AlienBees ABR800 Manual Petunjuk Pengoperasian - Halaman 8

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Notes and Additional Info on Ringflash Specifications

True Ws, Effective Ws, Lumenseconds and Guide Numbers: Quantity of electrical energy is measured in Wattseconds (Ws), also known

as Joules. This rating defines the amount of electrical power discharged with each flash. The actual amount of light (Lumenseconds -

Ls) produced for a given number of Ws can vary, depending on the efficacy of the unit or system. In par ticular, older "box and cable"

systems, in cer tain configurations, often produced considerably less usable light per Wattsecond than do highly efficient systems. In other

words, a highly efficient modern 160 Ws unit might yield as much usable light as an older or poorly designed 400 Ws unit. This disparity

gave rise to the adoption of the informal "Effective Ws" term by some manufacturers of highly efficient units. The Effective Ws term

is currently discouraged by most professionals due to its arbitrary nature. Most modern units and systems from reputable manufacturers

now offer similar efficiency factors and may generally be compared with reasonable confidence on the basis of True Ws - but there still

remain cer tain low efficiency systems which will boast a high Ws rating yet deliver less light than one might expect. The specification

that should be universally used is "Lumenseconds" as this is the only term that actually describes light output. In the absence of this term

being widely adopted, the best comparison available to the photographer is the measured output with specifically defined real-world

accessories. Regarding the common misconception that Guide Numbers are a good basis for comparison, this is more affected by the

angle of the reflector used than by the actual usable amount of light available, and is of essentially no value to the studio flash user. For

example, a given studio flash fitted with a narrow 50° reflector will yield a much higher guide number than the same flash fitted with a

wider 80° reflector, yet the amount of light emitted is the same in both cases.

Flash Duration (the industry standard for expressing flash duration is referred to as "t.5"): This term defines

the length of time it takes for 50% of the total light to be emitted from the flash. Many users confuse this

with the exposure time setting on the camera, but they are not comparable specifications. The light output

of a studio flash is not a sharply defined on-off function like the camera shutter. Instead, the light intensity

trails off after firing the flash as shown in the graph. The light does not suddenly shut off at the t.5 point,

but continues to diminish over time. As can be deduced from the graph, a flash duration of, say, 1/1000

second will not stop action as cleanly as would a camera shutter speed of 1/1000 second. A more usable

figure for determining the stop-action capability of a flash system would be approximately double the

specified t.5 flash duration (approximately 1/500 second for the "t.5 = 1/1000 second" example given).

Sync / Trigger Voltage: When using digital cameras, it is best to check on the maximum sync voltage allowed for your specific camera.

Some brands of flash units have sync voltages much higher than ours, some as high as 400 Volts that can damage digital cameras. The

sync voltage on all of our AlienBees™ flash units is under 6 Volts, safe for use with digital cameras.

Typical Output Measurements: Typical Full Power exposure at 10 feet (ISO 100) with the standard ring reflector is approximately f16 at

a coverage angle of 80°. The readings may vary when taken in various environments (with various room sizes and conditions) and with

various flashmeters, as different brands of meters can vary by as much as 1 whole f-stop in their interpretation of "correct" values.

Power Range: The flashpower and modeling lamp output are continuously adjustable over a stepless 6 f-stop variability, with typical

accuracy and repeatability of plus or minus 1/10 f-stop. The output is adjusted steplessly from Full down to 1/32nd of the total power,

in whole f-stop increments and everywhere in between, adjusted with the slide fader located on the back control panel of the flash unit.

The slide fader shows marked fractions and f-stop increments, allowing you to adjust the output of your unit simply and quickly.

Color Temperature: Most pro studio flash units produce a "color temperature" between 5000K and 6000K. However, choosing the

"correct" color temperature alone does not assure fully accurate color rendition and neutral colors in a scene will still show slight

colorcasts, or deviations from true neutral. While it is the nature of Xenon flashtubes to produce excellent color rendition, there

remain small discrepancies in the Xenon flash spectrum that require post processing (in Photoshop or RAW) if perfect color balance

and accurate neutrals are to be achieved. This is also true of "color corrected" or "UV coated" flashtubes. For tunately, with digital

equipment, this is easily accomplished. The accessories used, as well as the shooting environment, are usually of more concern than the

color temperature of the light source itself. Light modifiers such as umbrellas and softboxes will often change the color balance of the

light by as much as 600K, as will the room colors - even if they appear to the eye to be neutral. These, too, can be easily corrected in

Photoshop or RAW. With the exception of "constant color" lighting equipment, studio flash units that employ variable output power

typically vary in color temperature by about 400K as they are adjusted from Full Power down to 1/32 Power. Even the use of these

"constant color" lights will not eliminate the effects of accessories and environments. When flashes are mixed with different accessories

it is rare that perfect or consistent color balance will be achieved in the camera unless a "custom white balance" procedure is performed.

Perfect color balance can also be achieved using the RAW camera mode where test shots are taken with a known-neutral object (grey

card or pure white card or background) included in the scene. Following the test shot, the neutral object can be removed from the

scene or, better, placed in a corner of the scene where it can later be cropped out. When the test shots or real shots are opened in a

RAW program, it is a simple matter to use the "eyedropper tool" to sample the grey card to achieve a custom white balance. This will

calculate a perfect color balance regardless of the effect of the power levels, accessories, etc. If this is done on a test image, the resulting

balance setting can be applied to all shots in a sequence taken at the same power levels and with the same accessories. Since it is

common in a photo session to change power levels and accessories, the preferred method is to include a grey card in each shot when

possible. A similar procedure may be accomplished directly in Photoshop when shooting either JPG or uncompressed modes.

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