Post on 15-Feb-2020
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.