Exposure Mechanics

This is a discussion on Exposure Mechanics within the Exposure (ISO, Shutter Speed, Aperture) forums, part of the Photography Tips category; I've heard it explained before that shutter speed controls ambient, and aperture controls flash. I personally think that's a little bit oversimplified. For example, aperture ...


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  1. #1

    Join Date
    Mar 2011
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    I've heard it explained before that shutter speed controls ambient,
    and aperture controls flash. I personally think that's a little bit
    oversimplified.

    For example, aperture has to contribute to the exposure (to some
    extent) even when no flash is being used. Big hole, more light. Small
    hole, less light.

    But I'm curious as to why flash is able to get through to the sensor
    at high shutter speeds when something as bright as the sun cannot.
    What is it about flash that allows this to happen? In theory, what
    would happen if ambient matched flash?

    Is there nothing "natural" that is as bright as a flash?

    What is the color temp of flash?

    What are the foot candles of flash?

    What are the foot candles of the sun?

    Am I making sense?

    Thanks in advance.

    G.


  • #2
    Nikon Shooter
    Join Date
    Aug 2010
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    Eatontown, NJ
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    PHOTO EDITING OK
    First, you need to understand a bit of physics as it relates to photography.

    The duration of a pulse of light from your flash can be anywhere from 1/1000 of a second to 1/20,000 of a second. Since most camera's fastest shutter speed during a flash exposure is 1/250 or 1/200 of a second, basically, the shutter will be open before, during, and after the flash. It therefore has no effect on the exposure of the actual flash. This is because the shutter needs to be fully open to expose the entire sensor (or film) to the short duration flash.

    So- the only way to control the amount of flash light hitting the sensor is by opening/closing the aperture. This is why they say aperture controls flash exposure.

    Yes, the amount of ambient light is also being controlled with your camera settings, simultaneously with the flash exposure. But think of your normal dark area where you are taking a flash exposure, at say ISO 400- maybe f/2.8 @ 1/15. So, you set your camera to f/8 @ 1/100- this is a full six stops underexposed (for ambient light) and take your picture with flash. If the flash doesn't fire (you forgot to turn it on), you have a completely black picture. Shoot again with the flash on- nicely exposed.

    If ambient matched flash you would get a nicely exposed background and an overexposed subject (if the subject were in the same light as the background). If the subject is in shadow and you expose for the background, then light with flash to match the same exposure, you'll get a nicely exposed subject and background.

    This is the whole theory behind The Strobist blog... (http://strobist.blogspot.com) - read his Lighting 101 and On Assignment links and you will be enlightened!!!!

    Typical sun exposure- the good ol' Sunny f/16 rule... At any ISO, exposure on a bright sunny day is f/16 at a shutter speed of 1/ISO. So with ISO 100, f/16 @ 1/100 second. I can easily overpower that with a strobe, even a small one.

    Color temperature doesn't enter into this, as that relates to color quality. But usually around 5000K. Very close to a sunny day.

    I don't know foot-candles for strobes... they're usually measured in guide numbers (relates to aperture and ISO and distance to subject) or Watt-Seconds. What is the guide number of the sun?

    You're kinda making sense. I think you're confused on the first question because you didn't know about flash duration and how it relates to the mechanical shutter in a camera- but I'm just speculating on that.

    Did this help?

    One other thought... when I approach a studio type setting, or at least one where I get to set up and plan the shot a little these are the steps I take to get my exposure:
    - Set the camera to manual and adjust my ISO for the situation- indoors, usually ISO 400.
    - Set the shutter speed to my camera's (D5000) fastest flash sync speed of 1/200 second. Your camera may be different. Refer to the manual.
    - Set my aperture to get a decent background exposure. Using the fastest flash sync speed helps here because it will require the largest aperture to get proper exposure. So I've built in my equipment's largest "leeway" on exposure.
    - Take a test shot for the background. Adjust aperture and shutter speed (can only go slower first time through).
    - Turn on my strobes- experience helps here to set the initial power level. Usually around 1/2 power though.
    - Take a test shot. Check the picture and check the histogram. While this isn't a perfect method, am I close? Look for detail where I want/need it.
    - Adjust the power on the strobe to expose my subject. Was I too bright? Lower power. Not bright enough? More power or move the lights closer.

    Of course, the other method is to shoot with your strobes set to full TTL (instead of manual power) if that is an option with your equipment. TTL does a lot of the legwork. If I'm in a hurry and moving around (so scene and lighting is constantly changing) I'm fortunate enough to have the Nikon CLS system, and SB-900 and an SB-600. TTL is absolutely amazing with this equipment.

    I should add that all of this really works best if you can get your strobes off camera.

    So those are my thoughts.

    Regards,
    Marlo

    Check out my blog! --- http://marlomontanaro.wordpress.com


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