The Sunny 16 Rule is a guide that photographers can use to judge proper exposure settings in bright sun without the use of a light meter. The rule was developed several decades ago before there was reliable metering for cameras. The Sunny 16 Rule basically states that when using a camera in bright sun with the aperture set to f16 then the shutter and ISO should share the same reading. That is to say if you are shooting at ISO 200 you would set your shutter to 1/200th of a second.
This rule can be extended further to adapt to more lighting conditions provided you know your f-stops. For instance if you are shooting in the early morning after the sun has cleared the horizon but before about 9-10 o'clock (depending on latitude) you would open your aperture one stop to f11, this would also be your setting on hazy days. If you are photographing in the shade then the f-stop would be moved down to f8, if it is deep shade or the day is overcast you would be opening to f5.6. If it is an early morning with heavy cloud cover you may need to open the aperture as far as f4. If you wanted to photograph in BMNT or EENT then you would need a lens that could open to f2.8 or even f1.4 depending on how close you are to sunrise/sunset.
So why does a photographer that rails against the use of light meters and The Big 3 find the Sunny 16 Rule, a rule based on the exposure triangle, useful? I am glad you asked. Photography is all about understanding light. You are in fact creating an image out of light, that is what your CMOS/CCD/film plate does. I have noticed a tendency in photographers that rely on light meters and the exposure triangle to be controlled by these tools, instead of controlling the light. What I find happens is a photographer gets a reading that "tells them what the exposure is" instead of asking the question "is this how I want the light to act?" Remember that the equipment is metering for An 18% Grey World.
It is important to remember that light and shadow create mood. "Shadow creates drama" is a common expression among photographs, but it is only half true. If you meter for an 18% grey world and don't have much shadow then your picture, while it may be pretty, probably won't be very interesting. On the other hand if you deliberately over expose your beach photos by one stop, open the aperture to f11, then you can convey the kind of blinding light one frequently experiences at the beach. Or on the other hand you can underexpose a bright sunny day and then introduce light to your subject with the use of flash to add dram to an image, a common technique used in fashion photography today.
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