Transcript
APERTURE Malaysian Nature Society Photogroup
1
Contents • Review - Exposure Triangle • Aperture and Exposure • Aperture and Depth of Field • Circle of Confusion • Depth of Field • Hyperfocal Distance
2
Exposure Triangle ISO
Any photo exposure will involve all the elements from the Exposure Triangle namely, ISO, shutter speed and aperture. Changing any one of these requires changes in one or both of the other elements if the correct exposure is to be maintained.
APERTURE
SHUTTER SPEED 3
Aperture • Aperture therefore has two principle impacts on photographs :
Exposure Depth of Field
4
Aperture and exposure • What Is Aperture? • The main function of a camera lens is to collect light. • The aperture of a lens is the diameter of the lens opening and is usually controlled by an iris. The larger the diameter of the aperture, the more light reaches the film / image sensor. • Aperture is expressed as F-stop, e.g. F2.8 or f/2.8. The smaller the F-stop number (or f/value), the larger the lens opening (aperture).
5
Aperture and exposure The lens aperture is usually specified as an f-number, the ratio of focal length to effective aperture diameter. Therefore as the diameter increases, the ratio will decrease e.g. 100mm lens/25mm diameter equals f4 or 100 mm lens/50mm diameter equals f2. Note how a larger diameter (more light) gives a smaller f number. 6
Aperture and exposure Each full f-stop down or up doubles or halfs the amount of light reaching the sensor. The total light getting through the lens depends on the area of the open circle within the lens exposed by the aperture. Area of a circle varies according 2 to the square of the radius : πr
7
Aperture and exposure 2.8 x 2.8 = 7.84 (8) 4 x 4 = 16 (16) 5.6 x 5.6 = 31.36 (32) 8 x 8 = 64 (64) 11 x 11 = 121 (128) 16 x 16 = 256 (256) 8
Aperture and exposure
Therefore closing down by one full stop e.g. f4 to f5.6 will reduce the light reaching the sensor by half. Therefore opening up by one full stop e.g. f11 to f8 will increase the light reaching the sensor by two times.
9
CIRCLE OF CONFUSION Malaysian Nature Society Photogroup
10
Circle of Confusion In optics, a circle of confusion is an optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source.
11
Circle of Confusion In photography, the circle of confusion (“CoC”) is used to determine the depth of field, the part of an image that is acceptably sharp. Real lenses do not focus all rays perfectly, so that even at best focus, a point is imaged as a spot rather than a point. The smallest such spot that a lens can produce is often referred to as the circle of least confusion.
12
Circle of Confusion Focus on cat and light rays come to a point (focused) on sensor. For dog (behind cat) light rays come to a point (focused) behind sensor. For rabbit (front of cat) light rays come to a point (focused) in front of sensor. If not focused (if in front or behind) light rays form a circle instead of a point.
13
Circle of Confusion A standard value of CoC is often associated with each image format, but the most appropriate value depends on visual acuity, viewing conditions, and the amount of enlargement. A typical CoC might be 0.02mm for an 8 x 10 inch enlargement viewed at a distance of 25cms (10 inches). But an 8 x 10 inch enlargement viewed at a distance of 50 cms (20 inches) would only need a CoC of 0.05mm.
14
Circle of Confusion Do any of these circles look like points?
15
DEPTH OF FIELD Malaysian Nature Society Photogroup
16
Depth of Field In optics, particularly as it relates to film and photography, depth of field (DOF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image. A lens can precisely focus at only one distance at a time. The decrease in sharpness is gradual on each side of the focused distance, so that within the DOF, the unsharpness is imperceptible under normal viewing conditions.
17
Aperture and depth of field 200 mm -- f 4.2
200 mm -- f 4.2
18
Aperture and depth of field 200 mm -- f 4.2
200 mm -- f 5.6
19
Aperture and depth of field 200 mm -- f 4.2
200 mm -- f 8
20
Aperture and depth of field 200 mm -- f 4.2
200 mm -- f 11
21
Aperture and depth of field 200 mm -- f 4.2
200 mm -- f 16
22
Aperture and depth of field 200 mm -- f 4.2
200 mm -- f 22
23
Aperture and depth of field 200 mm -- f 4.2
200 mm -- f 32
24
Aperture and depth of field
25
Aperture and depth of field
26
Aperture and depth of field
27
Aperture and depth of field Sometimes it is important to obtain the maximum depth of field possible (e.g. landscapes) More often we want to restrict the depth of field so that the main subject is sharp but the background is out of focus and does not compete for attention with the main subject (200mm @ f11) 28
Aperture and depth of field Sometimes it is important to obtain the maximum depth of field possible (e.g. landscapes) More often we want to restrict the depth of field so that the main subject is sharp but the background is out of focus and does not compete for attention with the main subject (200mm @ f6.7) 29
HYPERFOCAL DISTANCE Malaysian Nature Society Photogroup
30
Hyperfocal Distance • Sometimes it is important to obtain the maximum depth of field possible (e.g. for landscapes). • The hyperfocal distance is the nearest focus distance at which the DOF extends to infinity. • Focusing the camera at the hyperfocal distance results in the largest possible depth of field for a given f-number. • When the lens is focused at this distance, all objects at distances from half of the hyperfocal distance out to infinity will be acceptably sharp.
31
Hyperfocal Distance
32
Hyperfocal Distance Focussing on a foreground object may still extend the depth of field all the way to infinity if the apperture is small enough.
33
Review - Aperture
• Aperture and Exposure • Aperture and Depth of Field • Circle of Confusion • Depth of Field • Hyperfocal Distance
34
APERTURE THANK YOU Malaysian Nature Society Photogroup
35
