http://free.wallpaperbackgrounds.com/photography/candle/68841-39891.jpg

• What does a lens do?

• Do I need a lens?

Pin hole camera – no lens!

• The small hole restricts the trajectories of the light rays – forces “point to point” transfer

• The smaller the hole, the sharper the image…. up to a point…. (more on this later)

• The distance between the hole and the image plane is the “focal length”

• The image is back to front and upside down (“inverted”), but this is true for any camera

“Focal length”

Pin hole camera – no lens!

• Big hole = blurred image

“Focal length”

Pin hole camera – no lens!

“Focal length”

• Small hole = sharp image

Pin hole camera – no lens!

“Focal length”

• Very small hole = blurred image……. due to diffraction

Diffraction

• Just like water, light “spills” past objects and sharp edges – this is diffraction

• Diffraction smears out the light rays in pin-hole cameras and blurs the image

• Diffraction is always present with pin-hole cameras (and lenses!) but only becomes important when the pin hole is very small*

https://johnvagabondscience.wordpress.com/2009/03/26/diffraction-wave-spreading-around-an-edge/

* How small? – When the hole diameter, d, is smaller than where f = the distance between the pin hole and the image and λ is the wavelength of light

Pin hole camera – Wide angle

Pin hole camera – Telephoto

Pin hole camera – Super zoom!

So what does a lens do?

• Allows images to be captured with MUCH wider apertures than pin hole cameras AND increases sharpness and contrast.

• Huge increase in light = faster shutter speeds and lower ISO • LOTS more benefits too…….

Light from The scene

Lens

Image plane

Bright, high contrast image

Light from The scene

Pin hole

Image plane

Dim, low contrast image

How does a lens work?

• Everyone knows a prism bends light:

How does a lens work?

• The amount of bend depends on the angles of the faces

How does a lens work?

• Imagine trimming the top and bottom of prisms:

How does a lens work?

• Like this:

How does a lens work?

• Now assemble them into a stack:

How does a lens work?

• Smooth off the edges and you have a lens:

• So a lens is a bit like a continuously varying prism….

So why does a camera lens look like this:

And not like this:

http://www.100open.com/wp-content/uploads/2011/10/magnifying-glass.jpg

A simple lens focuses slightly differently for each color:

Remember the prism analogy?

sensor

This is chromatic aberration and is one example of lens aberrations (distortions and image degradation)

Example lens aberrations:

Chromatic aberration

Example lens aberrations:

Barrel distortion

Example lens aberrations:

Pin cushion distortion

Example lens aberrations:

Coma

http://en.wikipedia.org/wiki/Coma_(optics)

Example lens aberrations:

Field curvature

Example lens aberrations:

Spherical aberration

Example lens aberrations:

Vignette

So a camera lens looks like this:

And not like this:

Because…….

It contains lots of glass to correct for all the aberrations

http://www.reflex-mania.com/anatomia-di-un-ottica/

More correction = more complexity = more $$$

Focal length • This determines the field of view

• Long focal length = narrow field of view (telephoto)

• Short focal length = wide field of view (wide angle)

Focal length

28 mm (wide angle)

Focal length

180 mm (telephoto)

Focal length • BUT the sensor size also matters, e.g.:

• 50 mm lens on a full frame sensor = standard view • 50 mm lens on a micro 4/3 sensor = mid telephoto

Focal length

Focal length

Big sensor = wide angle image

Focal length

Medium sensor = standard view

Focal length

Small sensor = telephoto image

F-numbers

Focal length

Dia

met

er

F-number = = ____________ Diameter

Focal length f __ D

F-numbers

The F-number determines: • Image brightness (not the same as exposure*….)

• Image depth of field

* - more on this in a bit….

F-numbers

• The image brightness depends only on the F-number

• These two lenses produce the same brightness! • The ratio of their focal length to their diameter is the

same (both are f/2.8)

F-numbers • Two lenses, same f-number • The shorter focal length clearly

collects less light, so why is the image brightness the same?

F-numbers • Because the shorter focal

length produces smaller image features

• Look at these examples:

F-numbers

28 mm, f/2.8

180 mm, f/2.8

• Short focal length – small diameter

• Collects less light

• Castle occupies only a small part of the image

• Castle light is squeezed into a small area

• Two lenses, same f-number

• Long focal length – large diameter

• Captures more light

• But the castle now occupies a larger part of the image

• The castle light is peanut-buttered over a much larger area

F-numbers • Why are F-number “stops”: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22…. • And not: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12……

F-numbers

• Because each “stop” represents HALF of the light from the preceding f-number

• i.e. f/4 gives half as much brightness as f/2.8 • i.e. f/5.6 gives ¼ of the brightness of f/2.8, etc….

• Half of the brightness corresponds to half of the lens AREA

F-number Relative lens area Relative lens diameter

1 1 1/1

1.4 1/2 1/1.4

2 1/4 1/2

2.8 1/8 1/2.8

4 1/16 1/4

5.6 1/32 1/5.6

F-numbers

• So each F-number stop represents the relative change in the effective diameter of the lens to give half the lens area of the preceding F-number.

f/1

f/1.4

f/2

f/2.8

F-numbers

• So each F-number stop represents the relative change in the effective diameter of the lens to give half the lens area of the preceding F-number.

f/1

f/1.4

f/2

f/2.8

F-numbers

• So each F-number stop represents the relative change in the effective diameter of the lens to give half the lens area of the preceding F-number.

f/1

f/1.4

f/2

f/2.8

F-numbers

• So each F-number stop represents the relative change in the effective diameter of the lens to give half the lens area of the preceding F-number.

f/1

f/1.4

f/2

f/2.8

F-numbers

• The F-number is controlled by a diaphragm inside the lens:

http://rweavernest.com/blog/2011/07/23/all-about-aperture/

Exposure

• Exposure is controlled by a combination of F-number and shutter speed (and ISO)

• For fixed ISO:

1/128 s 1/60 s

1/30 s 1/15 s

• “Sunny 16 Rule”:

• On a sunny day with the aperture set to f/16, the correct shutter speed is approximately the reciprocal of the ISO setting.

• “Looney 11 Rule”:

• For correctly exposed photos of the moon, set aperture to f/11 and shutter speed to the reciprocal of the ISO setting.

Exposure – Rules of thumb

ISO setting Shutter speed

100 1/100 (1/125) s

1000 1/1000 s

Depth of Field

• The lens F-number also controls the depth of field:

https://damienfournierdotco.wordpress.com

• Depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image. The depth of field depends on the lens focal length, the F-number, and the focus position.

Depth of Field

• The depth of field is shown on the lens:

http://www.luminous-landscape.com/tutorials/dep.shtml

• In this example when the lens is focused at 12 feet, everything will appear sharp from 6 feet to infinity for an F-number of f/11

MTF diagrams

• “Modulation Transfer Function”.

How well the lens reproduces an image

Worst

Best

Distance from center (mm)

Cen

ter

Edge

Thick lines = contrast Thin lines = resolution Black lines = wide open Blue lines = f/8 Dashed lines = concentric Solid lines = radial

MTF diagrams

• As a function of F-number:

0

10

20

30

40

50

60

0 4 8 12 16 20 24 28 32

Resolu

onorcontrast(L/m

m)

F-Number

4/3formatsensor

Fullframe(35mm)sensor

Mediumformatsensor

• Lens gets better as it is stopped down • Then gets worse as diffraction begins to dominate • “Sweet spot” gives best resolution and contrast

Types of lenses

• “Standard lens” • Gives a similar field of view as the human eye:

• 25 mm for 4/3 format sensor • 35 mm for crop frame • 50 mm for full frame • 80 mm for medium format • 150 mm for 4” x 5” field camera • 300 mm for 8” x 10” field camera

Types of lenses

• “Wide angle” • Gives an exaggerated field of view: • 10 to 24 mm for crop frame • 14 to 35 mm for full frame

• “Telephoto” • Gives a restricted field of view: • 50 to 600 mm for crop frame • 85 to 800 mm for full frame

Types of lenses

• “Macro” • Optimized for close focusing: • 1:1 is “true” macro • Image size on the sensor is identical to real life • Available from 50 mm to 180 mm focal length

• But also Extension Tubes and Lens Reversers

http://www.aliexpress.com http://www.polaroidstore.com

Types of lenses

• “Tilt-Shift”

• Shift lens up/down or left/right to correct for converging parallels

• Tilt lens to adjust the focus plane

• Hard to use but good for architecture and for “toy camera” effect. Can do much of this with software now.

http://www.usa.canon.com

Types of lenses

• “Zoom”

• Special lens that allows the focal length to be adjusted

• Standard “kit lens”

• Wide angle to telephoto (+macro)

• Most popular of all lens types

• Cheap zooms have variable F-numbers and poor image quality at maximum aperture

• Top quality zooms have fixed maximum aperture and image quality to rival (but not quite match) the best prime lenses

http://www.usa.canon.com

Image stabilization

• In-body image stabilization

• Sensor moves to compensate for image movement • Works with any lens • Can’t see the effect in the viewfinder unless the

viewfinder is electronic

http://asia.olympus-imaging.com/product/dslr/em5/feature3.html

Image stabilization

• In-lens image stabilizer

• Lens element moves to compensate for image movement

• Only works with lenses that have this feature

• Can see the effect in the viewfinder

• Electronic mapping

• Mainly point and shoot

http://www.usa.canon.com

Image stabilization

• All methods are capable of giving up to 4 stops of stabilization: • i.e. ¼ sec shutter behaves like 1/60 sec in terms of

blur.

• Turn off image stabilization when using a tripod, especially for long exposures at night.