EXPOSURE
Imagine light is like water. To create an image you need to fill your image (your water-bucket) with a specific amount of light (water).
Not enough light will give you an UNDER-EXPOSED, too dark image. Too much light will give you an OVER-EXPOSED, too bright image.
The main challenge in photography is to put the exact amount of light needed to get PROPER EXPOSURE.
Like water in the bucket is expressed in liters units, light is expressed in STOPS units.
You can increase/decrease light capture by playing with 3 parameters:
SHUTTER-SPEED (Tv): Duration - How long you open the eye? Time you open the captor.
1 stop Incrementation = 1/1000s, 1/500s, 1/250s, 1/125s, 1/60s, 1/30s, 1/15s, 1/8s, 1/4s, 1/2s, 1s, 2s, 4s, 8s
FAST = less light SLOW = more light
Set first when: MOTION Blur (slow) or Freeze (fast)
But not too high: slow Tv requires tripod.
RULE= if handheld, never go slower shutter-speed than your lens length (with a 50mm lens -> never go slower than 1/50s).
APERTURE (Av): Quantity - How wide you open the eye? Size of the captor.
1 stop Incrementation = f/32, f/22, f/16, f/11, f/8.0, f/5.6, f/4.0, f/2.8, f/2.0, f/1.4, f1.0
NARROW = less light WIDE = more light
Set first when: DEPTH-OF-FIELD Bckgrnd blur (wide) or Sharp (narrow)
But not too high: wide Av gives a very small in-focus zone.
ISO: Sensitivity - How sensitive is the eye? Light sensitivity of the captor.
1 stop Incrementation = 100, 200, 400, 800, 1600, 3200, 6400
LOW-ISO = less light HIGH-ISO = more light
Set first when: AMBIENT LIGHT Night (high) or Beach/Snow (low)
But not too high: high ISO brings noise in the blacks in picture and burns the whites if too much sun.
As there is reciprocity of the 3 light controllers, Tv, Av and ISO are linked together, like in a triangle. If we decrease one stop on the Tv (from 1/60s to 1/125s) we have to add 1 stop on the Av or the ISO (from f/11 to f/8 or from ISO100 to ISO200) to get the same amount of light in the image (light-exposure). Extremes are hard to get (frozen move with sharp background). Facing a situation we determine what we want to emphases: motion, depth-of-field or ambiant light. We set the first controller, then we see how we can play with the 2 others to get back the proper amount of light.
Goal: keep the overall exposure (avoid under/over exposition) and maintain the different light qualities (intensity, quality, colors, white balance...).
FROM AUTO TO MANUAL
This trade-off to achieve exactly what your image needs is only achievable in MANUAL MODE where you can set each of the 3 parameters. In automatic mode the camera take a quick light-metering and plays it safe by deciding that the overall exposure should be middle gray (crossing fingers that there is an equal amount of whites and blacks in the image). It usually gives you an decent image, as long as you stay in a simple situation but anyway never let you any artistic choice to make (blur, mood...). In semi-automatic modes (Auto-Tv and Auto-Av) the ISO value is taken out of the equation (by getting automatic/fixed), the camera takes a light-reading and decides by itself the amount of light needed (to get middle grey), the camera lets you then choose the Shutter-Speed (mode Auto-Tv) or the Aperture (mode Auto-Av) and computes the other value to match the amount of light needed. The path of a beginner is to learn full-automatic, then semi-automatic modes, to then understand manual mode.
e.g.1: Runner frozen in motion with background sharp, we set fast Tv (freeze) and narrow Av (sharp Bkgrd) BUT both gives us less light, we HAVE to boost the ISO. If the day is very sunny it might be possible to increase ISO, if it's dusk we will get noise by increasing the ISO. We might need then to compromise and forget about getting a sharp background, to not have to go with extremely high ISO (noise).
e.g.2: Children on a swing. You want to freeze the action so fast Tv. It's winter and there's snow everywhere, you need to keep the ISO low not to burn all this delicate white everywhere. You HAVE to use a wide Av to get your light. That means you might not be able to get a sharp background.
e.g.3: It's dusk/night and you want to shoot a landscape. You need to start with high ISO, but you don't want noise in the black so you cannot go very high. You still need to get more light. You cannot really get a wide Av as you want depth-of-field in the landscape. You HAVE to slow the Tv. That's why you need a tripod.
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SUNNY 16 RULE = Outside on a sunny day, the blue sky is always exposed at f/16, ISO 100, 1/100.
Camera position in sunny16: to sky with sun in the shot (-1stop=f/22), to sky and sunshine (f/16!!), 90deg from sky/sidelite (+1stop=f/11), 45deg from sky(+2stops=f/8), against sky (+3stops=f/5.6)
A variant broads the rule to different weather = f/22=snow/sand, f/16=sunny, f/11=slightovercast, f/8=regularovercast, f/5.6=heavyovercast, f/4=openshade/sunset