To our eyes a flash happens, well, in a flash! It appears instantaneous, but if you were to hook up an oscilloscope and the proper sensor you will see that a flash has a rise, a peak, and a fall. The rise is quite steep and the falloff starts almost immediately, but it then stretches out until the light totally fades. In most situations you don't need to be concerned at all with the flash duration. It is almost always faster than your shutter speed in normal sync mode (we will talk about high speed sync separately).
Measuring and describing flash duration
When you look at the specifications for studio flash/strobe units a flash duration is usually mentioned. Sometimes you will see two numbers listed, a t.5 number and a t.1 number. If neither is mentioned specifically, the duration is most likely listing the t.5 number. So, what do these numbers mean? t.5 measures the time it takes for 50% of the total flash power to be dissipated. The problem with this is that the flash continues to put out the remaining 50% of the light. One manufacturer, Speedotron, describes flash duration as "the time interval from the instant the flash reaches one-half of its peak intensity to the instant it decays to the same value." This number is not a good indicator of the flash being able to freeze motion. The t.1 number is the time it takes for 90% of the total flash to be emitted. This gives a better indication of the ability to stop motion. So, many studio strobe units have widely different flash durations. Let's look at some of the reasons why.
Pack & Head Systems
Even though I made the exposure after she was in his arms, her feet had very noticeable motion blur.[/caption] Large pack and head systems vary power by changing the number of capacitors connected to the flash tube. This is called capacitor switching. Lowering the power on these units will give you a shorter flash duration. As an example, my decades old Speedotron 2400 wattsecond power pack with a Speedotron 102 flash head has a duration of 1/225 second at 2400ws, but when switched down to 600ws the goes to 1/670. For example, I recently used this system to photograph a dancer jumping into the arms of another dancer. Even though I made the exposure after she was in his arms, her feet had very noticeable motion blur.
Monolights
A typical small monolight flash that doesn't mention IGBT in its description varies the voltage of the capacitors (where the energy is stored to fire the flash). This is called voltage lowering and these units will have a longer flash duration as the power is lowered. Color temperature will also be lower (more yellow) as power is lowered and the tail of the flash is extended. You can compensate for the color change via white balance settings when working on your raw files.
Speed Lights
Modern speed lights use IGBT (insulated gate bipolar transistor) technology. These units have a constant voltage, but are able to shut off the flash tube (called tail trimming) when the desired amount of light has been emitted. Flash duration shortens as the power is decreased. Cutting off the tail of the flash helps these units to freeze motion. The color temperature of a flash is affected by the voltage and current. A flash starts off more blue and becomes more red as it tails off. The color temperature of the flash is an average of the blue to red range. As the voltage is constant in the speed light the color temperature goes higher (more blue) as the power is lowered by cutting off the red that is in the tail of the flash. This is the reason digital cameras offer a Flash white balance preset, they tend to be more blue than daylight. With studio strobes the color temperature is more like daylight and the Daylight white balance preset is more neutral. Some photographers will select the Flash preset, though, as that will warm up the color of the light, similar to selecting a shade white balance.
IGBT Monolights
Recently IGBT technology has been improved to allow it to work with larger, studio style monolights to shut off the flash tube to control the power output. These systems combine the tail cutoff with control of the capacitor voltage to maintain color consistency . One brand of IGBT flash offers both a color and an action mode. In the action mode the flash color temperature will rise (more blue) when power is reduced to give shorter flash durations to freeze motion. At full power the action and color modes should produce the same flash duration. You need to go to half-power (or lower) to get the faster flash duration. As I started out saying, flash duration in most situations probably won't matter all that much. For portraits, headshots, family photos, and the like there isn't much movement going on. Even some movement is easily tolerated at a 1/300 second flash duration. However, it should be noted that any bright objects that might be moving in the scene will require a faster flash duration to be frozen by non-IGBT flash units. Think back to the chart of the flash duration. It quickly rises and slowly falls off. As it falls off (gets dimmer) the midtones and shadows no longer reflect enough light to register, but bright objects like sequins on a dancer's costume or jewelry the subject is wearing will continue to be bright enough to register during the latter stages of the flash's tail. IGBT units cut off the tail of the flash, so there is no light for the sequins to reflect.
High Speed Sync (HSS)
What about high speed sync (HSS)? First note that HSS is not available on all studio lights. It is only a recently introduced feature in studio lighting (though it has been around for a while in speed lights).High speed sync lets you use your flash at a higher shutter speed than the normal sync speed (normal sync varies by camera model, but is typically between 1/160 and 1/250 shutter speeds). The primary use of HSS is to be able to make photographs at wider apertures when using the flash outdoors in sunlight. It can also be used indoors to lower the output power of your flash if it cannot be powered down enough to give the aperture you want. For example, if you have your flash turned down all the way but it is still giving you f/5.6 and you want to use f/2.8 you can go into HSS mode and the power of the flash will drop about a stop for every doubling of your shutter speed above the normal sync speed. So, if your camera syncs at 1/200 and you are at f/5.6, you need to open up two stops to get f/2.8, so try setting your shutter speed to 1/800 to see if that brings the flash power down. Notice that there was no mention of freezing motion in the last paragraph. HSS is not a solution for action stopping photography. HSS works by putting out a rapid succession of low powered flashes in sync with the moving shutter blades in your camera. As you are probably well aware, a flash unit usually needs a second or two (or more) to recycle between flashes. In HSS the flash is firing multiple times within your 1/800 shutter speed. So the flash output power is cut way way way down to that it can recycle instantly. Yes, these are short duration flashes, but there are multiple flashes that may show some blurring in a fast moving subject. I have had discussions with photographers who say they are so grateful for high speed sync so that they can make photographs of a liquid pouring from a bottle into a glass." My bet is that they didn't even try it at normal sync speeds. Advertising photographers have been making such images for many decades before HSS was ever a possibility. All high speed sync is doing in this situation is driving the flash unit harder, using batteries and lowering the life expectancy of the flash tube while putting out a lot less light (requiring a higher ISO or a wider aperture).
The solution to freezing motion is to work in a darkened studio setting using a flash that offers a fast flash duration at or below the normal sync speed of your camera. You have to think about flash photography as making a double exposure within one exposure in the camera. You have ambient light that is affected by the shutter speed, the ISO, and the aperture. And you have the flash which is affected by the aperture, the ISO, and the power of the flash unit. In the studio we can cut out the ambient light and that takes the shutter speed out of the equation. The aperture is selected to give the depth of field that you want. So that is a set value and is also removed from the equation. You usually want to work at the lowest available ISO for image quality--100 or 200. So that is set, too. So your variable is the power of flash and the ability to freeze motion is also from the flash. With the studio darkened, you can have a shutter speed of 1 full second or 1/2 second or 1/200 second and nothing is going to be recorded on the film or sensor until the flash fires And that flash brightness determines the exposure and it is exactly the same at any of those shutter speeds. And the flash duration stops the motion. This is the way all flash photography worked in the early days of photography. Lenses didn't have timed shutters, itwas either closed or open. To make a flash photo you opened the shutter, you lit a fuse to trigger a tray of flash powder, the powder exploded lighting the scene, and then you closed thelens. Film speeds were so slow then (ISO 6 or even lower) that this could even work in daylight conditions. Unless there is so much ambient light available to affect the scene, the shutter speed for flash photography doesn't matter. To be safe, work in the 1/60 to 1/160 range of shutter speeds.
