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LECTURE 2: PERCEPTION. CAMERA AND EYE:
OUR LIMITS.
THE WAY PEOPLE PERCEIVE.
SOME BASIC HUMAN PERCEPTION SETTINGS.
Exact quantities vs. Changes : people perceive changes (contrast) in a better way; e.g. time, frequency, space or intensity.
PERCEPTION OF CHANGES: LIMITS.
- Frequency : Electromagnetical spectrum: involves both light and sound.
- Space : Size, distance, shape, texture… But things are not always what they seem.
- Time : Retinal persistence…
- Intensity : Weber-Fechner Law: the bigger the previous sensation, the higher change to be perceive (contrast).
PERCEPTION AND TECHNOLOGY.
- People receive stimuli: sound, light… (with some limits).
- Media technology tries to: imitate the way people perceive and taking advantage of this limits.
THE WAY PEOPLE SEE.
BASICS OF THE EYE.
- The eye gathers images: getting the light… Through the iris , pupil and lens.
- The eye fixes the image: Through the retina and fovea.
- The eye perceives the image: in the brain. Through an optical nerve.
- Process of sight: Cross the cornea / Pupil regulates the quantity of light entering / Lens (“cristalino”) leads the light to the retina to allow the focus / In the retina, is the fovea, are of most visual acuity and where the sharpest image is received / Also in the retina, are the “conos y bastones”, transforming the light stimuli in nervous impulses / The impulses arrives to the brain, where the image is interpreted.
THE WAY IS IMITATED WHAT PEOPLE SEE.
Term.
THE CAMERA OBSCURA.
A simple imitation of the process of the sight:
- Gets the image: through a hole.
- Fixes the image: on a surface.
THE CAMERA (BASICS).
Some light comes into some places for a period of time , over a surface , through a complex path.
- Light is captured by the diaphragm, sensor and viewer:
- The image is captured for a period of time , over a surface :
Term.
- The wavelength determines colors. Blue and violet has shorter wavelengths and higher frequencies; red and orange has longer wavelengths and shorter frequencies. Higher frequency = Higher energy.
LIGHT: PARALELISMS AMONG EYE AND CAMERA.
- Shutter = Eyelid : determines the quantity of time which allows light to enter the system.
- Diaphragm = Pupil : determines the quantity of light entering the system.
- Lens (“objetivo”) = Crystalline lens : leads the light to the focal area.
- Retina = Sensor : the focal area.
- The stereoscopic sight (separation between eyes) allows persons to appreciate distances, sizes, shapes, texture… and calculate positions and movements.
- Our sight is very limited: human visual acuity (among 1 and 2); temporal perception (to create movement in cinema)…
LIGHT CAN BE…
- Projected.
- Transmitted.
- Reflected : showing shape, size, texture, distance, movement, and color. The reflection of light is due to the bounce of it with a similar (but opposite) angle of the incidence of it.
Term.
- Refracted : The refraction is also a change in the trajectory of light, but it happens when it goes through one medium to another ( air to water ).
- (^) Absorbed.
BRIGTHNESS AND CONTRAST.
- Brightness : (reflected) light value of every point of an image. Can be distinguished though rods (eye), with less brightness rods have to work more.
- Contrast : difference between the maximum and minimum bright points; or two different color tones. Can be distinguished through cons (eye).
- Photopic vision : if we receive enough light to distinguish color.
- Scotopic vision : if we hardly receive light to distinguish bright.
- Differences of brightness can be perceived with the densitometry: zone system (Ansel Adams). But it is not a linear scale, not proportional :
Zone system and stops : Each stop down = Half of the light / Each stop up = Twice the light. It is based on film sensitivity (sensitometry): density (densitometry).
EXPOSURE.
The exposure is the amount of light per unit area (on an image). Correct exposure, then, is essentially the aperture setting that will best suit the scene brightness range. It is the quantity of light received by the film (photochemical) or the sensor (digital). The more accurate to the real scene, the better. The bright is the quantity of light reflected by the objects ( overexposed vs. underexposed ). The camera parts involved in exposure are:
Term.
- Higher shutter speed: lack of sequentially (image may have jerkiness, stroboscopic effect).
- Lower shutter speed: lack of definition (blurred movements). The Law of Reciprocity/Law of Bunsen-Roscoe shows that the iris aperture and the shutter speed are compensated at the time of exposing an image (ATTENTION: in video at 25 fps, should not be recorded at a shutter speed slower than 1/50, because of the blurriness. The rule is that the shutter speed should be 1/(double) of the fps ).
Term.
- Sensitivity/Sensor/Gain : control the relation between signal and noise (grain). ISO (ASA) : It is the post-exposure, in the sensor. It is light after being taken. In the eye: is what rods has to do when is not enough brightness. Collects the light and forms the image. In analog cameras, the sensor is the film where the light image was printed. The camera sensor is formed by a matrix of pixels sensible to light: each one collects the light information of a point of the scene. With a higher number of pixels, a higher precision of light representation details is possible. But the higher the ISO, the higher the sensitivity (risk of noise, grain; it depends on the ratio signal/noise ).
Gain measures the sensibility of the sensor; makes the image clearer, but at the sensor. Modifying this parameter can help to gain more luminosity in dark environments (although it should be the last parameter to be touched). It is measured in decibels (dBs). It is equivalent to the ISO in photography. The normal scales of noise (grain) are: low gain (0 dBs); medium gain (3 to 9 dBs); high gain (9 to 18 dBs).
Term.
HOW TO MEASURE REFLECTED LIGHT.
Each surface has a reflectance. There exist some tools to measure light coming into the camera:
- Camera light meter: different measurement strategies: point, center, matrix …
- Zebra stripes : only on digital cameras. Show areas based on brightness: 70% caucasic skin well exposed, 100% overexposed.
- Histogram: X axis (kind of tone) and Y axis (number of pixels with a certain value).
Term.
LECTURE 4: FOCUS: LENSES. FOCAL DISTANCE
(DOF).
CAMERA OBSCURA
(REMINDER).
- Gets the image.
- Fixes the image: on a focal plane. Problem: lack of luminosity.
EYE (REMINDER): HOW IT SOLVES THE PROBLEM?
With a lens , that REFRACTS the light, redirecting the light to a focal plane :
- Fovea : interprets the sharp part of the image; the central view.
Term.
First, make sure which is the real point of focus. Then, test the focus ring right and left, to find the focus limits (when circles of confusion are more visible).
DEPTH OF FIELD (DOF).
It is a range of real distance , in front and behind a point of focus, seen as focused on a scene.
THERE ARE THREE SETTINGS (AT LEAST) TO CHANGE THE DOF.
- Aperture (F number) :
- Less exposure = More (longer) DOF (e.g. F.8).
- More exposure = Less (shorter) DOF (e.g. F 2.8).
- Object distance :
Term.
For a same lens, the shorter the distance, the shorter the DOF (and vice versa).
- Focal length ( f number) :
More focal length: shorter depth of field.
Longer f , shorter DOF:
All are combined among them.
Term.
USE OF THE ZOOM.
For distance can be used a set of prime lenses of the same camera. It allows to change the focal length with only one lens (or more), through a control tool. All the lenses are inside a barrel.
Basic zooms work with a focusing lens , allowing to get a sharp image from a point of focus. When the zoom control is moved, another lenses are added inside the lens barrel. Its aim is getting a different focal length , but it does not change the point of focus ; it is only modified the DOF around the point of focus.
PRIME LENS VS. ZOOM LENS.
- Prime lens : has only one focal length shown on the lens.
- Zoom lens : has two values: minimum and maximum.
Is a prime lens useful?: The camera has to be moved back or forward; the angle of view cannot be changed. But lenses might have mistakes, called aberrations : zoom lens has a bigger number of lenses, so aberrations are more likely.
TYPICAL LENS ABERRATIONS (AFFECTING IMAGE SHARPNESS).
Term.
- Flare : brilliance or flash, originated inside the lens.
- Spheric effect : convergence in the lens´ borders.
- Chromatic : is noticed in the border of the objects, because of the different velocity of color frequencies among hugely different tones.
Term.
Part of the incident light is absorbed, while the other part is reflected. Light has different colors (it depends on many factors: the object surface, material, angle of reflection, texture…). The angle of bounce will be the almost always the same as the original ray, but at the opposite direction. Types of reflection :
- Specular : all or most of the light waves are reflected to the same direction.
- Diffuse : light waves are reflected to different directions or angles.
COLORS: DIFFERENT LIGHT FREQUENCIES.
Light = Wave = Frequency. Warm colors have less energy (lower frequency/higher wavelength), while cold colors have more energy (higher frequency/shorter wavelength).
WHICH WAVES ARE COLOR WAVES? WHICH SPECTRUM DO HUMANS
PERCEIVE?
Term.
LIGHT IN THE EYE.
Cones , on the retina, have a chemical reaction when they receive specific light frequencies.
- Photopic vision : cones distinguish colors (if enough light).
- Scotopic vision : rods distinguish brightness (if not enough light).
HUMAN EYE CHROMATIC SENSITIVITY.
Term.
