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MEDIA TECHNOLOGY
1. OPNENING A CAMARA
A camera is a rough imitation of the eye: the “camera obscura” (dark room)
Process Part of the camera Where the light comes to the camera (and how) Aperture Shutter How the sharpness of the image is defined Lens Where the image is composed Sensor (focus) Where the image is stored Card, hard disk, etc. (Storage device) Where the image is reviewed before and after the image is taken Viewer/Screen LIGHT IN A CAMERA BRIGHTNESS AND CONTRAST Brightness: reflected light value of every point of an image. Contrast: difference between maximum and minimum bright point or two different colour tones. ZONE SYSTEM AND STOPS Stops: changes of the light in the camera (opening up/closing down a stop). Each stop down is equal to capture half of the light Each open up is equal to capture twice de light SETTING BRIGHTNESS ON A CAMERA EXPOSURE The exposure is the amount of light per unit area (on an image). It is the aperture setting that best suits the scene brightness range. The exposure is defined by several settings: Aperture: diaphragm size Shutter: exposure time ND filters: decrease exposure Gain: sensitivity APERTURE: F NUMBER The aperture size of the diaphragm to let the light come to the camera. Every increase/decrease of the aperture is called a stop (f-stop).
F =
f
d
When you open up 1 stop, you are capturing twice as much light and when you close down 1 stop you are capturing half of the light. f: focal length d: iris aperture diameter
Apertures usually go by this sequence: F1. F1.2, F2, F2.8, F4, F5.6, F8, F11, F16, F22, F32. However, there are some camera that allow to open up or stop down fractions of a stop. For example: Each 1/3 stop: F2.8, F3.2, F3.6, F Each 1/4 stop: F2.8, F3.1, F3.4, F3.7, F
F = π × ( √ 2 )
x SHUTTER SPEED The shutter speed is how much time the shutter uncovers the lens. Every image is made by receiving an amount of light for a short period of time and it is fixed on the sensor while the diaphragm is covered. When you twice the speed you get half of the light coming into the camera and vice versa. Ex.: 1/120 = twice faster than 1/60 = half of the light = 1 stop down 1/30 = twice slower than 1/60 = twice the light = 1 *stop up Compensation: to get the same overall exposure level, the smaller the aperture, the slower shutter speed although the effects are not really the same. SHUTTER SPEED MATTERS Lower shutter speed means lack of definition (blurred movements) if there is something moving Higher shutter speed means lack of sequentially. Images may have jerkiness (stroboscopic effect) Default shutter speed: shutter speed = 1/twice the number of fps EFFECTS OF EACH LIGHT CONTROLS Shutter affects the movement on the image (blurring images, jerkiness…) Aperture affects how much depth of the image is in focus (next episode) ND FILTERS Neutral density (ND) filters allow you to decrease the amount of light received, therefore it allows you to use a bigger aperture and slower shutter speed. LIGHT AFTER BEING TAKEN: SENSOR On a digital camera, light is received on a sensor as a matrix of pixels. It transforms light signal into electrical signal. It is the same on the camera focal plane. SENSOR AND SENSITIVITY After taking an image in a camera, we can convert this image into a brighter one. This is called increase sensitivity – we add some extra electrical energy to each pixel the image is clearer. INCREASE SENSITIVITY ON A PHOTO CAMERA ISO (International Organization of Standardization), with both analogue and digital cameras, is the light sensitivity of either the film or imaging sensor. When you change the ISO on a digital camera, you’re rendering the sensor more or less sensitive to light. The higher the ISO number, the higher the sensitivity. ON A VIDEO CAMARA SENSOR
2. MAKE A SHARP IMAGE
Choose what part of your image should be clear, well defined and how. In a digital camera, the focal planes are on the sensor. This is at the back side of the camera. It is usually smaller than the real image. Light should be directed (“refracted”) to it. Light is redirected (refracted) to the sensor through a lens located at the front of the camera. It might be composed by more than one lens. PRIME LENS A camera may have a single lens prime lens (aka fixed lens) with a fixed focal length (distance form lens to sensor). The result is that a sharp image of the point of focus is received on the sensor and thus the image is on focus. FOCUSING Focusing is moving the focal ring to choose what part of the image is the point of focus. In other words, it is ‘telling’ the camera what the object’s distance is. Ideally, it means the light rays coming from any object that are refracted just to the sensor (neither before nor behind the sensor). Every lens has a minimum distance to focus, which is the minimum distance form object to lens. Closer than that, this lens does not allow to focus. If you are focusing on a right way, each point on the image will be seen as a sharp point. If you are not focusing on a right way, light rays are not coming to the sensor, but in front of it or behind it. Then, each point on the image is seen as a bigger and blurred circle circle of confusion (CoC) HOW TO FOCUS Next to the point of focus (ahead and behind), there is also a range of distance if any other object is inside this range, it seems also focused. This range is called depth of field: the distance (from a camera) before and after the point of focus where the image is also sharp
SEETINGS TO GET A DOF
APERTURE
The less the exposure the more DOF and vice versa. OBJECT DISTANCE For a same lens, the shorter the distance the shorter the DOF and vice versa FOCAL LENGTH (f) The longer focal length, the shorter DOF PDC: MORE PART BEHIND The are in-focus is mostly behind the point of focus. If you focus to infinity: 1/3 of focused area is ahead the point of focus 2/3 of focused area is behind the point of focus FOCAL LENGTH TYPES STANDARD: The focal length is the same size as the diagonal of the sensor. It gets a similar DOF to the human eye. WIDE ANGLE: Wide angle of view Objects look smaller Scene looks deeper You can place a camera very close to the object, although it might distort the image Image stabilization is easy It allows more open F numbers: it gives a lot of DOF, so you can lose part of it by using a bigger aperture TELEPHOTO: Narrow angle of view Objects look bigger Scene looks flatter and closer You must place a camera further to the object Image stabilization is tougher It needs closer F numbers to increase DOF, so you will need enlightened scenes
HOW TO FOCUS WITH A ZOOM
1. Consider a good placement to put your camera before taking a shot 2. Zoom in as much as possible to the distance object you want, and get the right focus Choose a point of focus (when image should always be in-focus) Zoom in to get the shortest focal length there = smaller available DOF – Thus, you must be more precise to find the point of focus Move your focus ring both to left and right side, to find the limits of the range. Where you do not get noticeable circles of confusion – Thus, you will make sure that the image is well focused 3. Zoom out to frame and shot You will not change your point of focus; you will only increase DOF. Never set up focus again!!!
3. COLOR
Color waves are electromagnetic waves. Color is related to the frequency of light. The lower the frequency the closer to red and the higher the frequency the closer to blue/violet. LIGHT COLOR Each light has a dominant color wave and some other added color waves. If you do not enough light you cannot distinguish color. You can create a same color with different combinations of other colors. A WAY TO DEFINE COLOR (“COLOR MODEL”) Each color can be defined through three variables (features): Hue (tone): length of the dominant wave. Ex.: red (700nm) Saturation: rate of white light used to produce this color o 0% of white light: very saturated. This color is not mixed with another white light source. o 100% of white light: very clear color Red: 0% of white light Rose: red + higher % of added white light Some objects or phenomena might add some white light: mist, light reflected on a window, the light angle of reflection… Luminance (= lightness): relation between light stimuli (intensity) and the light perception on our eyes ANOTHER COLOR MODEL (RGB) Red (R) + Green (G) + Blue (B) are primary colors. Mixing only two of these colors you obtain secondary colors. Mixing all these three colors, you can get any other color. *If we consider hue/saturation and luminance (Y) White: 100%Y = 30%R + 59%G + 11%B Red (luminance 30%) + Green (luminance 59%) + Blue (luminance 11%) HOW DO PEOPLE PERCEIVE COLOR? It is a question of light reflection. Part of the incident light is absorbed and another part of the light is reflected. The light has color. The color of an object depends of many factors (object, surface, material, angle of reflection, texture…) LIGHT INSIDE THE EYE Rods (bastoncillos) allow us to perceive brightness. Cones (on the retina) have a chemical reaction when they receive specific light frequencies perceive color.
HOW TO MAKE A WHITE BALANCE
- Make sure you have a well exposed image and choose the lightest part of the image
- Use a white object as a white piece of paper
- Close shot as much as possible, and unfocus. The camera operator should only see a white frame. Get a non-specular refraction.
- Find a memory on your camera, it will keep the white balance adjustment in the future.
- Click the white balance button. If the final result is white, ok. Otherwise, review the light (there might be too much light, or too few light), and repeat the process. SOME WB REMINDERS Some WB mistakes if you do not set it up properly o Balance indoor and shoot outdoor more blueish o Balance at noon, shoot at sunset more reddish o Balance in a sunny day, it turns overcast more blueish If you have more than a source of light o Try to avoid one of them o Or use a color filter for one of them With an orangish filter (if you want to decrease CT) With a blueish filter (if you want to increase CT) IDENTIFY COLORS Color model: a way to represent a subset of color values. It does not include the whole set of colors of a color space. Gamut: range of colors that a device is able to show YCrCb (o YUV) Consider an image, use RGB color model. Amount of luminance for each color to get white color: Y = 0,30R + 0,59G + 0,11B Using YCrCb color model, it only requires: o Get values of Y, R and B luminance o Hues of red (Cr) and blue (Cb) After that, to edit or play a video, G value might be calculated o Based on the aforementioned formula o Why? To save space when saving data about color on a video file
6. SOUND (I). A BRIEF INTRODUCTION
ENERGY, VIBRATION, WAVES
Energy makes something vibrate (produce sound). The vibration is transmitted through waves in some medium (matter), and it gets weaker and weaker. An ear is able to get them and they become electrical energy. THE EAR It amplifies the sound. It never disconnects. It can neither focus nor frame the sound, the ear perceives many different sounds at the same time. What we really perceive I not the stimulus (the waves), but a feeling based on it. We only distinguish sound if the waves are vibrating (into the ear) for a minimum amount of time: minimum 40 milliseconds and maximum less than a second. We can not accelerate sound. If so, we might likely not be able to perceive it properly. FROM SOUND SOURCE TO THE EAR SOUND TRANSMISION: The sound needs a matter to be transmitted. It will not be transmitted on vacuum. SOUND REFELCTION: Some sounds are directly transmitted from the sound source to the ear. But some other sound waves are reflected on the surface, so they come to our ears after. HOW THE EAR PERCEIVES DELAYS The Human ear may perceive delays of 50 mms among different waves from the same source of sound. It is called reverberation (slight persistence of the sound once the original source has stopped emitting it). Echo happens when the delay is so long that the waves from the same source are perceived as different sounds. SOUND ABSORTION Some materials convert sound into different types of energy (for example, heat), so part of the sound is not transmitted anymore. Soft, pliable or porous materials absorb sound on a better way (bigger coefficient of absorption) while dense, hard, impenetrable materials reflect sound. SOUND PROPERTIES FRECUENCY Low frequencies correspond to bass sounds and therefore have bigger wavelengths. High frequencies correspond to treble sounds and therefore have smaller wavelengths.
lenght =
frequency
Hertz ( Hz )= number of waves per second
The more bass audible sound for a human being is 20Hz The more treble audible sound for a human being is 16kHz. Some people raise it up to 20kHz The older you get, the shorter your range of audible frequencies FREQUENCY AND MUSIC: The frequency of a musical note is called pitch. Is not actually the same as frequency as it is a feeling.
7. SOUND SYSTEMS AND MICS
A healthy human ear may perceive (and distinguish) sound around you and create a spatial perception, but no sound system has been able to emulate this skill. SOUND SYSTEM MODELS Mono system: a single sound source reproduced. You use a unique sound track. You can only distinguish if a sound source seems farther or closer to you by its intensity. You cannot perceive transversal spatial position. It may be reproduced through two headphones if both are sending the same sound form from the same source. Stereo system: It allows you to create a sound space by using two different front speakers. Every speaker (headphone) sends sound from a different source of sound. They work together to make you feel spatial sensation through sound. You can create two spatial sensations: sound sources getting closer or farther and sound sources moving from left to right. You need two different recording tracks. Surround system (5.1): 3 front speakers, 2 latera speakers and 1 subwoofer (to magnify bass sounds). HOW TO PICK SOUNDS The very basic sound tools are pick up devices (ex.: microphone) and amplifiers. A pick up device gets sound that we really want to pick up and noise (undesired sound). There is no perfect microphone, it depends on any required use. HOW DOES A MIC WORK? A mic is a transductor. Sound (mechanical energy) hits on a membrane and it is turned into electrical energy. There are two main ways (working principles) to do so: moving coil principle and electrostatic principle. Moving coil principle: sound pressure hits a membrane and it is turned into electricity through a magnet and a coil. The higher the pressure, the louder the sound. Electrostatic principle: two plates separated, fed by power. It requires to add some voltage to each plate ( phantom , 48V). One plate is fixed and the other one is mobile. Sound pressure over mobile plate changes the distance between them changes the voltage. The higher sound pressure, higher change of voltage. Changes of voltage are recorded.
MICROPHONE SPECIFICATIONS
Dynamic range: the difference between the loudest and the quietest SPL which responds linearly. Or the loudest and the quietest SPL that a mic may perceive. Sensitivity: how much skillful is a mic (or a sound system) to get (or play) a sound. The more sensitive, the more amplified a same sound will be taken by a microphone (more sensitive = higher amount of electrical energy produced by a sound on a mic). It is measured in dBFS (digital) or dBV(analogue). These values are negative. The lower the number (without a sign), the more sensitive. Ex: -26dBFS is more sensitive than -40dBFS Signal/Noise relation: every sound recording includes noise from the sound source and from the microphone itself. Signal Noise Relation (S/R o SNR) shows the difference between the sensitivity of a microphone and the intensity of the noise that this microphone is capturing simultaneously. The higher a sound signal, the more likely the noise is also amplified. Frequency response: fidelity from the sound intensity to the sound really captured. Microphones do not always answer with the same fidelity to different sound frequencies (as happens with our ears) Directionality: polar response (in degrees) in which the microphone has a higher sensitivity. There are different kinds of microphones: o Omnidirectional: it picks up sound with equal sensitivity at all angles. Disadvantage: it picks up also noise from all angles and, if you have speakers close to it, it may cause feedback. o Bidirectional: it picks up sound coming from the front and the back of the mic, but it does not pick up sound from the left and the right. It may be good to record two sound sources at the same time. o Cardioid: its polar pattern looks like a heart. It is highly sensitive in front of the sound source but it is now sensitive at all behind it. This is a good choice for a handheld mic, as it avoids any sound from someone holding it. o Supercardioid: the front angle is more reduced than cardioids, but it also lets you pick up some sound behind it. This kind of mic should be held on a different support than a hand. It allows to pick up farther sound than cardioids. o Hypercardiod: similar to cardioid, but with a higher sensitivity area behind it Electrical power: there are three kinds of mics that mar, or may not need electrical power o Dynamic microphone: it does not require any electrical power; it is much less sensitive and it works worse with treble sounds. But it works better on noisy areas and it allows movement around difficult areas. o Condenser (or capacitor) microphone: it requires an external power supply, through batteries or through cable ( phantom supply). It has a smaller capsule. It is more sensitive, mainly to treble sounds. o Electret microphone: it includes an electrical charge inside, so it does not require any external energy supply, but if the charge ends you can not replace it. It is very sensitive and it is usually cheaper. Impedance: once the sound is transduced into electrical signal, it must be sent to a cable (or wireless). But the same microphone circuit has a resistance to send this electrical power = impedance. It is measured by ohm (Ω). Lower impedance (under 500 Ω) larger available cables.
XLR (Cannon): male connector gives sound and female connector takes sound Jack: female connector gives sound and male connector takes sound (usually).
8. DIGITAL SOUND
SOUND SETTINGS (for 1 second) Frequency: how many times a sound wave moves per second? Dynamics (dynamic range/SPL/intensity): What is the difference between the louder time and the quieter time of a sound wave? Channels: How many channels are we going to use? More channels more need of data SAMPLING – FREQUENCY Getting sound frequency for digital sound (a sample = 1 Hz) How many samples, at least, are required to sample a sound wave with different frequencies? Sampling rate MINIMUM SAMPLING RATE: NYQUIST FREQUENCY As a rule of thumb, the higher the number of samples, the higher sound fidelity is obtained. But the higher this number, the higher the required size to save the sound. The minimum sampling frequency is twice the sound frequency range Nyquist frequency ALIASING Effect of an inaccurate sampling rate. For instance, if you record with a sampling of 11,025 Hz the maximum frequency recorded is 5,512 Hz. All sounds over this frequency will be recorded as 5,512 Hz. It results in sound defects. QUANTIZATION – DYNAMICS Each digital audio sample is internally saved as a set of bits. The number of bits for each sample is called bit depth. Each bit more means twice the range of possible values for each audio sample. For instance: 1 bit corresponds to a sample audio among 2 values, 2 bits to 4 values, etc. All the audio samples on a same file are using a same bit depth. The higher the bit depth , the higher the dynamic range – the maximum SPL range available on any sampling. Every 1 bit extra ~ 6 dB bigger dynamic range. Ex.: 16 bit = 96 dB NUMBER OF CHANNELS Mono: 1 channel Stereo: 2 channels o Each channel works on a different way. o It means you have to double all the required information.
