Week 7 - 15 Questions

Light and Lenses

1. The speed of light is medium dependant. Which means it changes speed depending on what it
is travelling through. Does it get slower or faster travelling through denser mediums. Yes or
no?

It travels slower and can be calculated by the refractive index of a material. Glass is about 1.5
so the speed light travels is c/1.5

2. If light enters glass at an angle other than 90 degrees what happens to it? Explain.

It bends due to refraction. As light slows entering glass it changes direction. At 90 degrees it
is still refracted but does not change direction.

3. Light travels in waves. What is the measurement for this?

Wavelength – visible light is within the range of 350-790 nm

4. What is the degree of the refraction of light dependant on? And which colour is refracted the
most?

The degree of refraction is dependant on the refractive medium and its wavelength. Violet is
refracted the most.

5. What is the angle of incidence?

The angle at which a light ray approaches a medium.

6. What is the refractive index of material?

A number that is used to calculate the speed that light traverses a medium. Glass is about 1.5
so the speed light is traversing it is c/1.5

7. What is the difference between a convergent and divergent lens?

In a convergent lens the light rays are bent inwards and cross at the focal point. In a divergent
lens the light rays are bent outwards.

8. What is the focal length of an optical system?

The distance from the front of the first lens through to the focal point.

9. What are circles of confusion?

A lens that is focused on an object does not render everything sharp. Depth of field will show
some things as sharp and some things as blurred. The circle of confusion are those parts of the
image that are no longer sharp as perceived by the human eye or the largest blur spot that the
human eye would still see as a point.

10. What is the angle of view of a lens?

The angle of the scene that the lens will include in an image.

11. What is the covering power of a lens?

The usable area of the image circle a lens makes.

12. Do lenses alter perspective?

Yes – long lenses shorten perspective and wide angle increase perspective.

13. What is geometric distortion?

When lines appear to be bent. Common forms are barrel distortion or pincushion distortion.

14. What is the equation to calculate correct viewing distance?
And, PROBLEM to solve:
You need to make a vertical portrait using 24x36mm format which will be used on a roadside billboard
where the photo will be 5m high and it will be ideally seen from 50m. What lens focal length should
you use to maintain ‘correct’ seen perspective?
Given: image height = 36mm
print height = 5000mm (5m)
viewing distance (VD) = 50,000mm (50m)
Unknown: magnification (M) = print / image
= 5000 / 36 = 139x
focal length (f) = ?

15. For the following lens types, write a description of each
using no more than 10 words for each lens.

Wide angle – field of view greater than what a human eye sees

Zoom – a lens with variable focal lengths

Long focus – focal length greater than the diagonal of the sensor

Telephoto – lens is shorter than the focal length

Retro focus – regular focal length with wide angle attachment

Quasi fish eye – image covers the entire sensor

Fish eye – field of vie greater than 100 degrees

Catadioptric - mirror used to increase focal length but maintain small overall length

Macro - produces images sizes greater than 1:1

Teleconverter – increases focal length of lens with a 1 stop drop normally

Supplementary or close up lens – accessory attached to front of lens to alter focal length

PC or tilt shift lens – perspective correction lens