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    Flash photographybasics: every common

    question answered

    By jmeyer

    05/02/2013

    TTL? Rear Curtain Sync? Guide Number? All the flash photographybasics you need to know are right here, as we answer all thecommon questions to all the common problems photographers haveabout using flash, as well as share some of our best flashphotography tips.

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    Image by Rolfo Rolf Brenner

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    Sometimes my SLRs flash pops up and sometimes it doesnt.Why?

    When youre in fully automatic green square mode, the small flashbuilt into the majority of DSLRs will activate automatically when lightlevels are low.

    However, in the advanced P, A, S, M modes you can pop the flashup manually whenever you like by pressing the button marked with aflash symbol.

    This is useful for adding a little extra illumination, such as abrightening up an outdoor portrait. Bear in mind that youll have to

    stick with a shutter speed that isnt faster than the maximum flashsync speed.

    Maximum flash sync speed?

    Despite the light from a flash being emitted very rapidly, you cantfreely choose the fastest shutter speeds your camera offers whenyoure using a flash.

    This is due to the way in which the two shutter curtains inside atypical SLR work. These open and close to allow the imaging sensorto be exposed to light.

    When you press the shutter-release button, the mirror flips out of theway and the first curtain opens to allow light to hit the sensor. This isfollowed by the second curtain, which closes across the sensor to

    end the exposure.

    At very fast shutter speeds, the second curtain starts closing beforethe first curtain has finished opening so the image is recordedthrough a rapidly moving gap rather than all in one go.

    Because the burst of light from a flash is so brief, it would only berecorded on the narrow part of the sensor thats clear of the curtainswhen the flash fires, and the exposure would be uneven across thepicture.

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    The flash sync speed is the fastest shutter speed at which theshutter is fully open, ensuring that the exposure is even across theframe.

    Historically, this has been 1/60 sec, but many current DSLRs takeadvantage of the fact that flash duration is extremely short and offerhigh sync speeds of 1/250 or 1/500 sec.

    Ive heard that I should use the built-in flash as a last resort. Is

    this true?

    Yes and no. The trouble is that the small size of the flash unit and itsposition on the camera means that the light it produces is direct andharsh.

    Other limitations include the risk of red-eye, due to the closeproximity of the flash to the lens, and the fact that you need to be

    fairly close to the subject, as the Guide Number of a pop-up flash iscomparatively low.

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    Whats the Guide Number?

    The Guide Number (GN) is a measure of the maximum power of a

    built-in flash unit or a flashgun the bigger the number, the morepowerful it is.

    The Guide Number = distance x f/stop at ISO100. We know it allseems to be getting complicated again! Knowing the Guide Numberis useful if youre shooting with manual flash, because it helps youcalculate the aperture you need to set, or the distance the flashneeds to be from the subject in order to get a good exposure.

    However, these days, exposure can be taken care of automatically

    by TTL flash metering.

    TTL flash metering? Things really are getting complicated

    TTL or Through the Lens flash metering actually makes thingseasy! The latest intelligent versions, such as iTTL on Nikon and E-TTL on Canon, fire one or more brief flash bursts or pre flashes arefired before the main flash.

    The camera registers the amount of light returning through the lens(hence the name), and then works out the right amount of light topump out for the main flash exposure.

    However, just like normal exposure metering it can be fooled by

    dark, light or reflective backgrounds, and you may need to adjustthe flash exposure to compensate.

    How do I alter the TTL flash exposure?

    The camera will automatically control the flash exposure forconsistent results in TTL mode, no matter what aperture or ISO orshutter speed (as long as its slower than the flash sync speed) youchoose.

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    That is, as long as the flash has enough power to reach the subject.If youre too far away, your flash-lit subject will appear too dark, andyoull need to move the flash closer, choose a wider aperture and/orincrease the ISO setting.

    If you want to fine-tune the flash exposure, the easiest way is to useflash Exposure Compensation (FEC). Look for the button markedwith a zig-zag flash symbol and +/-. See the previous page foralternative ways to control the brightness of your flash.

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    Controlling flash exposure

    Three quick and easy ways to ensure perfect results by adjustingthe exposures of your flash pictures

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    1. Flash Exposure Compensation

    This allows you to adjust the output from the flash, making the

    subjectlit by flash either brighter (positive compensation) or darker(negative compensation). Flash Exposure Compensation can be

    applied on the camera, or on a dedicated flashgun attached to thecamera.

    2 Flash Exposure Lock

    Some cameras (like Canons 7D) have an FEL (Flash ExposureLock) or FV (Flash Value Lock) button. Ensure the main subject is inthe middle of the picture, then press the Flash Lock button. Theflash will fire without an exposure being made, but the exposuresetting will now be locked in. Simply reframe and take the picture.

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    3 Flash Exposure Bracketing

    If timing isnt crucial, try taking a series of exposures at a range ofFlash Exposure Compensation settings to improve the chances ofgetting the perfect ratio of flash to ambient light. Advanced flashgunsand cameras offer Automatic Flash Exposure Bracketing.

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    How to get flash in sync

    Slow Sync is useful when shooting in low light, allowing a longexposure to be combined with a brief burst of flash. Rear CurtainSync allows you to fire the flash at the end of an exposure, ratherthan at the start.

    Normal flash

    Automatic camera modes default to a flash sync speed of 1/60 sec,which is too short to allow much ambient light in at night.

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    Slow Sync flash

    By using slow-sync flash, a longer exposure can be used to allowambient light to register on the sensor as well. Use a tripod too.

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    Front Curtain Sync

    In a long exposure, subject movement recorded after the flash fires

    will appear as a trail in front theyll seem to be travellingbackwards!

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    Rear Curtain Sync

    The results look more natural, but its harder to predict where the

    subject will be when the flash fires at the end of the exposure.

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    Flash photography cheat sheet what you need

    from a flashgun

    Now that weve broken down some of the misconceptions aboutflash photography, lets take a look at what you need from adedicated flashgun.

    In our flash photography cheat sheet below weve illustrated some ofthe most useful features for photographers and what you can dowith them.

    Simply click on the infographic to view the larger version of thischeat sheet, or drag and drop to your desktop.

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