Shutter Speed

Controls the exposure time. It must be reasonably fast. 1/15 sec, 1/30, 1/60, 1/120, 1/250, 1/500,

… The slower shutter speeds need good

camera support, like a tripod. You can shoot close-up portraits of people who can hold still at 1/4 sec, but it’s risky. Better to use 1/8 sec and faster if your subject doesn't remain still..

Slow shutter speed reveals motion, shown particularly by the dancer's blurred hands.

Low light levels require a slow shutter speed for proper exposure.

A medium shutter speed is all that is needed when the subject is not moving very fast.

A point and shoot camera will automatically select a fast shutter speed in bright sunlight

Pin-point sharp, stop-action photography is made possible by very fast shutter speeds, but focusing must be accurate

A fast shutter speed was used to freeze this bike rider in mid-air.

Must be able to expose all parts of the film equally throughout the exposure Between-the-lens shutter: near the lens Focal-plan-shutter: expose the film strip

by strip, can get deformed images of the rapidly moving objects.

Two types of shutter

Stops or Aperture

An opaque barrier with a hole — a device to limit the width of a light beam.

By varying the diameter of the aperture, you control the amount of light passing through the lens into the camera to expose the film.

A large aperture (or wide lens opening) lets in more light than a small one, and can deliver the same brightness from a dimly-lit subject as a small aperture will deliver from a brilliantly-lit subject.

f-number

f-number = focal length / diameter of stop’s hole

example: f = 50 mm, d = 12.5 mm

Then f/d = 4, which is written as f/4.

For a fixed focal length, the f-number tells you directly the size of the stop.

For the same f-number, the image has the same amount of intensity.

It seems contradictory but is important for you to remember that a larger ƒ-stop number designates a smaller lens opening, and vice-versa. For example, an aperture of ƒ/22 is a

very small lens opening, whereas an aperture of ƒ/1.4 is a

relatively huge opening, and will let a great deal more light pass through the lens.

f-number

The bottom row of numbers (16, 11, 8, etc.) on this lens shows the aperture settings of the lens, which is currently set at ƒ-5.6. The numbers are

inscribed on an adjustable "aperture control ring."

An aperture of f/1.4 is a wide opening!

An aperture of f16 is a small opening!

F-number and intensity of light

If you increase the f-number by a factor, you cut down the light by the square of that factor. The area is proportional to diameter

squared. Double the f-number, get ¼ of light!

Why f-number involves the focal length? For a fixed stop, larger the focal length,

less the intensity of the light.

a change of f-number to the next in the sequence corresponds to a factor of 2 change in light intensity,…0.7, 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32,…

Example:What is the difference in light intensity

betweenf2 and f22? f2 lets in more light, f22 lets less, the difference is (22/2)2 = 121

F-number sequence

Exposure For a given lighting condition, the exposure

is the product of the light-intensity times the exposure time, where the light-intensity is related to the f-number. Light intensity can be measured using exposure

meter. If film receives the correct amount of light, the

resulting picture reproduces the subject in all of its proper tones and colors. Such a picture is said to have been "properly exposed."

One can get the same exposure by increase the f-number and the exposure time by the same factors.In a bright sunny day… f/11 and 1/60 sec f/8 and 1/120 secGives the same exposure If an object is moving, you want to have

shorter exposure time.

Choosing Exposure

High shutter speed froze this explosion's debris in mid-air

A small aperture provided this image with plenty of depth of field

Film

Chemicals which react when exposed to light Silver compound of chlorine, bromine or

iodine(silver halides), discovered in 1830s

Light strikes the silver halide (crystal), breaking chemical bond and yielding metallic silver Nuclei Latent image: distribution of the nuclei.

Positive and negative images If the film is developed so that the final

image is white where the original subject is bright, we have a positive image.

Two methods of development Physical development (Dagurre and

Talbot, used in photocopiers) Chemical development (what we used

mostly) Fixing the image

Using hypo to wash away the unexposed halides.

H & D Curve

How to record the color between white and black (gray)? Use crystals of different sizes. Large

crystals are easily exposed by weak light and small crystals are only active at very strong light.

The reaction of film, or the density of the nuclei is a function of the exposure. The relation can be shown by a curve, called H & D curve.

Speed, Contrast and Latitude

Speed.The initial rise (toe) of the H&D curve represents measure the speed of the film. Lower the exposure at the toe, faster the film.

Contrast: the slope of the curve determines the sensitivity of the film to the different exposure. Larger the slope, bigger the contrast.

Latitude: How long is the steep part of the curve? It represents the range of exposure.