Viewing Distance and Speed of Light

Light Meters

With the invention of the in-built light meter, some photographers have laid down their hand-held meters and are opting to just point and shoot like cavemen, however they are forgetting the importance of this very versatile tool.  When on location, it is often not the most efficient use of time, fidgeting with external light meters; however in the studio it is another story.   A light meter can help you achieve the best tonal range in your photographs without having to change it with

Reflected and Incident Light

There are two ways of measuring the amount of light in a scene.  The first is measuring the amount of reflected light emanating from the subject.  This measurement is taken either from a single spot in the scene or an average metering of the reflected light of the scene.  This measurement is used a standard for the entire picture and will make calibrate the lights and darks of the image based on this standard.  Reflective light meters are the only type of metering system capable of measuring light from a distance and thus are the only type of meters built into cameras today.

The second method of measuring light is measuring the “incident” light, which is the light that is being shone on the subject.  This type of light measurement is far more accurate than measuring reflected light because incident light readings are independent of diluted light reflected off any subject which could, depending on the colour, have a higher than average level of reflectance.  For example:  A woman wearing a white t-shirt will have a greater level of reflectance than that of a woman wearing a black t-shirt.  An incident light meter will measure the light shining ON the t-shirt, not the light bouncing off it.

How it works

The average hand-held light meter is built with a plastic spherical dome placed on top of a light sensor that records the intensity of the light by calculating its proximity and its general luminescence.  These days they are built with computer chips to record the measurements taken and provide you with the measurement on a readable LCD screen.  In general, you are able to program the meter at a certain shutter speed, ISO and/or aperture size and the measurement taken indicates which f/stop or shutter speed you should use to get the correct exposure.  

Colour Fringing

When a ray of light is shone through glass, the light is bent at a different angle because the glass is denser than air, thus slowing down the speed at which the light is travelling through the glass.  This process is called “retraction”.   Light is made up of different coloured light, all of which are on different wavelengths and bend at different angles.  It is the job of the lens to bend these rays of light back to one point to create a sharp image, however sometimes not all the rays are focused completely at the same point as the others.  This can cause what is called “chromatic aberrations” in your image, which are certain colours “leaking” into other parts of the image.  Lens manufactures have developed ways of fixing these aberrations by using aspherical elements to bend stray light to the singular point. You should look at buying these types of lenses as they are sharper than most other lenses.   

Sensor Arrays

Consult Sheet Given In Class

Lens Types

WIDE ANGLE
24mm focal length or lower, fits more into the frame of the image (hence WIDE angle).

ZOOM
A lens that has the ability to change it's focal length (zoom in and out).

TELEPHOTO
Generally 70-300mm zoom, the telephoto lens magnifies the subject at the expense of the frame of vision and the depth of field.

RETRO FOCUS
Difficult to figure out... will revisit .

SUPPLEMENTARY LENS
A simple converging lens that screws on to the actual lens to allow closer focusing.

QUASI FISH-EYE
Extreme wide angle, captures image with all of image frame.

FISH-EYE
Extreme wide angle, captures image circularly within image frame

CATADIOPTRIC
Reflects light back and forth within lens to capture image.

MACRO
Allows very close range focal lengths.

TELECONVERTER
Secondary lens used to increase focal distance of primary lens.

ASPHERICAL LENS

PC LENS / TILT SHIFT LENS
Changes the plane of focus and the perspective.

LENSES CONTINUED (Focal length and Perspective)

As  a rule, lenses do not alter perspective.  In general, any distortion seen in an image is due to the image being viewed from the wrong distance;  view it too close and the perspective appears flat, view it from too far away and the perspective is exaggerated.

This can be proven optically by enlarging the center area of an image taken with a wide angle lens that corresponds with the same area photographed with a longer lens.

VIEWING DISTANCE

A comfortable viewing distance would be equal to a normal reading distance and can usually be calculated by measuring the diagonal of the print.  The correct viewing distance however could be calculated by the focal length of the lens used multiplied by the magnification of the image.

LENSES (refraction of light)

A lens can be thought of a series of prisms which combine to bring the light to a single point or "to focus".  Thus, lenses can be classified as one of two types: Convergent and Divergent:

A Converging Lens bends Incident Rays from infinity so that they intersect at a common point.
A Divergent Lens bends Incident Rays from infinity backwards so that they intersect at a common point. (in this lens, the glass is concave instead of convex)

We call the point at which the rays intersect the "Point of Focus"

REFRACTION (aka. the Pink Floyd Theorem.... or Snell's Law)

The Speed of Light = 299 792 458 m / s

However, through denser mediums such as glass or a beer, the speed of light is slowed down.  If it enters the medium at an angle other than 90 degrees, then the light is bent.  this is called REFRACTION

Refraction of light: 

Blue light is refracted more than Green or Red because these colours are all on different wavelengths.

BLUE >refraction GREEN >refrac RED

as demonstrated by Pink Floyd... 

As you can see red has the least amount of refraction, while green and blue are refracted more. 

REFRACTION INDEX (n) OF MATERIAL:

A figure that represents a material's ability to bend light by slowing it's speed .
Air = 1.00
Glass = 1.5 - 1.8
Beer = 1.34

Catch Up Blog

Exposure

Exposure is dependent on 4 MAJOR FACTORS

-          ISO rating

-          Subject lighting

-          Subject reflectance (also called the “level of reflectance”)

-          Your own personal preference

ISO Ratings

The ISO ratings in Digital SLRS originate from the concept of film sensitivity.  Film with a high sensitivity to light has a high rating.  Film with a low sensitivity to light has a low rating.

Before ISO there was:

DIN (Deutshes Institut fur Normung)

ASA (American Standards Association) – which ISO is based on.

From 1987 -  ISO (Internal Standards Organisation)

ISO ratings range from 25 to 3200 depending on the camera (Canon 400D = 100 – 1600).  A doubling of the rating makes the film (or sensor) twice as sensitive.

Eg:   ISO 400 is twice as sensitive to light as ISO 200.  Therefore, ISO 400 requires ½ the exposure as ISO 200.

Assume ISO 100 requires 125 at f.8, adjust the shutter speeds for ISO 800, 400, 100 and 50. *remember, 400 is MORE sensitive to light, therefore the shutter speed should be FASTER.

 200 = 2x125 = 250^th/sec

400 = 2x250 = 500^th/sec

800 = 2 x 500 = 1000^th/sec

50 =  125 ÷ 2 = 62.5 (round that down to nearest speed) = 60^th/sec

Adjust Aperture for ISO 800, 400, 100 and 50

If 100 = f.8

200 = f.11

400 = f.16

800 = f.22

50 = f.5.6

Digital Cameras use a sensor to convert light to an electronic signal.

ISO ratings are used, and same exposure theory applies, the difference is that film has a FIXED sensitivity and can’t be changed once the film is in the camera, whereas the sensor sensitivity can be manipulated by amplifying the signal.

A HIGH ISO can lead to image degradation.  In film it increases the image “grain”,  with a sensor it increases the image “noise”.

USE THE LOWEST ISO IN A GIVEN SITUATION

Level of Illumination

A high level of illumination allows use of a faster shutter speed and a smaller aperture.

A low level of illumination needs a slower shutter speed and a WIDER aperture.

Subject Level of Reflectance

Camera uses a “Reflected Exposure Meter”

-          Measures light reflected from the subject

-          Calibrated to always render the subject as mid-tone.

-          Auto or manual results in same rendering. 

CAMERA BASICS

Aperture (Av): (def.) - Hole/opening that varies in size (depending on how much light it is required to be let through).  It is controlled by a mechanism called the "Iris Diaphragm", which allows more control than in the past.  F. Stop is the term used to describe the size of the hole.

Smaller aperture sizes such as f/22 let in less light and allow for a greater depth of field (more of the shot will be in focus), whereas a larger aperture size such as f/1.4 will let in MORE light but will have a much shorter depth of field.

ON MY CAMERA (Canon EOS 400D), these are the Aperture Sizes available with the Kit Lens (smallest to largest hole):

f/29< f/25< f/22< f/20< f/18< f/16< f/14< f/13< f/11< f/10< f/9< f/8< f/7.1< f/6.3< f/5.6< f/5< f/4.5< f/4.0


Shutter (Tv): (def.) - Mechanism that begins and ends the exposure.  Different speeds let in different amounts of light and are usually expressed in seconds or fractions of a second.  
There are 2 different kinds of shutters: Focal Plane Shutters, which operate directly in-front of the film/sensor, and BTL (leaf) shutters, which operate inside the lens much like a diaphragm.

ON MY CAMERA the shutter speed times are as follows:

30"> 25"> 20"> 15"> 13"> 10"> 8"> 6"> 5"> 4"> 3.2"> 2"5> 2"> 1"6> 1"3> 1"> 0"8> 0"6> 0"5> 0"4> 0"3> 1/4> 1/5> 1/6> 1/8> 1/10> 1/13> 1/15> 1/20> 1/25> 1/30> 1/40> 1/50> 1/60> 1/80> 1/100> 1/125> 1/160> 1/200> 1/250> 1/320> 1/400> 1/500> 1/640> 1/800> 1/1000> 1/1250> 1/1600> 1/2000> 1/2500> 1/3200> 1/4000>

Exposure: (def.) - Total amount of light captured:

                Exp. = Time (shutter speed) x Intensity (aperture size)
                   = Tv x Av

HISTORY OF THE CAMERA

1267 - ROGER BACON (english photographer) writes about the "camera obscura" as depicted by 10th century writings of arab scholars. 

Camera: (lat.) "vaulted room" 

Obscura: (lat.) "Darkness" 

1490 - LEONARD DI VINCI (artist) describes the room in detail.

A Camera obscura was a room what was very dark with a primitive lens on the top which projects an image (using mirrors) onto a concave surface.  It was originally used to see what was going on around the towns they were erected.  See - Jerez, de la Frontera in Spain. 

1557 - GIOVANNI BATTISTA DELLA PORTA suggests its use as a drawing aide, and the first movable camera obscura appears. 

1676 - First reflex mirror camera produced. 

1685 - First Telephoto lens produced. 

1826 - First Permanant captured image in recorded history taken by JOSEPH NICEPHORE NIEPCE.  It was an 8 hour exposure, created with Bitumen of Judea on a Pewter Plate.  It was the view from his bedroom window.  The process he used (Heliography) has now been recorded as the first successful example of what we now call photography.

Later on, Niepce's collaborator LUIS DAGUERRE continued the work mapped out by Niepce and created what would become known as the "Daguerrotype", a style of photography common in the early 1800s.