Transcription

1

2 What does a lens do? Do I need a lens?

3 Pin hole camera no lens! Focal length 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

4 Pin hole camera no lens! Focal length Big hole = blurred image

5 Pin hole camera no lens! Focal length Small hole = sharp image

6 Pin hole camera no lens! Focal length Very small hole = blurred image. due to diffraction

7 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* * 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

8 Pin hole camera Wide angle

9 Pin hole camera Telephoto

10 Pin hole camera Super zoom!

11 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 Image plane Light from The scene Lens Image plane Pin hole Dim, low contrast image Bright, high contrast image

12 How does a lens work? Everyone knows a prism bends light:

13 How does a lens work? The amount of bend depends on the angles of the faces

14 How does a lens work? Imagine trimming the top and bottom of prisms:

15 How does a lens work? Like this:

16 How does a lens work? Now assemble them into a stack:

17 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.

18 So why does a camera lens look like this: And not like this:

19 Remember the prism analogy? A simple lens focuses slightly differently for each color: sensor This is chromatic aberration and is one example of lens aberrations (distortions and image degradation)

20 Example lens aberrations: Chromatic aberration

21 Example lens aberrations: Barrel distortion

22 Example lens aberrations: Pin cushion distortion

23 Example lens aberrations: Coma

24 Example lens aberrations: Field curvature

25 Example lens aberrations: Spherical aberration

26 Example lens aberrations: Vignette

27 So a camera lens looks like this: And not like this: Because.

28 It contains lots of glass to correct for all the aberrations More correction = more complexity = more $$$

29 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)

30 Focal length 28 mm (wide angle)

31 Focal length 180 mm (telephoto)

32 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

33 Focal length

34 Focal length Big sensor = wide angle image

35 Focal length Medium sensor = standard view

36 Focal length Small sensor = telephoto image

37 Diameter F-numbers Focal length F-number = Focal length = Diameter f D

38 F-numbers The F-number determines: Image brightness (not the same as exposure*.) Image depth of field * - more on this in a bit.

39 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)

40 F-numbers Two lenses, same f-number The shorter focal length clearly collects less light, so why is the image brightness the same?

41 F-numbers Because the shorter focal length produces smaller image features Look at these examples:

42 F-numbers Two lenses, same f-number 28 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 180 mm, f/2.8 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

43 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

44 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/ /2 1/ /4 1/ /8 1/ /16 1/ /32 1/5.6

45 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

46 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

47 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

48 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

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

50 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

51 Exposure Rules of thumb 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. ISO setting Shutter speed 100 1/100 (1/125) s /1000 s

52 Depth of Field The lens F-number also controls the depth of field: 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.

53 Depth of Field The depth of field is shown on the lens: 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

54 Center Edge MTF diagrams Modulation Transfer Function. Best Thick lines = contrast Thin lines = resolution How well the lens reproduces an image Black lines = wide open Blue lines = f/8 Worst Dashed lines = concentric Solid lines = radial Distance from center (mm)

55 Resolu on or contrast (L/mm) MTF diagrams As a function of F-number: /3 format sensor Full frame (35mm) sensor Medium format sensor F-Number Lens gets better as it is stopped down Then gets worse as diffraction begins to dominate Sweet spot gives best resolution and contrast

56 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

57 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

58 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

59 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.

60 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

61 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

62 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

63 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.