Function of a lens

Optical picture

• Camera obscura l. – Dark room• Law of optics was known from ancient

times. Camera obscura was an aid for drawing for artists.

• Daniel Barbaro presented first camera obscura which had glass lens during 1568.

Traditional film camera

• Light which has reflected from the target draws picture on the film

1. Light travels through the lens.2. Aperture controls the amount of light

going through the lens.

IRIS & LENS

SHUTTER

Normal camera

Digital Camera

• Optical principle same as in film camera

• Film is replaced with light sensitive CCD-cell (CCD=Charge-coupled device,CMOS)

Object Lens• As it’s simplest an object lens is just single

lens, which assembles light rays from a single target to intersect at one point behind the lens.

• Spherical lenses cause some distortion for the edge-rays. To correct this problem aspherical lenses have been developed.

• More information

http://www.mir.com.my/rb/photography/opinion/html/aspherical.htm

Focal Length of a Lens

• Focal length is measured in millimeters.• In the case of single lens focal length is

distance from lens to film or CCD-cell, CMOS,Foveon.

FOCAL L

• Normal lens is defined as a lens, which focal length is equal to diameter of produced picture.-> Varies depending on the size of the film or the CCD-cell.

• Example: Normal lens (50mm) of the kino film camera is equivalent to about 10mm lens in digital camera (depending on the size of the CCD-cell).

• Usually focal length istold by the kino size.

Different types of lenses• Fisheye lenses

Under 15 mm. With the shortest focal lengths picture is round. Perspective (angle of view) is 150-180 degrees.

• Special wide angle lenses15-20 mm. Distort perspective strongly: Straight lines seem to bend and objects close to camera seem excessively bigger than objects further. Perspective around 110 degrees.

• Wide angle lenses20-28 mm. Distort perspective in similar way as special wide angle lenses, but not as much. Perspective is 80-110 degrees.

• Slightly wide angle lenses28-35 mm. Distortion very small, but still too big for taking portraits.

• Normal lenses45-55 mm. Perspective is comparable to human field of vision. General lenses.

• Small tele lenses70-150 mm. Distances between different levels seem smaller. Perspective shrinks. For taking portraits and object pictures. Perspective 15-30 degrees.

• Tele lenses180-400 mm. Pespective shrinks very strongly. Sensitive to small moves of camera during picture taking. Demands fast shutter speed (1/250s or less) or tripod. For nature, sports etc. photographing, where getting close to target is difficult. Perspective 8-11 degrees.

• Zoom lenses24-250 mm. Focal length of lens is adjustable. Negative point when compared to non-adjustable lenses is poorer luminous intensity and resolution. More expensive lenses have better quality. General lenses.

Special lenses• Mirror tele lenses

500-1000 mm. Because construction of mirror lenses is based on light travelling through the mirrors (focal length gets longer due to reflections), the external size of the lens doesn’t grow proportionately to the growth of focal length. Very strong shrink in perspective. Need of tripod. For nature, sports etc. photographing.

• Macro lensesShort distance lenses. Some general (for example zoom) lenses have capability for macro shooting. Even 1:1 photographs possible without additional equipment

• Perspective control lenses24-35 mm. Used to fix distortions in straight lines (bending) when shooting with relatively wide angle lenses. Used specially in architectural photographing.

• Tele converters and wide angle convertersOptical devices, which are placed between lens and camera or in front of lens. Changes focal length of the lens, simultaneously decreasing other capabilities. Wide angle converters are used relative often with digital cameras, as they don’t have interchangeable lens.

Effect of focal length

• The shorter the focal length, the greater deep focus area.

• The shorter the focal length, the easier it’s to avoid of small movement of camera (shaking etc.) during shooting.

...and same visa versa

• The longer the focal length, the smaller the deep focus area.

• The longer the focal length, the easier it’s to get small shake of camera to ruin your photograph.

• As focal length grows -> Wideness of perspective decreases.

• As focal length grows -> The target grows (when compared to picture size)

Luminous intensity of a lens

• From luminous intensity can be seen how much the lens needs light comparing to other lenses.

• Lenses luminous intensity is the biggest apperture divided by the focal distance.

• Example: f:1,4 has very high luminous intensity, f:8 very low luminous intensity

Diaphragm• Diaphragm is a device, which can

be used to increase and decrease the size of the aperture, which light goes through.

• The diaphragm is usually located inside the lens between individual lenses.

Diaphragm

• Diaphragm is used with the aperture to adjust the amount of light that reaches the film.

• In addional to the amount of the light diaphragm has an effect on the deep focused area of the picture.

• In digital cameras diaphragm is not a necessary.

• The bigger the value of diaphragm the smaller is the aperture.

• While adjusting the diaphragm each of the steps either half or double the amount the light that comes through the lens:

f:2.8->f:5.6 halves the amount of the light,

f:16->f:11 doubles the amount of the light

• Diaphragms of all cameras havethe same basic settings. 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64 etc.

• The values of diaphrams have been got by dividing the focal length by each of the aperture at that setting. -> Example: From aperture 5.6 always the same amount of light gets out. The object has not effect for that.

Diaphragm and shutter

• By adjusting the diaphragm and the shutter speed the wanted lighting can be gotten.

• The sensitivity of the film will adjust the amount of the lightning on the film.

• In digital cameras the sensitivity can be adjusted from the camera setup.

• If diaphram is increased (aperture decreased / closed) must exposure time increse.

• Also if the exposure time seems to be too long, diaphram setting has to be decreased (aperture increased / opened)

• Diaphram aperture and shutter speed have inverse relation each other.

• For example f:8 and 1/125 have similar lighting than f:5.6 and 1/250.

• Resolving power (or resolution) of the lens is at it’s worst when the the shutter speed is atit’s maximum or minimum.

• The shutter speed has the effect for shaking of the camera.

• Rule can be kept that 1 divided by the object focal length is the longest time which can be shot without camera holder. (no need to worry about shaking)

• In genaral it means that with a normal camera (50 mm normal lens) that time is about 1/50 (~1/60)

Deep focused area

• The deep focused area grows when diaphragm is increased (aperture size decreased) .

What does it mean?

• Depth of field• sharpness• deep focus area (focal area)• focal length• accuracy in capturing subject• aperture/exposure

What happens to aperture?

• in the case the right exposure is 1/60 sec

• then because of movement the exposure must be adjusted to 1/125 sec

• What is the correct aperture?

Links

• http://www.kodak.com/global/en/consumer/glossary/glossaryContents.shtml

• http://www.photonhead.com/exposure/simcam.htm• http://www.photocourse.com• http://www.shortcourses.com

• In addition to adjusting diagraphm the shooting ratio has effect on the deep focused (focal area) area. The shorter the focal length of the lens -> the longer the deep focused area and visa versa.

• The further the lens has been focused the bigger the deep focused area.

• The closer the lens has been focused the shorter the deep focused area -> In macro shooting small aperture has to be used to be able to have focused object.

• Because of the physics, one third of the deep focus area is located front of the level where lens has been focused and two thirds behind it.

Focus• Lens is focused by moving the group of

individual lenses inside it. • In Automatic cameras the lenses are

moved with motor, which can be found from either object lens or the body of the camera.

• Automatic focusing doesn’t work well in all conditions. One example of that is poor lightning conditions.

• The problem with digital cameras is usually that the marketing properties are more of concern than the quality of the lens.

• The low quality lens may affect distortions on the picture, which can’t be corrected afterwards.

Picture distortions

• Pincusion- and barreldistortion

- Can been found especially from cheap zoom lenses.

• Problem with the cheap lenses are usually uneven light distribution.

• As a result of that, the corners of the picture can be more darker, which can be seen easily when shooting objects which are of a equal, even colors - like facade of the building or paper documents.

• Most of the digital cameras don’t have a proper protection against light rays. On some cameras the surroundings of lens can even reflect light.

• Light, which comes from the forward sector can spoil the image contrast and make exposure errors or reflections to the picture.

• To decrease amount of reflection and to improve the contrast the lenses are usually covered by multilayered foil (film).