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Exposure, Stops, EV, GN |
ISO film speed:
- Multiply by two to get a full stop faster (200 » 400)
- Divide by two to get a full stop slower (200 » 100)
- full stop scale: 100, 200, 400, 800, 1600, 3200, 6400
Shutter speed: A shutter speed of '30' -- is 1/30 of a second
- Multiply by two to get a full stop faster (30 » 60)
- Divide by two to get a full stop slower (30 » 15)
- full stop scale: 2, 4, 8, 15, 30, 60, 125, 250, 500
Aperture: An aperture of '8' -- is written as f/8:
- Multiply by 1.4 to adjust a full stop slower (8 » 11)
- Divide by 1.4 to adjust a full stop faster (8 » 5.6)
- full stop scale: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32
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What is Exposure? Proper exposure is a combination of factors:
- ISO film speed (eg: 200)
- shutter speed (eg: 1/100)
- aperture (eg: f/2.8)
The Key:
The key to understanding exposure is thinking in terms of a quantity of light
entering the camera and exposing the film, or sensor -- a change of 'one stop'
(or 1 EV) means twice (or half) as much light. This makes it easy to change
any two variables (one up a stop, the other down a stop) and still keep the
same exposure.
Same Exposure; Different Picture:
However, even though exposure may be the same, that does not mean that the
resulting picture is the same. And this is where photography turns
into an art. Fast shutter speeds can stop motion. Slow shutter
speeds can cause some objects to blur (like water in a waterfall). Wide
open apertures (f/1.4) create a very small depth of field, great for
portraits (face in focus; everything else blurred). Small apertures
(f/22) create a larger depth of field, great for landscape photography
(everything in focus).
Understanding Aperture:
The lens focal length (in mm) and aperture diameter
(in mm) are combined into a single 'f-number', the ratio of the lens focal
length to the aperture diameter. Namely, a lens at 200mm at "f/4" will have
an aperture diameter of 50mm (200/4). So, the larger the F number, the
smaller the actual aperture opening. A lens at 200mm at "f/20" will have an
aperture diameter of 10 mm (200/20).
Understanding Aperture '1.4' multiplier:
Because the aperture is a circular hole, the circle area determines how much
light passes through to expose the film/sensor. To double a circles area, the
circles diameter/radius changes not by two, but by the square root of two
(or approx 1.4). To understand, the area of a square (4×r2) is similar to
the area of a circle (PI×r2). A 10x10 square is 100 units. What do you
multiply 10 by to get a square with (nearly) 200 units? The answer is 1.4
-- So, 10×1.4, or 14 is 14x14 or 196 units -- (nearly) double the 100 units.
Aperture tip: Remember that aperture is written as f/#, which is
a reminder to you that increasing the aperture number (#), which is in the denominator
(the number under the divide sign),
decreases the size of the circular aperture opening (f/32 is a small;
f/2.8 is large).
Or, remember that Pinhole camera that you made as a child actually made
decent photos -- its depth of field was excellent. Maybe it had an
effective aperature of f/100, or maybe even f/333. So a very small
aperature hole in mm (large aperture number) means good depth of
field.
Flash GN: The GN (or 'Guide Number') for the flash built into the D300
is "17/56 (m/ft)", or 17 meters/56 feet at ISO 200. The formula for
calculating maximum flash range is very simple. Divide GN by f-number.
So, at f/5.6, the flash range is 56/5.6, or 10 feet. One way to
increase flash range is to use a higher ISO. As per the manual,
"For each twofold increase in ISO sensitivity, multiply the Guide
Number by 1.4". So, at f/5.6 and ISO 400 (200x2), the range is 14
feet (10×1.4), or at ISO 800 (200x2x2), 20 feet (10×1.4×1.4), and so on.
SB-800: The performance of the Nikon SB-800 external flash unit is
so much better than the built-in flash because of the zoom head built into
the SB-800. As zoom angle changes, so does the flash head inside the
SB-800. This allows the SB-800 to provide GN's from 17/56 (m/ft) at
14mm all the way to 56/184 (m/ft) at 105mm. And when combined with
a higher ISO, the range extends even further!
Exposure Metering: You are ultimately responsible: Most modern
cameras have really good automatic exposure metering, but they don't
always get it right. When that happens, it is up to you to correct the
exposure. Don't blame the camera! Most cameras can meter either (1)
the entire scene/frame, (2) mostly the center of the frame, or (3) just a
spot at the focus point.
'Whole frame' metering
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'Spot' metering
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So, if the exposure for a photo is wrong, it
is more likely that you were using the wrong exposure mode. So, switch
metering modes, and try again (and understand what part of the frame is
being used for 'metering').
 Wiki - Focal Plane Shutter
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