The Digital World of Photography by Jeff Montanye Lesson 229 – Flash Intermediate
flash metering principles Earlier electronic flashes required the photographer to perform distance calculations by hand. First you focused then looked at the lens for the distance then dial in your ISO on the calculator. You controlled the flash exposure by setting your aperture to the calculated value.
TTL Flash metering Early flashes would emit a single flash of light at it’s brightest level and use the aperture of the camera to control the amount of light entering the lens. The aperture was either controlled manually or automatically depending on the flash unit.
TTL Flash metering This method can still be used with modern cameras when placed in manual flash mode in the menu.
flash metering principles Later, the first generation of automatic electronic flash units relied upon external sensors to determine the flash exposure setting. These sensors, mounted on the front of the flash unit, simply recorded the flash bulb’s light reflected back from the subject, and cut off the power when enough light for a satisfactory exposure was determined.
TTL Flash metering Modern flashes use the internal light meter of the camera and adjusts the intensity of the flash. This is called TTL (Through The Lens) metering
Flash metering principles • There are a few different types of TTL flashes: • E-TTL (evaluative TTL) • A-TTL (Average TTL) • D-TTL • I-TTL • Which one you use depends on the camera. You simply buy the flash type for your camera and it will work properly
TTL Flash metering Without flash turned on, the camera meters ambient light bouncing off the subject into the camera then adjusts the aperture and shutter speed to accomplish a proper exposure.
TTL Flash metering However, when flash is turned on, the camera does not care what the aperture or shutter speed is set at. It will adjust the intensity of the flash to light the closest subject properly.
TTL Flash metering The problem is that the camera needs light from the flash to get a measurement before the picture is taken.
Flash metering principles So the camera does a quick pre-flash just as you squeeze the shutter release, measures the light bouncing into the lens from the flash, adjusts the flash intensity, lifts the mirror and opens the shutter, then flashes the flash again at the proper intensity. This is called TTL (Through the Lens) flash metering.
Flash metering principles • This creates a simple fact that is very important for you to understand: • There are two exposures happening here: • Ambient light which is controlled by aperture and shutter speed. • Flash light which is controlled by the intensity of the flash.
E-TTL 1. Shutter button pressed halfwayAuto focusing and metering are executed by the camera. The existing light is thereby metered by the camera’s metering sensor. 2. Shutter button pressed completely.Pre-flash is fired, and the reflected light is metered by the camera’s metering sensor. 3. The meter readings of the existing light and the pre-flash are compared and the main flash's output is calculated and stored in memory 4 The reflex mirror goes up, the first shutter curtain starts to open, the main flash fires, the film is exposed, the second shutter curtain, and the reflex mirror goes back down. 5. The flash exposure confirmation lamp lights.
Class Challenge (Instructor only) Turn the flash on. Set the camera’s mode to Shutter Priority and set the shutter speed to 2 or 3 seconds. Set the flash to second curtain Take a picture. Notice that the flash fires twice. The first flash is the test flash to get an exposure. The second flash is the actual flash
Flash Pulse Photos taken without flash use aperture and shutter speed to control the exposure. Photos taken with flash use the flash pulse width to control exposure.
Flash Pulse The flash is always the same brightness. When the photo needs more light, it simply keeps the flash on longer. This is known as the pulse width of the flash. Since flash is momentary bursts of light or pulses, the human eye cannot see the difference between a short pulse and a long pulse.
Flash Pulse Think of your flash as a flash light. The bulb is always the same brightness. The longer you have the light on the subject, the more it gets exposed. The difference is that flash times are very fast. A pulse from the flash appears to be the same time weather it is one, two, three or four times the length.
The Light Race The pulse with is calculated automatically by the camera in TTL mode. However, light disperses quickly as it travels away from the camera. Objects twice the distance away need four times the light.
The Light Race To prevent items close to the camera from being over exposed, the flash will only produce a long enough pulse to properly expose the closest objects to the camera. Thus creating dark backgrounds and poor exposures when there is depth in the photograph.
The Light Race Some solutions to this problem are: Use a bounce flash Get closer to your subject Adjust flash composition Use more than one flash Rearrange Your subject
The Light Race The pulse width can be adjusted manually using flash compensation.
Flash Intensity (Compensation) Both fill-in and slow-flash use the full intensity of your flash. In both cases the flash is fired off regardless of the lighting. Your camera selects aperture and shutter speeds as if the flash will not be used. This may cause some over-exposure of close-up objects. To fix this, it may be necessary to reduce the power of the flash. If the flash is too dominant, its effect can be weakened with flash compensation. You can, for example, weaken the flash with, what corresponds to two thirds of a diaphragm step (2/3 EV). So that the people in the image look less ”burnt out”.
Flash Intensity (Compensation) Flash exposure compensation lets you manually adjust the amount of flash illuminating the subject without changing the camera's aperture or shutter speed. This is an ideal way to balance flash and natural light Flash exposure compensation can be set to a minus setting to make the main subject darker or to a plus setting to make it brighter. A setting on some cameras that automates this procedure is flash bracketing that takes a series of pictures at different flash intensities.
Flash Compensation Use your camera’s menu system or touch button to adjust the flash compensation.
Flash Intensity Use your camera’s menu system to adjust the flash compensation.
Class Challenge Setup a subject (such as the silk flowers) next to a battery operated candle. Take multiple pictures at various flash intensity settings. Examine the final images and explain the results.
Shutter Curtains The shutter in your camera is what covers the film or CCD when you are not capturing an exposure. All SLR cameras have a shutter. The shutter is made up of curtains, first curtain, and second curtain. When you take a picture, you press the shutter release button. The first curtain quickly lifts up and away, allowing the light from your subject to expose the CCD. After the calculated exposure time is up, the send curtain lifts up and covers the CCD up again. This is known as a Focal Plane Shutter.
Flash Sync • In standard flash sync. mode the flash fires as soon as the whole film frame is fully exposed to the incoming light. This time period is limited to the flash sync speed of your camera which is typically 1/60th of a second. On some cameras the sync speed can be increased in Shutter Priority Mode.
Sync Speed Sync speed is the fastest shutter speed you can use with flash, period. You cannot use a faster shutter speed than the sync speed with flash. If you try, your camera will override you electronically when the flash is on. A shutter's rated top speed is irrelevant because it doesn't work any faster than the rated sync speed with flash. Today’s modern cameras can achieve sync speeds of up to 1/250 in Shutter Priority mode.
Sync Speed To determine your camera’s max sync speed, place your camera in Shutter Priority, turn on your flash and try to raise the shutter speed. How high will it go? Some cameras give you the ability to set your flash sync speed.
Class Challenge Turn the flash on. Set the camera’s mode to Shutter Priority and set the shutter speed to 1/125 of a second. Try to take a picture. Did the shutter speed change? Try it at 1/500 of a second and take a picture.
Balancing Flash and Ambient Light By now you should understand that ambient light and flash light use different techniques to capture a proper exposure. Think of it as a double exposure: Flash controls the foreground. Aperture and shutter speed control the background
Flash Pulse vs Shutter Speed The flash pulse is too fast to be effected by shutter speed thus shutter speed can be adjusted to lighten or darken background objects.
Flash Pulse vs Aperture The aperture will reduce the amount of light received from the flash as it is closed down. In TTL mode, the flash will automatically increase it’s pulse width to compensate. In manual mode, the exposure from flash will vary with aperture.
Balancing Flash and Ambient Light • Now you should understand that you can: • Lighten and darken the foreground by increasing or reducing the flash pulse with flash compensation. • Lighten and darken the background by reducing or increasing shutter speed.
Slow Sync Slow sync is when you purposely slow the shutter speed down in low light situations to obtain a background exposure from ambient light at the same time flash is fired.
Slow Sync Methods There are three ways to control slow sync speeds: • Use the slow sync option on your camera • Use exposure compensation • Control the Shutter speed manually
Slow Sync No matter how you do it, if you want good pictures in low light a tripod is a must-have piece of equipment. You can raise your ISO but it is not recommended.