Understanding Exposure

Three things control exposure
• Shutter speed 
• Aperture
• ISO (ASA) is the “film” speed
Reciprocity - In photography, reciprocity is the inverse relationship between Shutter Speed, Aperture, and ISO

Shutter Speed (how long the shutter is open)
Varies from camera to camera
30 sec. to 1/8000 sec.
Choice of shutter speed is usually determined by the speed of movement within the scene
Can be used to stop the action (fast shutter speed)
Can be used to create motion effects (slow shutter speed)

Aperture
Aperture is the opening in the lens
Varies from lens to lens
Measured in f-stops
Small f-stop = large opening
Large f-stop = small opening
Aperture size determines depth of field

Depth of Field
How much of the image (from front to back) will be in focus
Determined by Aperture
Small aperture = greater depth of field
Large aperture = narrow depth of field
F22 (smallest opening) = greatest depth of field
F2.8 (largest opening) = narrowest depth of field
Depth of field is used in controlling “selective focus”
Depth of field is affected by the distance from the lens

ISO
Range varies from camera to camera
Choice of ISO is determined by scene lighting or speed of action
Increasing ISO adds noise to the image
Use the slowest (lowest number) ISO whenever possible

Reciprocity
Reciprocity is used to explain the F/stop to Shutter Speed to ISO relationship. f16 at 1/125sec 100 ISO is the same as f11 at 1/250sec 100 ISO is the same as f8 at 1/500sec 100 ISO...

It's the total AMOUNT of light that counts
The interaction between the Aperture and the Shutter Speed determines the volume of light hitting the film. The aperture and shutter speed settings go in "opposite" directions. If you have a tiny opening, you need to have the shutter open a very long time to allow enough light in to properly expose the shot. Conversely, if you have a large opening, you need the shutter open for a short time to let in the right amount of light in to properly expose the shot.

Compare this to a faucet. If you have the valve opened only slightly (a tiny aperture), the faucet needs to run for a long time (shutter speed) to fill the sink. And if the valve is opened all the way (large, or wide-open aperture), the faucet needs to run for far less time in order to fill the sink.

Aperture, Shutter Speed, ISO Reference
So now that you have a better understanding of aperture, shutter speed and ISO how about some real world reference.

Shutter Speed

• Slow shutter speeds less than 1/60 second can cause camera shake and blurry images; use a faster speed, an IS (Image Stabilized) lens, or a tripod.

• To exagurate motion, use shutter speedless than 1/30 of a second.

• Use fast shutter speed (e.g. 1/1000 sec or greater) to freeze motion.

• Possibly use a slow shutter speed and a technique called panning to show motion.

Waterfall

Pan Blur

Fast Shutter Speed

Stars

Aperture

• Large apertures e.g. f2.8, f4.0 are generally used when you want the background (behind your subject) to be out of focus.

• Small apertures e.g. f11, f16 are best used whenever you want most of your scene to be in sharp focus.

• Large apertures (e.g. f2.8, f4) are often used for portraits photography and food photography.

• Small apertures (e.g. f11, f16) are often used to photograph landscapes, and anything that requires a wide area of acceptable focus. 

large d

large depth

Shallow depth

Shallow

ISO

• ISO 100-400 for outdoors

• ISO 400-1600 for indoors

• ISO 1600 or greater for when you need to record as much ambient light as possible with a faster shutter speed.

• At ISO 400 you may begin to see noise.

• At ISO 3200 you might find the noise to be overwhelming the image.

• There are no hard rules!

iso noise

Bracketing
Bracketing exposure means taking one frame at indicated setting, one frame at 1-stop under-exposed, one frame at 1-stop over-exposed.
Bracketing shutter speed:
1/60, 1/125, 1/250 sec.
Aperture stays the same.
Bracketing aperture:
f5.6, f8, f11
Shutter speed stays the same.