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"