SLIDE 3 Aperture
– Light on unit area of sensor per second ) is proportional to size of the area. – Inversely proportional to the square distance to the sensor – Proportional to the square of the diameter of the
– Inverse square distance to the sensor (~ focal length f)
13
Area = π f 2N ⎛ ⎝ ⎜ ⎞ ⎠ ⎟
2
N = f D
- 2 x D (doubling the aperture), its area
(hence the light that can get through it) increases by 4X (because area)
- [same focal length]
- As the distance to the sensor is doubled,
the area intersecVng the cone increases by 4 so the light falling per unit area decreases by 4X [changing focal length]
Simple Geometry
§ Twice the diameter means four times the area. § Stop down à higher f-number § Fast lens allowing a low f-number à you can
use lower shutter speeds in lower light situation
§ Light captured by a lens is proportional to the
area of the aperture. (circle)
14
1 1 1 2
Area = π D 2 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟
2
= π f 2N ⎛ ⎝ ⎜ ⎞ ⎠ ⎟
2
N = f D
Aperture
- Lens opening given by # f-number
- A relative aperture size e.g., “#”, called an f-number,
written f/#, reflects the fact that it is computed by dividing the focal length by the absolute aperture (D).
– Aperture of a 100 mm lens at f/2 is a
- Circle of diameter 100/2 = 50mm.
– Aperture of a 50 mm lens at f/2 is a
- Circle of diameter 50/2 = 25mm
- At a given f/stop, e.g., @ f/4 all lenses allow the same
amount of light.
- Greater f-number (smaller hole)
– And less light per unit area reaches the image plane (irradiance), watts/m2
15
#∗ D = f
f = focal length D = diameter of opening
lower f-number long lenses fat & expensive More ``glass’’ required.
D = f #
# = f D
h:ps://en.wikipedia.org/wiki/F-number
Aperture
General Idea: To maintain the same f-number a longer lens needs a larger diameter to produce the same illuminance ( lumen/m2) on focal plane (longer lenses has a magnifying effect) [example coming too see this better] We will continue to use
– Aperture of a 100 mm lens at f/2 is a
- Circle of diameter 100/2 = 50mm.
– Aperture of a 50 mm lens at f/2 is a
- Circle of diameter 50/2 = 25mm
16
Wide Open (full)
#∗ D = f
f = focal length D = diameter of opening
D = f #
Allowing Light with Aperture
- f/2 on a 50 mm lens (N=2) 2 = 50/D (D=25 mm)
- f/2 on a 100 mm lens (N=2) 2 = 100/D (D=50 mm)
17
N = f D
N = f-number = (f/#) f = focal length D = diameter opening
Aperture
- f/2 on a 50 mm lens (N=2) 2 = 50/D (D=25 mm)
- f/2 on a 100 mm lens (N=2) 2 = 100/D (D=50 mm)
18
N = f D
N = f-number = (f/#) f = focal length D = diameter opening Doubling both the absolute aperture diameter (D) and the focal length(f) cancel (b/c reciprocity); leaving the same relative aperture size (N). In this example, both lenses are f/2. [ 50 * (X) / (25 * (X) ]
1x 2x
