. The resolution on LCD's is just too low to accurately depict what you will see in the final image. Even if you do a bunch of "test pops" to see what your image will look like with your eyes. You still won't really see what your camera sees since the sensors in cameras read light differently that the human eye does. The human eye compensates and adjusts to give your brain what it wants to see. You can't trust your eyes in a situation like this. Most of the time after a few test pops, the scene will look over gelled and you think it will turn out terribly. This usually isn't the case though once you ingest your shots and see them on a color corrected screen.
Once you get the hang of things. You can make other adjustments like set the WB in your camera to match temperatures i.e. shooting w/ CTO gelled flashes in a gymnasium to tungsten on your camera. I prefer to leave it WB alone in a situation like this. I can ingest all my images and make global changes and call it a day in a few minutes in post. This is purely my preference though.
How to make a gel and secure it to a flash:
The first thing you want to do is take your gel sheet and cut it down to size. All it needs to do is be the same size or larger than the flash head your using. Here are two images for examples: You'll notice in the images I show you that the gels aren't perfectly cut. I did this on purpose to show you don't have to spend some ghastly amount of time cutting a gel to perfection. The only goal of cutting a gel is to cover the entire area that's transmitting light. That's it. If the gel hangs over the edge a bit, it hangs over.
CTO 3/4 gel cut to size above flash:
Gel cut to size. Staged above flash head:
Now that you know one way to attach gels to a speedlight. Lets talk about what it looks like when you use a gel and the virtually infinite combinations you can use to result in some great (and sometimes anywhere from strange to downright ugly) light coloring modifications.
What gels really look like when they are used:
Here's a head on shot through an umbrella with just a bare flash for reference:
This is a common gel to use when balancing for tungsten (gymnasium) light, and natural sunlight.
Here is a 1/4 shot using a "Window Green" gel:
This is a very common gel to use when balancing for florescent lighting. Florescent lights give skin a sickly green look. This isn't a big deal unless you have a fluorescent light and a "white" light from your flash. Then you have two different worlds of colors and it's almost impossible to correct in post production. It's possible, but do you want to spend hours on a shot or one simple temperature adjustment in post production? Up to you...
Here is a 1/4 CTO ON TOP of another 1/4 Window green gel:
Here is a 1/4 CTO side by side with a 1/4 Window green gel: (Green on camera left, CTO on camera right)
There are many ways to store gels from empty Altoid cases to just leaving them in a side pocket of your lighting bag (ugh). Gels are very slick and tape easily comes off of them. So when I'm not using a gel, I fold the tape over that I was using to secure the gel to the flash onto itself.
After I have folded the tape over so it doesn't stick to anything else. I store it in a folding business card case like this one. Then the gels tuck away neatly into the case and I close it up.
It's that simple.
Gels are incredibly useful tools for lighting. Once you start to understand how to light things, your next step will be controlling the temperature of the light so it looks consistent (or not if that's your goal). After you start effectively using gels, you will start identifying non-gelled flash images and see why it looks the way it did. This is also (In my opinion) when you start becoming a lighting "professional".
For me, it's all a bout the details in photography. This is one of the most important details to understand.
Correctly modified light temperature also helps in post-processing more than you'll ever realize. If you have two color temperatures in a shot, no layer mask gradient is fine enough to compensate for color differences. Light just acts differently than pixels and you can't get them to match. So instead of having a shot that's OK, you'll have a shot that's what you wanted and everyone walks away happy.