Lecture 2-Light and the Camera Flashcards
How does film process light.
It uses a light sensitive emulsion (silver halide) attached to a polyester base, different layers of the emulsion react to different wavelengths of light.
What are digital imagers sensitive to? And how does a bayer filter work?
Sensitive to brightness, a bayer filter is used to separate different wavelengths of light onto different photo-sites in a bayer pattern, more photo-sites=higher resolution
What is a bayer filter mosaic?
A bayer filter mosaic is a colour filter array for arranging RGB colour filters on a square grid of photosensors.
If an aperture has a f stop of f 2, is there more or less light coming through the lens than if it has an f stop of f 32?
More light at f 2
If an exposure time is 1/1000 is that faster or slower than one of 1/8? And if it is faster does it let in more of less light?
1/1000 faster, faster lets in less light
Does ISO 100 have more or less light than ISO 6400?
Less light, it’s much lower.
Define illumination:
The amount of light falling onto a subject or area.
What does the symbol ‘E’ represent, and how is it measured?
E- the amount of incident light, measured in lux- metric (Europe and TV), fc (footcandle- US and film industry)
1 fc = 10.76 lux
1 lux = 0.093 fc
What does the aperture (iris) do?
Controls the amount of light transmitted to the imager. Measured in f/T stops.
Stops: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16…
Transmission: 1 1/2 1/4 1/8 1/16 1/32 1/64…
What is the f-stop and what is the
T-stop?
f-stop is a number based on design specification.
T-stop is a number based on actual measured transmission.
If the illumination in a scene is 200 fc, how much light is hitting the imager with an aperture setting of f 2?
F 2 = 1/4 of the light transmitted
200 * 1/4 = 50 fc
How much more or less light is hitting the imager if the aperture is changed from f 4 to f 11?
F 4 to f 11 is 3 stops less
1/2(^3) = 1/8th of the original amount of light is reaching the imager.
What is sensitivity measured in?
ISO units
What does a fast speed of film mean compared to a slow speed of film?
How do fast and slow films differ?
Fast= more sensitive, slow= less sensitive, fast film can record at lower levels of illumination but has larger grain and lower colour fidelity. Slow film requires higher levels of illumination but has smaller grain and higher colour fidelity.
What does EI stand for?
Exposure index- the speed rating you choose- ISO rating
How do you make an ISO rating for the imager lower or higher?
Lower: using filtration or pulling the exposure
Higher: pushing the exposure
How do you push the exposure?
Amplification- a chemical process in film, by using gain to boost the signal in video.
What is latitude?
The dynamic range of the medium in f-stops.
What is the relationship of ISO to exposure?
Doubles, 6dB = 1 stop
ISO rating: 25 50 100 200 400
Rated sensitivity: X1 X2 X4 X8 X16…
What is the relationship of parameters?
Fc= 25 x f^2/ t x EI Lux = 269 x f^2/ t x EI Fc = footcandles f = T= stop setting t = exposure time EI = Exposure index (ISO)
What T-stop setting is required to properly expose a scene in a feature film project with an illumination level of 384 fc using a Red Epic digital cinema camera?
Red epic is rated at ISO 800 Film is 24 fps, exposure time = 1/48 sec f = (square root) (fc x t x EI) / 25 (384 x 1/48 x 800) / 25 = 256 Square root of 256 = 16 Answer= T 16
What T-stop setting is required to properly expose a scene in a UK TV project with an illumination level of 1, 055 lux using a Sony F3 camera in progressive mode?
Sony F3 in progressive mode is rated at ISO 400 PAL is 25fps, exposure time = 1/50 sec f = (square root) (lux x t x EI) / 269 (1,055 x 1/50 x 400) / 269 = 31.38 Square root of 31. 38 = 5.6 Answer T 5.6
What does a camera lenses speed and length mean?
Speed is the largest aperture the lens has (e.g. T 1.8)
A fast lens has a larger maximum aperture than a slow lens.
Length is the focal length (mm)
Telephoto refers to a lens with a longer focal length
Wide angle refers to a lens with a shorter focal length
What is a telephoto lens and a wide angle lens?
Telephoto lenses compress the relative positions of objects based on depth. Wide angle lenses expand the relative positions of objects based on depth, extreme wide angles distort the image.
What us the Circle of confusion (CoC)?
Areas of the image that are rendered larger than the actual points (i.e. Blurred areas) the minimum circle of confusion represents a sharpness limit.
CoC is also a measurement of resolving power, imagers have different CoC values based on size, resolution etc. CoC values are used in calculations of depth of field.
What is the depth of field? (DoF)
The area along the Z axis that appears to be in acceptable focus, based on aperture, focal length of lens, object distance and CoC. The smaller the aperture, the greater the DoF, the smaller the imager size the greater the DoF. Use f-stops for DoF calculations.
What is the hyperfocal distance? (H)
The closest distance where both objects at infinity and close objects are in acceptable focus. When a lens focus is set to H everything from 1/2 of H to infinity will be in acceptable focus. H varies according to the f-stop, opening up (smaller stop) by two stops will double H. Closing down by two stops will reduce H by half.
What is the equation for hyperfocal distance?
H= f^2 / (f x CoC) H= hyperfocal distance F= focal length of lens f= f-stop CoC= size of circle of confusion
What is the hyperfocal distance for a 100mm lens on a camera with an imager with a CoC of 0.03 mm at f 4?
At f 16?
At f 4: (100x100) / (4x0.03) = 83,333mm = 83.3 m
At f 16: (100x100) / (16 x 0.03) = 20,833 mm = 20.8m
Two calculations for the depth of field:
ND= (HxS)/(H+(S-F)) FF = (HxS)/(H-(S-F)) ND= Near distance FD = Far distance S = distance from subject to camera H= Hyperfocal distance F= focal length of lens
What are the DoF focus distances for a 100mm lens at f 4 focusing on a subject 2m away?
ND= 83x2/83+(2-0.1) = 1.96m FD= 83x2/83-(2-0.1)= 2.05m
What are the DoF focus distances for a 100mm lens at f 16 focusing on a subject 2m away?
ND= 1.83m FD= 2.20m
If an imager is larger what is the DoF?
Smaller (greater need for precise focus)
