If I want to get a large group of photographers together, I just have to run a fill flash workshop. I do these things not for massive profit, but to help photographers cut through the crap and fog to be able to mix flash and ambient light easily.
I've seen so much dreck and poor courseware on this subject, I cover it to help clear frustration so let's get started. As usual, I will start with some basic lighting science. Don't run, you probably already know this stuff.
When we make an ambient light exposure, we are typically doing so based on a reflected light meter that is measuring the subject, doing some math and giving a reading that will give a balanced exposure to the best of the meter's ability. Works great most of the time. But what about those situations where it is not working because of heavy shadows or very bright elements? What about when the ambient light level is very low and we have to go to a longer exposure on a tripod but some areas just aren't lit all that well?
This is when we start to think about using a flash to fill or supplement the ambient light. This is different from when the flash is the dominant light source, or when we want the flash to overpower the ambient light. Then we read a couple of articles, or watch a video, or open the owner's manual and in sadly short order decide it's just not that important because the explanations are confusing.
To see how we can mix flash and ambient, and leverage as much of the technology we own as possible to make getting to optimal lighting easy, there are a few things you need to understand a few things.
First you want to be comfortable using your camera in Aperture Preferred mode. Why Aperture Preferred? The reason is that it's a bit less work than full manual and also because MOST manufacturer's will allow you to select the aperture for the depth of field you require and even when a flash is connected, let the shutter speed float to the proper combination of shutter speed and aperture at your selected ISO for a properly exposed ambient light picture. This mode is typically referred to as AUTO sync. To be technical, the camera drags the shutter speed as slow as needed to make a proper ambient exposure but still fires the flash when the shutter opens. This is a real advantage. Let's suppose you are shooting your kitchen because you are selling your home. You determine that f/11 gives you the depth of field that you want, and without a flash, the proper exposure for the ambient light requires a shutter speed of 1/15 second. No problem, camera on tripod and away you go. But the image is a bit flat, there's no shine off the appliances or countertop and areas under the cabinets and lower to the floor are seeping into blocky shadows. When you turn your flash on in this mode, the camera MAINTAINS the proper exposure for the ambient light and fires the flash as well to supplement the ambient light using TTL flash metering.
Second, the aforementioned TTL flash metering. TTL or through the lens flash metering means meter the flash off the sensor just like you would ambient light. TTL is amazing and works wonderfully over 85% of the time. It uses (typically) a small pre flash to determine the flash duration to give a correct flash exposure. Because the flash duration is MUCH shorter than the shutter speed duration, you get a nicely exposed image so long as the shutter speed is at your flash sync speed or less. Used in conjunction with Aperture Preferred and Auto Flash Sync, you get a balanced combination of ambient and flash light.
For many images, you can stop here. It's simple and it works. Except for those times when it's not quite right. The ambient light reading may be tricked by the light coming through a window, or there might be a very reflective surface that really pounds back the flash. This is why the third thing to learn are the two types of exposure compensation.
Exposure compensation in most cameras comes in two flavours. Ambient exposure compensation and flash exposure compensation. In some cameras, the same setting controls both. This is worse than having neither so I hope this does not describe your camera. Ambient exposure compensation lets us choose to over or underexpose the ambient light image. If shooting in aperture preferred mode it manipulates the shutter speed. If shooting in shutter speed preferred mode it manipulates the aperture. If shooting in Program mode, it will manipulate both. If shooting in Manual, it does nothing at all if you are very lucky. I say this because some cameras still allow exposure compensation to work when shooting in Manual. This usually leads to frustration, confusion and camera to floor contact at high speed. So let's hope this doesn't describe you.
Flash exposure compensation does not change shutter speed or aperture. It changes flash duration. Remember that exposure is just light hitting a sensor. The longer the light hits the sensor, the more light hits the sensor and so there is more exposure. Simple right? Well flash is generated by striking a gas, typically Xenon in a sealed tube with an electric charge. This excites the molecules enormously and they give off light. The molecules are only excited for a very short time to prevent bad things from happening like explosions or burned out tubes. Even if our flash sync speed is 1/250 of a second, a full pop flash duration may be no more than 1/1250 of a second, five times less time. By changing the duration of the charge, the flash system changes the duration of the flash pop. So when you hear people talking about turning the power down or up on an electronic flash, what they are really talking about is reducing or lengthening the flash duration. A shorter flash duration means less light, a longer flash duration means more light. Still simple, right?
Well that's the long road to flash exposure compensation. Dial in less "power" to the flash, by setting it at ⅛ power and you are really just shortening the duration. Dial in some overexposure and you increase the flash duration for more light, but never more than the maximum that the flash can push at one time. So when folks ask me what flash to get, I always say the most powerful one you can afford, not because you will always need it at full pop, but because of the versatility in output that a more powerful flash will bring you.
Let's go back to the kitchen. We have light streaming in through the window. Our ambient light meter reading is fooled a bit by that bright window and exposes for it, leaving part of the kitchen a bit dark. Dialing in some positive ambient exposure compensation brings up the dark area but blows out the window. Ah crap. Time to get ready to play in post processing or haul out some graduated ND filters. Or, get your TTL flash.
Put the flash on the camera in TTL mode. Make sure that the camera flash synchronization is set to AUTO and the exposure mode to Aperture Preferred. Take an image with the flash off and you'll get again what you got before, good exposure at and near the window but with the rest of the room fading to shadow. Turn the flash on and point it towards the darkened area. Take the shot. Now you have a balance of ambient and flash and it make be just perfect. Not just perfect? You want the flash to look less flashy? You want to blow out the window a little bit to hide the muddy yard but still have decent ambient light in the kitchen? Here's where the compensations can help. Maybe increase the ambient exposure compensation to +⅔ That will increase the shutter speed to allow more exposure in the kitchen, at the cost of some blow outs on the window. Too much shine off the face of an appliance being lit by the flash? Dial in -⅓ flash exposure compensation and shoot again. Better? Great! Still more needed? No problem. You manipulate the ambient exposure compensation to achieve the ambient exposure you want and the flash exposure compensation to get the flash exposure you want. Bringing the two together gives you a much more balanced lighting structure without you having to go to a complex multi-light setup.
Now let's go outside. Cousin Pam is visiting and you need to make photos of her with the family. Why? Because you are the family photographer by decree. The family loves that bright overhead sun, that's perfect for photos isn't it? You drink heavily for a moment and get Uncle Bob to hold a diffuser over the heads of the people to cut some of the harsh sunlight. This is twofold, the light is more diffused and Uncle Bob isn't in your way with his Canon 5000D Mark 27 telling people what to do and how to pose. Hmm still not great. Put the sun behind the people so it is hitting them in the back, the only person with the sun in his or her eyes should be you. Now you have even flatter light on the faces and may be even a nice rim light behind their heads provided by your 93 million mile distant primary light source. But you still need to put catchlights in the eyes and pick up the glitter cousin Julie has all over her face and may be lighten up Gerry's face where he stopped the ball with his eye socket. Put your flash in the hotshoe. Now check your shutter speed and so long as it is not over your flash sync speed, turn the flash on, set it to TTL and shoot. You will get a nicely exposed image for the ambient light that TTL flash will fill in the shadows and put some catchlights in. Too bright in front, dial down the flash with some negative flash exposure compensation, say -1 and shoot again. Looks great! You're a rock star. Want to drop the light from the back a bit, to make the faces more dramatic? Dial in -2 on the ambient light exposure compensation and give it a try. You will likely want to try a few different settings of flash and ambient compensations but by doing so you will make a better image and demonstrate your expertise.
In this sample gallery you will find a series of images shot in the kitchen including the patio doors and stuff on the table. They give you a sense of what happens when you add fill flash as described above, and then how you can use ambient exposure compensation and flash exposure compensation to achieve more usable images. They will not win any awards and are not meant to. No processing has been done to these images other than RAW to JPEG conversion and export from Lightroom CC. They go from just ambient as metered, to ambient as metered with TTL bounced off a white ceiling with no compensation, to ambient as metered with -1 flash exposure compensation, to ambient as metered with +1 flash exposure compensation, to ambient as metered -1 exposure compensation with +1 flash exposure compensation to finally ambient as metered -2 exposure compensation with +1 flash exposure compensation. By experimenting with the settings we can get the feeling of light through the window providing illumination but without completely blowing out the highlights and by adding the fill at +1 bounced bring the levels up inside the room without it screaming "I was lit by flash".
Now at this point, there are fine readers who are saying, "hey we could have put the flash on manual and played with the output settings in scenario #1" They are correct. They also said "hey we could have put the flash and camera in manual and played with the settings to get great images in scenario #2". They would also be right. There are many routes to solving the challenge of mixing flash and ambient, I've covered the simplest. Aperture Preferred mode with AUTO sync speed and TTL flash. There are others, but before you nail yourself to a tree, give simple a try. You'll be able to make images that you haven't done before, you'll be able to do so quickly without a lot of fuss, people will be impressed as all get out, and no techno-babble was required.
Until next time, peace.