Lecture 2-Light and the Camera

Question 1

How does film process light.

Answer 1

It uses a light sensitive emulsion (silver halide) attached to a polyester base, different layers of the emulsion react to different wavelengths of light.

Question 2

What are digital imagers sensitive to? And how does a bayer filter work?

Answer 2

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

Question 3

What is a bayer filter mosaic?

Answer 3

A bayer filter mosaic is a colour filter array for arranging RGB colour filters on a square grid of photosensors.

Question 4

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?

Answer 4

More light at f 2

Question 5

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?

Answer 5

1/1000 faster, faster lets in less light

Question 6

Does ISO 100 have more or less light than ISO 6400?

Answer 6

Less light, it’s much lower.

Question 7

Define illumination:

Answer 7

The amount of light falling onto a subject or area.

Question 8

What does the symbol ‘E’ represent, and how is it measured?

Answer 8

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

Question 9

What does the aperture (iris) do?

Answer 9

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…

Question 10

What is the f-stop and what is the

T-stop?

Answer 10

f-stop is a number based on design specification.

T-stop is a number based on actual measured transmission.

Question 11

If the illumination in a scene is 200 fc, how much light is hitting the imager with an aperture setting of f 2?

Answer 11

F 2 = 1/4 of the light transmitted

200 * 1/4 = 50 fc

Question 12

How much more or less light is hitting the imager if the aperture is changed from f 4 to f 11?

Answer 12

F 4 to f 11 is 3 stops less

1/2(^3) = 1/8th of the original amount of light is reaching the imager.

Question 13

What is sensitivity measured in?

Answer 13

ISO units

Question 14

What does a fast speed of film mean compared to a slow speed of film?
How do fast and slow films differ?

Answer 14

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.

Question 15

What does EI stand for?

Answer 15

Exposure index- the speed rating you choose- ISO rating

Question 16

How do you make an ISO rating for the imager lower or higher?

Answer 16

Lower: using filtration or pulling the exposure
Higher: pushing the exposure

Question 17

How do you push the exposure?

Answer 17

Amplification- a chemical process in film, by using gain to boost the signal in video.

Question 18

What is latitude?

Answer 18

The dynamic range of the medium in f-stops.

Question 19

What is the relationship of ISO to exposure?

Answer 19

Doubles, 6dB = 1 stop
ISO rating: 25 50 100 200 400
Rated sensitivity: X1 X2 X4 X8 X16…

Question 20

What is the relationship of parameters?

Answer 20

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)

Question 21

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?

Answer 21

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

Question 22

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?

Answer 22

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

Question 23

What does a camera lenses speed and length mean?

Answer 23

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

Question 24

What is a telephoto lens and a wide angle lens?

Answer 24

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.

Question 25

What us the Circle of confusion (CoC)?

Answer 25

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.

Question 26

What is the depth of field? (DoF)

Answer 26

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.

Question 27

What is the hyperfocal distance? (H)

Answer 27

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.

Question 28

What is the equation for hyperfocal distance?

Answer 28

H= f^2 / (f x CoC)
H= hyperfocal distance 
F= focal length of lens
f= f-stop
CoC= size of circle of confusion

Question 29

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?

Answer 29

At f 4: (100x100) / (4x0.03) = 83,333mm = 83.3 m

At f 16: (100x100) / (16 x 0.03) = 20,833 mm = 20.8m

Question 30

Two calculations for the depth of field:

Answer 30

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

Question 31

What are the DoF focus distances for a 100mm lens at f 4 focusing on a subject 2m away?

Answer 31

ND= 83x2/83+(2-0.1) = 1.96m
FD= 83x2/83-(2-0.1)= 2.05m

Question 32

What are the DoF focus distances for a 100mm lens at f 16 focusing on a subject 2m away?

Answer 32

ND= 1.83m
FD= 2.20m

Question 33

If an imager is larger what is the DoF?

Answer 33

Smaller (greater need for precise focus)