Sunday, August 23, 2009

TTL Flash FAQ for digital cameras

Updated: May 8, 2010
Number of times updated: 9 (mainly to #14 - mixing ambient and flash)

This FAQ guide is a super-simplified intro to flash. It assumes no knowledge about flash but requires that you already have some understanding of exposure.

1. Why use a flash?

Properly used, a flash is another way to control the image and improve the lighting of your image.

2. What do you mean by proper way? Don't you just turn it on?

No. Proper use of flash requires understanding how flash metering and controls work in relation to your exposure metering and controls. Just as with exposure for existing light, if the power of the flash is incorrectly set, then your image will be over- or underexposed. In addition, flash can serve a variety of functions (as the main or key light, fill, background, hair, rim light, etc.) and the ideal flash exposure differs depending on the function of the flash.

3. Isn't your flash exposure simply determined by your exposure settings?

Ambient exposure and flash exposure are two different concepts. (Ambient exposure is the amount of light recorded on the image if you had not used the flash. Flash exposure is the amount of flash recorded in the image.)
Your exposure settings for the ambient light (aperture, shutter speed, ISO) may or may not affect your flash exposure.  In manual mode, ambient exposure controls do affect your flash exposure.  In TTL mode (see #4 below), ambient exposure controls generally don't affect flash exposure.  In either case, learning to control your flash exposure is a separate skill.

4. How is flash exposure controlled?

It depends on your flash mode (manual, auto, or TTL). Let's get something straight: flash output (the amount of light from the flash) and flash exposure (the amount of flash that is recorded in the picture) are two related but different concepts. Mostly, what we care about is flash exposure. There are several ways a flash exposure is determined. Generally, they are:
a. Manual - user controls flash exposure by specifying the flash output (usually on the flash itself). With manual flash, there is no built-in metering (unlike switching the camera to manual exposure mode, which at least gives you a light meter). With manual flash exposure, you either use the guide number (the flash power rating), a handheld incident light meter, or trial and error to determine the correct flash exposure. Using manual flash is outside the scope of this FAQ. FYI manual flash power is sometimes expressed as an f/stop which is an indication of the aperture required for a correct exposure (according to a light meter). So, setting the flash at f/8.0 means putting out twice as much power as setting it for f/5.6. Sometimes expressed as a fraction, e.g., 1/2 power, 1/4 power, etc. Usually in full stops. The latter is less informative because different flashes have different light outputs for the same amount of power.
b. Auto - You don't control flash output per se. Instead the flash (not the camera) uses its own sensor to control flash exposure. You initially specify on the flash the aperture and the ISO you're using. When you press the shutter, the flash fires and detects its reflection through a sensor on the flash. (On more advanced flashes made for your particular camera, the camera automatically communicates the aperture and ISO to the flash.)  When the flash sensor detects that there's "enough light" based on the aperture, the ISO, and the reflection,  it cuts off the power.
c. TTL ("through the lens") - You also don't control flash output per se. Instead you use the camera to control flash exposure. When you press the shutter, the camera tells the flash to fire a test flash. The camera (not the flash) detects the reflection from the test flash coming in though the lens. Based on the amount of reflected light, aperture, and ISO, the camera tells the flash how much power to put out. All this happens in a split second before the shutter even opens. When your shutter opens, the flash power has already been set by the TTL system.

5. In 4b and 4c, you didn't mention shutter speed as affecting the amount of flash. Why?

The flash only lasts 1/1000 to 1/20,000 of a second, so changing the shutter speed generally doesn't affect the amount of flash. 
Note 1: shutter speed becomes relevant if the shutter speed is so high that the shutter starts to close before the flash is fired. That maximum speed before this issue occurs varies from camera-to-camera and is called the "sync speed." Some cameras have X-sync exposure mode. That is a shortcut way of choosing shutter priority exposure at the sync speed.
Note 2: although shutter speed doesn't affect the amount of flash, it is relevant for controlling the ratio of existing light to flash. See #14 below.
Note 3: Modern flashes have a way of increasing the sync speed at the expense of power/distance through high speed sync.

6. I'm using a filter (e.g. a neutral density filter). Will that affect the flash metering results?

With Auto, yes because there's no way for the flash to know whether you're using a filter or other accessory. With TTL, no (because the test light comes in through the lens).
6b. I'm using a large diffuser that's blocking part of my external flash.  Will that affect the flash metering?
If you have a diffuser or accessory attached to your external flash, and it's large enough to block the sensor of your external flash, Auto won't work correctly.  However, TTL will generally function without problems because the flash metering is done through the camera's lens, not a sensor on the external flash.  However, TTL may screw up if your accessory directs some of the flash into the lens (instead of bouncing off from the subject).
6c. My flash is off-camera (i.e., removed from the camera) and positioned closer  to or farther  from the subject than is the camera.  Will that affect the flash metering?
If you are using TTL, then the flash metering should not be affected by any difference between the flash-to-subject and camera-to-subject distance.
If you are using Auto, then the flash metering will likely not be accurate.  If the flash is closer to the subject than the camera (e.g., the flash is 3 feet away from the subject while the camera is 10 feet away from subject), then the flash exposure will likely be underexposed and vice-versa.

7. Do my ambient exposure controls (aperture, shutter, ISO) affect the TTL flash exposure? What about exposure compensation?

With TTL, generally no.  Within a wide range of ambient exposure settings, the flash exposure should be the same regardless of ambient exposure controls. For example, if you're using f/5.6 @ 100 ISO a TTL flash will put out more flash power than if you had used f/2.8 @ 400 ISO, so that the flash exposure in both cases is about the same. Exposure can start affecting the flash exposure if the exposure settings force the flash to exceed its maximum or minimum power. If you're at f/64 and ISO 100 and 30 feet away from your subject, your flash is not going to put out enough power for the 'correct' amount of flash exposure. If you're at ISO 1600, f/1.4, and 5 feet from your subject, you will have way too much flash exposure. In addition, although ambient exposure settings in many cases don't affect flash exposure itself, you have to balance ambient and flash properly for the photo to look nice. See balancing ambient and flash in #14 below.
Using exposure compensation may or may not affect your flash exposure.  With Canon, the flash exposure ignores the exposure compensation and is unaffected.  With Nikon, exposure compensation will affect flash exposure.  For example -1 stop EC will reduce the ambient exposure by 1 stop and the flash exposure by 1 stop as well.  There are advantages to both approaches.
7b. How do I know if I will exceed the minimum or maximum power from the flash?  What can I do about it?
One way to tell is through the distance range displayed on the backs of some flashes.  Some flashes will show you the minimum and maximum range in TTL mode for any given exposure setting.  Note however that those ranges work only if you are using the flash directly.  If you are bouncing the flash, the distance range won't be reliable.  Knowing that there is a maximum and minimum power, what can you do about it?  If you want the highest maximum range, set the shutter at your camera's maximum sync speed or less, use a wider aperture, and/or use a higher ISO.  Note: using a wider aperture and higher ISO will of course affect your ambient exposure.
If you want to decrease the shortest minimum range, you can either decrease your aperture, decrease your ISO, or if your camera has high speed sync enabled, choose a shutter speed above the sync speed (which will rapidly decrease the effective power of your flash).

8. TTL sounds like a reasonable way to set flash exposure. What could go wrong?

A number of things can cause TTL to set the wrong exposure:
  • If the reflectiveness of your subject is not average, then you can get over- or under-exposure. For example, if your subject is wearing black velvet, then your camera will try to put out as much flash as it can to try to bring the velvet to gray, and the subject will be overexposed. If your subject is wearing a reflective vest, then the camera will cut off the power very early because of the efficient reflection of the vest, and the subject will be underexposed. Those are extreme cases, but there are more common situations that are analogous, such as having a shiny object near the middle of the image.
  • In some flash systems (e.g. Nikon), the camera measures the preflash reflection toward the middle of the image. If your subject is off-center, the TTL system doesn't know that and will try to set the flash exposure for whatever's in the middle of the image, usually resulting in overexposure of the subject if the object in the middle is further away.  One solution for this kind of situation is to frame the subject in the middle, lock the flash exposure (in Nikon cameras, with the FV Lock), recompose, and take the shot.
  • If there is a reflective object in the foreground, it can cause overexposure of that object and underexposure of your subject.  Sample:

One solution for this kind of situation is to zoom in to the subject until the problematic foreground image is out of the frame, then use FV Lock.
  • In addition, as discussed above, the correct amount of flash differs depending on the purpose of the flash (as key/main, fill, hair, rim, or background, etc.).

9. If the flash is overexposed or underexposed, what can I do to adjust flash exposure?

With TTL, you use flash exposure compensation ("FEC"). If there's not enough flash, you increase FEC, and vice-versa. With TTL, changing your aperture, shutter or ISO won't change your flash exposure (unless your exposure settings are such that they exceed the minimum or maximum power for the flash).

10. OK I understand how to set the flash exposure. Am I good to go?

In addition to the amount of power from the flash, the basic variables include quality (hard vs. soft), color, shape, ratio to existing light, and direction. More advanced techniques (not covered in this FAQ) include: adding more than 1 flash, and adjusting those variables for each flash, using a stroboscopic (repeating flash), and more.

11. Why and how do I change the quality of the light from the flash (hard vs. soft)?

Hard light (with hard, well-defined, high contrast shadows) is more dramatic but potentially unflattering if not used correctly. Softer light is generally more flattering for people and makes it easier to achieve an acceptable image. To soften light, increase the apparent size of the light source. To increase the apparent size of your flash, you can bring it closer or use bounce techniques or use a flash modifier such as a softbox, umbrella, or any number of flash modifiers out there.  The larger the apparent size of the light source compared to the subject, the softer the light will be.
  • Using bounce techniques:  instead of directing your flash at the subject, you direct it at a large surface such as a wall or ceiling, and use the reflection to light your subject.  Note: this will eat up much of your flash's power because of the extra distance the light has to travel and because the surface is likely not a perfect reflector.  Thus, you usually have to select a higher ISO like 800 or more.
  • Using umbrellas: to attach a flash to an umbrella, you’ll need an umbrella holder (aka umbrella swivel). An umbrella holder is equipped with: 1) a hole to insert a photographic umbrella, 2) a shoe for the flash, and 3) a way to insert a 5/8 stud, which is the standard size for lightstands. Some brackets use a tripod 1/4-20 mount instead.
  • Using softboxes: there are a variety of softboxes out there – some made for studio lights and others made for hotshoe flashes. To use the ones made for studio lights, you'll probably need some sort of adapter.  The ones for hotshoe flashes use different ways of attaching the softbox such as velcro.
  • Using flash modifiers: there are a myriad of flash modifiers that are supposed to soften the light from the flash. If you’re concerned about softening your flash, just ask yourself whether the flash modifier is making the light from the flash larger or not. If it’s not, it doesn’t soften the light. For example, some people think the sto-fen omni-bounce diffuser softens the flash. Actually it doesn’t because it doesn’t change the size of the light. It does help bounce the light around indoors, so the softening effect of the sto-fen diffuser is from the bounced light, not the diffusion. Outdoors, where there’s nothing to bounce from, there won’t be a softening effect.

12. Why and how do I change the color of the light from the flash?

Three reasons: to correct existing light conditions, to change the emotion of the image, or to use color as an element of your composition. If existing light is not neutral and you want to make it neutral, you need to change the color of the flash to match the existing light so that when you correct for white balance, both flash and existing light will have the same color. Second, you can use colored flash to change the emotion of the image (e.g., red is hot, blue is cool, green is sickly etc.).  Third, you can use colored flash to actively control the color of the subject, shadow, and/or background for composition purposes.
Three ways: 1. through gels; 2. through your white balance; 3. add or subtract colored lights.
  • Gels: Gels are colored sheets of plastic (like colored transparencies), velcroed or taped or somehow attached to the flash, like this.
  • White balance: You can gel the flash for a particular color, use white balance to ‘correct’ that color, and the existing light will become the complimentary color. For example, if you gel your flash orange (with a CTO gel) and adjust your WB for incandescent light, the flash will look neutral while the existing light will look blue.
  • Mixing colored lights: Another way to change color is to add or subtract lights. For example, if you have a red light and a green light, the areas where they are both shining on will look yellow.  More on adding and subtracting colored here: Color addition: http://www.physicsclassroom.com/class/light/U12L2d.cfm and Color substraction: http://www.physicsclassroom.com/class/light/U12l2e.cfm

13. Why and how do I change the shape of the light from the flash?

Why: changing the shape of the light changes the shape of shadows and highlights from the flash. Shadows can add three-dimensionality to an image and can add drama or become an element of composition.
How: Diffusers, snoots, grids, gobos, cookies, and other similar devices. Homemade or store-bought.
  • Diffusers spread out the light from the flash.  Some flashes have built-in diffusers.  There are also a number of flash modifiers out there that act as diffusers. See #11.
  • Snoots are like tubes that you attach to the flash to make the light beam narrower. Longer snoots = narrower beams.
  • Grids have a similar function to snoots but they composed of many smaller tubes (e.g., in a honeycomb or grid pattern). Generally, the resulting light is even narrower than snoots. Longer and smaller-tubed grids have tighter beams. Louvers are similar to grids but they channel light in one axis but not the other.
  • Gobos or flags: something placed to partially block the light beam from the flash. For example, gobos are used to reduce lens flare.
  • Cucoloris or "cookie": a surface with a pattern of holes to let the light beam shine through with a particular pattern (e.g., to simulate shadows of foliage).
  • Found objects: many objects around us, whether opaque, translucent or transparent, can be placed in between the flash and the subject to shape the shadow and or highlights from the flash.  For example, a common technique is to shine the flash through a glass of water to project an ethereal highlight on the background.

14. Why and how do I change the ratio of ambient light to flash?

Why: allowing enough ambient light to appear in the image preserves the atmosphere of the scene you’re shooting.  Conversely, having no ambient light (as in a background that is turned black) can direct the attention to the subject or simplify the composition.
How: With TTL, you adjust the ratio of ambient light to flash by:
1) adjusting the ambient light exposure: To adjust ambient light exposure, just change the exposure controls (aperture, shutter, ISO). As discussed above in #7, changing those controls generally doesn’t change the flash exposure in TTL mode but doing so will change the amount of ambient light exposure on the image.
2) adjusting the flash exposure:  You can also change the ratio by adjusting the flash exposure (which for TTL is done by adjusting FEC).
3) controlling the amount of flash that reaches the background (lighting depth of field*): other than through controlling the direction of the flash (toward or away from the background) or using flags, this can be done by controlling the distance between the flash, subject, and background. If the flash to subject distance is much shorter than the subject to background distance, then the flash exposure of the subject will be much greater than the flash exposure of the background due to the inverse square law.  Example of this technique here.
(*Note: usually depth of field means the part of the image that is in focus.  In lighting terms, however, depth of field means the part of the image that is illuminated. See this Strobist lesson)

15. Why and how do I change the direction of the light from the flash?

Why: changing the direction also changes the shadows. See #14. Changing the direction also allows selectively highlighting parts of the image.
How: You need an external flash. With your pop-up flash alone, you basically can't change direction unless you use reflectors or mirrors.
Changing light direction with an external flash depends on how the external flash is connected to the camera: the camera’s hot shoe (“on-camera flash”), with a cord, or use a wireless connection.
a. On-camera flash: to change direction, you need an external flash with bounce and/or swivel (ideally both).
b. TTL cord: look for a TTL cord specifically made for your camera brand. One end attaches to your camera’s hotshoe. The other end attaches to your flash’s hotshoe. Then you put the external flash on a lightstand, flashbracket, or hold it with your hand.
c. Wireless TTL connections:
i. optical – each major camera maker has its own way of implementing wireless TTL via optical preflashes (like morse code), although some flash systems allow more sophisticated control than others. Generally, you’ll need a commander, and a flash that is capable of acting as a receiver for such systems.
ii. Radio – wireless TTL via radio is relatively new. To date, wireless TTL via radio are featured in products available from quantum (freexwire system), pocketwizard (control TL), radiopopper (PX system).

16. How do I choose an external flash? Can I get just any flash for my camera?

You’ll maximize the features of the flash if you pick one that’s specifically designed by your camera maker (or a third party version of such a flash). For example, if you have a Canon DSLR, search for “Canon flash.”  To be really sure, confirm that the flash is compatible with your specific camera model.  For example, the SB-900 is not fully compatible with Nikon film bodies or Nikon cameras not compatible with Nikon's CLS (Creative Lighting System).
The features you’ll probably want are:
a. power – flash power is rated by guide number. The higher the better, especially if you intend to bounce the light.
b. wireless mode – to allow wireless TTL assuming you have a compatible commander.
c. bounce and swivel – bounce means the flash head can be tilted upward to bounce the beam from the ceiling to the subject (to soften the light). Swivel means the flash head can be rotated (to bounce the beam from walls). Note the maximum swivel angles.
d. high speed sync – allows you to exceed the sync speed at the expense of flash power.
e. optical slave mode – this is different from optical wireless TTL. With this feature, some flashes can be triggered when they sense the camera’s popup flash (or other flashes) being fired (usually, it's limited to manual mode). Useful when you don’t have other wireless connections available but the disadvantage is that there's no TTL and your flash can be triggered by other cameras or flashes or pre-flashes from a  TTL type pop-up flash.
f. zoom head – some flashes have beams that can be zoomed. This is useful for controlling the shape of the beam. Zooming the flash makes the beam narrower and maximizes the flash’s power and vice-versa.
If you want to choose some non-dedicated flash, then at a minimum:
a. Assuming you’re connecting the flash to the camera, you have to check the voltage of the flash. Older flashes have very high voltages that can fry your digital camera.
b. Next you should make sure the flash’s hotshoe can fit on your camera. Sony/Minolta use a different sized hotshoe from other brands. Other than Sony/Minolta, I’m not aware of any other uniquely-sized hotshoes.
c. Check whether the flash is advertised as being compatible with your camera brand.

17. Where do I go from here?

To try out some of the ideas here, check out the TTL Flash Tutorial.  For another tutorial on TTL techniques, check out http://www.planetneil.com/tangents/flash-photography-techniques/ or the book On-Camera Flash. For technical explanations of how TTL works exactly, read the Nikon CLS Practical Guide and Flash Photography with Canon EOS Cameras. For aesthetics and lighting ideas, I look at parenting magazines, movies, movie posters, high-budget TV shows and ads, and the work of skilled photographers.