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Index 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
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 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 overall exposure.

Same overall exposure can result in different photo: However, even though exposure may be the same, that does not mean that the resulting photo 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/11) create a larger depth of field, great for landscape photography (everything in focus). To use a slow shutter speed on a sunny day, you will need a ND (neutral density) filter. Wiki - Neutral Density Filter

close subject at f/16 - subtle background blurring
close subject at f/5.6 - noticeable background blurring

ISO: The ISO (or 'film' speed) for a DSLR adjusts the amount of 'gain' used when reading values from the image sensor. High ISO can be incredibly useful in low-light situations -- to capture a subject without a flash, and without blurring. But the tradeoff is 'grain' (errors) in the photo. For example:

D300 ISO Comparison
(100% crops, high ISO noise reduction set to 'normal')

ISO 200
ISO 400
ISO 800

ISO 1600
ISO 3200
ISO HI1 (6400)

ISO HI1+1 (12800)
ISO HI1+2 (25600)
ISO HI1+3 (51200)

Lens diaphram creating aperture
Aperture: An 'aperture' is typically created by a physical diaphram located in the lens (seen right). An aperture is written as "f/#" -- and denotes the physical diameter of the aperture relative to to the focal length of the lense (so it is NOT a absolute size). By far the easiest way to see and remember this is how apertures are actually written: as "f/#", which means that the physical size of the aperture is the lens focal length ('f') divided by the specified number. So, with a 200mm lens at "f/4" means an aperture opening of 50mm (200mm/4). And a 50mm lens at "f/4" means an aperture opening of 12.5mm (50mm/4). So, when you see "f/4", think literally that means 'f' (focal length) divided by 4.

Understanding Aperture full stop '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 1.414). 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.
TIP: Blurring with low apertures will only happen on cameras with large (film-like) image sensors. Many 'point-and-shoot' cameras notoriously have 'everything' in focus (caused by a very small image sensor).
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 (photo below left), (2) mostly the center of the frame, or (3) just a spot at the focus point (photo below right)

Lockheed SR-71A Blackbird
'Whole frame' metering
  Lockheed SR-71A Blackbird
'Spot' metering
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'). Or better yet, just correct mild exposure variations in post processing.

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!

Wiki - Focal Plane Shutter

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