A Little Ditty About Flash Exposure Compensation

Discussion in 'Photography' started by ceg4048, 8 Aug 2008.

  1. ceg4048

    ceg4048 Expert/Global Moderator Staff Member

    Messages:
    8,952
    Location:
    Chicago, USA
    This is a follow on to our discussion about f-stops and exposure in this recent thread:=> viewtopic.php?f=14&t=2459

    To recap about the definition of exposure, we'll remember that the "correct" exposure is one in which the combination of aperture size and shutter speed duration captures just the right amount of light to show the subject's brightness as a nice average brightness. This isn't always possible as nature doesn't always cooperate. Taking pictures at night or in dim light often requires artificial light. We can use a flash to help but flashes tend to make harsh images.

    There are a variety of ways to soften the flash's light, such as bouncing, light boxes and so forth but there is a basic way in which you can actually tell the flash not to be so bright and harsh. This is called flash exposure compensation and all that means is that the flash lowers it's energy output.

    Unless you are taking a picture in pitch black conditions, normally there is at least some light falling on the scene, so we want the flash to add in just enough light to augment what is already there. There is an automatic feature called "fill flash", but we want a bit more control.

    Remember that if we start at an f-stop of f5.6 and then open up to f8 this is a 1 stop decrease in light transmission and this 1 stop halves the light entering the camera. A further reduction to f11 means that 1/4 of the light is recorded. In this same the flash lowers or raises it's output by halving or doubling it's duration or intensity. Most flash units have this ability to "compensate". A -1 compensation lowers the intensity by the equivalent of a full stop, while a +1 raises the intensity by a stop. Here is a sequence of shots showing normal power, -1, -2 and -3 which is the limit on this unit.

    Normal Flash Exposure. When the flash is mounted the camera defaults to a shutter speed (you can select this value in the menus) normally 1/60th of a second. The camera and flash put their heads together to determine how much intensity and what duration to fire the flash. This is a perfect exposure for this scene. But that's because it's uniform.
    [​IMG]

    The flash unit is set to -1 compensation. This means 1 full stop less and half the light. This is still not bad and cuts down a lot of reflection and glare while still keeping nice saturated colors.
    [​IMG]

    This is -2 compensation. This means 2 full stops less and a quarter the light. Things have gotten muddy and we've lost details in the darker areas.
    [​IMG]

    This is -3 compensation. This means 3 full stops less and eight times less light from the flash. Now it's completely murky but this illustrates what a three stop difference looks like.
    [​IMG]

    The compensation increments are actually in 1/3 stops so I can use + or - 1/3, 2/3, 1, 1.3, 1.6 and so forth to the upper and lower limits. I used the full stop increments because it's easier to see what's happening. These fractional increments makes it easier to fine tune the effect you want.

    There is one more option you can use with the flash. There is a setting on your DSLR called "Rear Sync".
    Normally the flash fires the moment the shutter opens. This causes the light from the flash to dominate the photo, but the result is that the objects near the flash are well lit and everything else goes dark. In Rear Sync mode the camera forces the flash to hesitate. The shutter speed, instead of being automatically set to 1/60th is allowed to be slower so the light from the scene can be recorded first. Then, as the shutter is about to close, the flash fires. This gives you both the flash light and the ambient light. Here is what the mask looks like in Rear Sync mode. Compare the colors to the first shot. The differences are subtle but you can see that the ambient light is more yellow so the wood of the mask as well as the rope have an nice amber tone. It's worth playing with this feature.
    [​IMG]


    Here is another example of full flash mode versus compensation using my favorite sculpture Mr. Bojangles. The first picture is full mode and looks harsh. In this shot I actually don't want to show the rest of the elements of the scene, but full flash gets light in places I don't really want and creates harsh shadows.
    [​IMG]

    This is -2.6 flash compensation and it hardly looks like I used a flash. I've isolated Mr. Bojangles and obscured the background. Of course the image can be considered a bit dim so oone could play with the compensation values to get the look one wants.
    [​IMG]

    Hope this sheds some light on the subject. :D


    Cheers,
     
  2. aaronnorth

    aaronnorth Member

    Messages:
    3,955
    Location:
    worksop, nottinghamshire
    thanks Clive really helpful, shame i camt try it :lol: :lol:

    i understand ISO etc etc now ;)
     

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