Flash Photography Primer Mar 4 09

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Text of Flash Photography Primer Mar 4 09

    • Why Use Flash?
    • Stop action.
    • Can freeze motions that are too fast for the eye to see.
    • Lots of light to give good depth of field and fill in shadows caused by sunlight or other bright light sources.
    • More comfortable than working with hot lights.
    • Same color as daylight.
    • Lots of power/light can be obtained from a small package.
    • Portable.
    • Controllable with light modifiers.
    • Working with Flash
      • Guide Numbersare a way to determine correct exposure.They incorporate the sensitivity (ISO or EI) of the film or sensor, the distance from the flash to the subject, the power output of the flash and the desired f/stop.Typically, guide numbers are used to calculate the exposure for flashes with manual power level settings.
      • UV (ultra violet) correctionis usually in the form of a coating or filter placed on or in front of the flash tube.This correction minimizes or eliminates the effects of ultraviolet light that the flash tube may produce when it flashes.
      • Recycle Timesrefers to how long the flash will take to charge up and be ready for the next flash.This can range from instantaneous to several seconds depending on a number of factors.
      • Color Temperature .Most electronic flash units are rated between 5,000 and 5,800 Kelvin.This is basically the same color temperature as the sun on a cloudless day between 10:00AM and 3:00PM.
    • Working with Flash
      • Flash Duration .Rather than changing the amount of power dispensed into the flash tube, usually the duration of the flash is adjusted while the output level of the flash remains constant.Flash durations can range from several hundredths of a second to several hundred thousandths of a second. Reciprocity Law failure should be considered in these cases.When using an automatic or TTL flash close to a subject with a large aperture and a relatively fast ISO/EI, flash durations can be extremely short.This can be very helpful in stopping very rapidly moving subjects.If the light from the flash is the only source illuminating the subject some spectacular results can be obtained.Harold Edgerton did a great deal of pioneering work in this area and is often considered the father of electronic flash.
    Photos on this page are provided courtesy of the estate of Harold Edgerton
  6. 7. A FLASH PHOTOGRAPHY PRIMER The Inverse Square Law Light intensity falls off with the square of the distance Example 1: A single light in a dark room will cast 4 times the amount of light on a subject when it is 2 feet from the subject as it will when it is 4 feet from the subject. Example 2: A single light in a dark room will cast 4 times the amount of light on a subject when it is 5.6 feet from the subject as it will when it is 11 feet from the subject. Example 3: A single light in a dark room will cast 4 times the amount of light on a subject when it is 16 feet from the subject as it will when it is 32 feet from the subject. ( Do you see a pattern emerging???Hint:Think in terms of f/stops.)
  7. 8. A FLASH PHOTOGRAPHY PRIMER Watt/Seconds vs. BCPS Watt/Seconds is a measurement of stored electrical power.BCPS (Beam Candle Power Seconds) is a measurement of actual light output. The reflector and its finish (polished, stippled or matte), shape, and size have a direct effect on the amount of light that is output from a flash head.Hence, you can achieve different BCPS output by changing the reflectors and/or light modifiers with the same amount of watt/seconds.
  8. 9. A FLASH PHOTOGRAPHY PRIMER How to Use Guide Numbers The key to using flashbulbs (or any manual flash system) is the concept ofguide number . The guide number expresses the amount of energy contained in the flash in a way directly useful to the photographer, and relates distance covered to lens f-stop, as follows: F = G / Dwhere F is the lens f-stop, G is the guide number, and D is the distance. Whereas for electronic flash (strobe lite), the guide number depends only on the film speed, for flashbulbs a guide number is stated for a certain film speed, shutter speed, and film sensitivity (B&W or color).The reason that shutter speed enters the equation is that a flashbulb flashes over a relatively long period of time, and shutter speeds of faster than 1/30s cut off some of the light from the bulb.
  9. 10. A FLASH PHOTOGRAPHY PRIMER How to Use Guide Numbers On just about any hot-shoe flash capable of manual, there's a guide number calculator built in.
  10. 11. A FLASH PHOTOGRAPHY PRIMER How to Use Guide Numbers The four flash exposure variables are: F/stop, distance, power and ISO. You plug in any three, and the calculator spits out the fourth.Play around with your buttons a bit and you will see how yours works. What I like to do is to already know my ISO, my desired shooting aperture and an estimated flash-to-subject distance. Now, by setting up my GN calculator, I just dial in the different manual power settings until my desired f/stop lines up with my flash-to-subject distance.
  11. 12. A FLASH PHOTOGRAPHY PRIMER This GN calculator is set forfull power manual at ISO 200.It is telling you that, at30-40feet, you would get aboutf/4out of this flash. And if you set the flash to 1/2 power, you'd get f/2.8 out of it at that distance. Here's the cool thing: If you zoom the head -- even on this old-design flash -- it will move the dial and adjust the result. Full power 200 ISO
    • When the flash is triggered, a burst of light leaves the flash head and travels to the subject.
    • Some of the light is absorbed but some will reflect off the subject and return to the sensor window on the flashs automatic sensor.
    • When the circuitry in the flash senses enough light at the sensor to yield the desired exposure it will instantly cut off the light being output by the flash head.
    • With a thyristor flash (like the one pictured below) any unused stored electrical energy will be conserved thereby keeping recycling times as short as possible.
    How does an automatic flash work?? S
    • There are three types of flash meters:
      • Incident
      • Reflected
      • Spot (a reflected light meter that measures very small areas (spots) of the scene before it)
    • Flash meters can be triggered by the actual flash of light emitted by the flash unit(s).Additionally, you can trigger the flash unit(s) with the meter by connecting them to the flash meter with a sync cord.
    How to use a flash meter
    • To use a flash meter, follow the steps below:
    • Select the triggering mode you wish to use.
    • Set the ISO or EI into the meter.
    • If using an incident meter, hold the meter at the subject with the receptor (a white dome) pointed at the light source(s).
    • If using a reflected or spot meter, aim the meters receptor (light sensing area) at the subject from the camera position or at an 18% gray card that is illuminated by the same light that is falling on the subject.
    • Take a reading and transfer the settings to your camera and lens.
    • Expose.
  14. 15. A FLASH PHOTOGRAPHY PRIMER How to use an Automatic flash Most automatic flashes have multiple auto setting options.The one pictured to the right has four.They are color coded yellow, red, blue and purple. In the example to the right, a yellow setting on the flash sensor will give correct exposures with the lens set to f 2.8 between 50ft and 4.5ft.A red setting will give correct exposures at f5.6 between 25ft and 3ft from the subject.Blue would be good @f11 from 12ft to 1ft and purple f16 between 8ft and 1ft.
    • Syncro-sun or flash fill
    • When flash is the only light source it is the duration of the flash that serves as a shutter speed. The actual camera shutter speed is not a factor with electronic flash under these conditions.
    • As the ambient light level increases to the point where it can affect the exposure, the camera shutter speeds DO become significant.It is in these conditions where it is helpful to understand how to utilize flash fill or synchro-sun techniques.
    • Procedure
    • If you are outdoors or in a brightly lit environment and want to use flash to lighten up (put detail into) the shadows, here is how to do it with an electronic flash:
    • Determine the exposure for the existing (no flash) light.You will get a shutter speed and an f-stop based on the amount of ambient light and your working film speed (EI/ISO). The shutter speed wont have an effect on the flash exposure (as long as its longer than or equal to the maximum sync speed of your camera/lens).The number youre interested in here is the f-stop.
    • Use guide numbers, a flash meter, automatic or TTL settings to adjust the flash output to give you the same f-stop you achieved in step 1.This will result in a one to one flash to ambient ratio.
    • By varying the flash f-stop relative to the ambient exposure f-stop you will be able to control the ratio of how much effect the flash has on the shadow areas.