DE 2990: PHOTOGRAPHYLec 4: LensesLec 4: Lenses

Dr. Rohan MunasingheBSc, MSc, PhD, MIEEEDepartment of Electronic and Telecommunication Engineering

Faculty of EngineeringUniversity of Moratuwa 10400

Contents

• Focal length issues

• Normal, Wide angle, and Telephoto lenses

• Zoom lenses

• Macro lenses• Macro lenses

Learning Objectives

After this lecture you should be able to

• Tell what focal length/lense is to be used for what

you want to photographyou want to photograph

• Tell what you get from a given lense

• Tell what is macro photography

Focal Length

• Focal length affects on two quantities

– field of view

– magnification.

Focal Length

� <35mm “short”, or “wide angle”

FoV �, DoF �, low magnification

focusing is less critical

� 35- 65mm “normal”

similar to the magnification of the eye

� >65mm “long”, or “telephoto”

FoV �, DoF �, high magnification

Focusing is criticalFocusing is critical

Normal Lens in Film Cameras

• A lens is called normal because it sees a scene just as the human eye sees (same magnification)

• A normal focal-length makes the objects appear in the same distance relationship as you perceive them same distance relationship as you perceive them ordinarily.

• It happens when the focal length of a lens is close to the diagonal measurement of the film format

Film format Film diagonal (mm) Normal Lens (approx.)Film format Film diagonal (mm) Normal Lens (approx.)

35mm (24 x36) 43mm 50mm

2 ¼ x 2 ¼ inch 90mm 80mm

4 x 5 inch 163mm 150mm

• solid-state image sensors are much smaller than the

smallest film

Normal Lens in Digital Cameras

1/3”

1/2”

2/3”Image sensor

Width(mm)

Height(mm)

Diagonal( mm)sensor (mm) (mm) ( mm)

⅔″ sensor 8.8 6.6 11

½″ sensor 6.4 4.8 8

⅓″ sensor 4.8 3.6 6

• 7mm is the actual focal length of the lens in the digital

camera, that gives the same angle of view as a 50mm

lens does on a 35mm cameralens does on a 35mm camera

• Canon S45 camera 3×

• 7.4~21.3mm zoom (normal to 3×)

Effective Focal Length

• Need to know this when you want to use a film camera

lens with your digital camera

• A lens projects a circle of light onto the imaging plane, and • A lens projects a circle of light onto the imaging plane, and

the size of the film or image sensor determines how large

an area of that circle is captured

• On a digital camera of

15.1X22.7mm sensor size,

any film camera lens will

assume 1.6× additional

magnification

35mm film

22mm

sensor

• Optical Zoom

– changes the amount of the scene falling on the image sensor

– Every pixel in the image contains unique data so the final photo is sharp and clear

Zoom Lenses

data so the final photo is sharp and clear

• Digital Zoom

– Take a part of the normal image falling on the sensor and then save that part as is, or

enlarge it to fill the sensor by adding new pixels to the image using interpolationpixels to the image using interpolation

– You can get exactly the same effect just by

cropping a normal image in a photo-editing program and then enlarging it

Wide Angle Lenses• Short focal length and great DoF

• Focusing isn't critical because of the great DoF10mm, 110o FoV

Telephoto Lenses• Long focal lengths, narrow DoF

• Heavy: Harder to handhold.

300mm lens

1.4××××

extender

840mm effective

300mm lens

2××××

extender

2××××

extender

Telephoto LensesWhat you see through 85mm

• Capture details of an object,

but shallow depth of field

makes focusing more critical

Telephoot 400mm Telephoto 400mm

• High magnification makes blur

with very slight movements

Macro Lenses• Short and fixed object

distance

• Magnification more than

physical size Canon physical size Canon

PE65 (1~5×)

• Photograph at close

range using a tripod and

flash

• Extremely narrow DOF

Macro Photography

• Have you seen the complex details of bees and flies? The

shapes and patterns on the wings and all such details this

close?

Spiral Antennas of a FlyHave you seen it this close before ?

Eye of a Butterfly, Have you seen it this close?

Image Stabilization: IS lenses

• Some lenses now have image stabilization built-in. Lenses

with this feature has built-in gyros to sense the vertical

deflection of the camera and micro-motors that instantly

shift a special image stabilization lens group vertically to shift a special image stabilization lens group vertically to

compensate for the motion and keep the image steady on

the sensor

• Break the old rule that you should never handhold a lens

using a shutter speed slower than the reciprocal of the lens

focal length (e.g 80mm ⇒ 1/80). Can go for two more stopsfocal length (e.g 80mm ⇒ 1/80). Can go for two more stops

• Don’t use IS when using a tripod.

Choice of Lens

• The choice of lens depends on what you plan to photograph

� Wide-angle lenses� Wide-angle lenses

� buildings, landscapes, interiors, and

on the street

� Telephoto lenses

for tight portraits, wildlife photography, � for tight portraits, wildlife photography, sports photography

� Normal lenses are a compromise

� gatherings, weddings, etc