chapter 3 the lens. pinhole lens light, hitting a solid barrier with a very small hole, admits...

Chapter 3

The Lens

Pinhole lens

Light, hitting a solid barrier with a very Light, hitting a solid barrier with a very small hole, admits straight rays of light small hole, admits straight rays of light that make an image when it hits a flat that make an image when it hits a flat surface on the other side of the hole, like surface on the other side of the hole, like film.film.

Without a prism, the image is flipped Without a prism, the image is flipped upside down. See page 38.upside down. See page 38.

Using a Pinhole Camera

What does a glass lens do?

Admits more light and gives a sharper Admits more light and gives a sharper image.image.

Modern lenses Modern lenses are based on the CONVEX are based on the CONVEX lens.lens.

They are thicker at the middle than a They are thicker at the middle than a concave lens.concave lens.

Convex lens.

Canon 500mm F4 L

Refracting light

Light passing through glass causes the light to Light passing through glass causes the light to refract or BEND.refract or BEND.

The light will bend to strike the film at the The light will bend to strike the film at the focal point.focal point.

See diagrams on page 39.See diagrams on page 39.

Refracting light to the film

What lenses do

Refract lightRefract light

Collect light raysCollect light rays

Focus light rays onto light sensitive materialFocus light rays onto light sensitive material

How lenses differ

The most important way lenses differ is The most important way lenses differ is their their focal lengthfocal length..

Focal length is the distance between the Focal length is the distance between the rear nodal point(rear of lens) and the focal rear nodal point(rear of lens) and the focal plane, or the FILM.plane, or the FILM.

Angle of view

Focal length controls the angle of viewFocal length controls the angle of view

A longer focal length A longer focal length increases the increases the magnificationmagnification, but , but decreases the angle of decreases the angle of viewview..

See page 41 for examples.See page 41 for examples.

17mm lens has very wide view but small 17mm lens has very wide view but small magnificationmagnification

500mm lens smaller angle of view, but 500mm lens smaller angle of view, but increased magnification.increased magnification.

Your 35mm camera

What is a “normal” lens?What is a “normal” lens?

50mm (about 2 inches)50mm (about 2 inches)

What is normal?

A “normal” lens (regardless of film size) willA “normal” lens (regardless of film size) will

Approximate what the human eye seesApproximate what the human eye sees

Differs by the size of the film (35mm, 120, or Differs by the size of the film (35mm, 120, or 4x5)4x5)

Is about the same as the diagonal measurement Is about the same as the diagonal measurement of the film.of the film.

What lens to use

If I were photographing a wild, maybe If I were photographing a wild, maybe dangerous, animal with a 35mm camera dangerous, animal with a 35mm camera what lens would work best?what lens would work best?

2828

3535

5050

8585

200200

A long lens is useful when you cannot get A long lens is useful when you cannot get close to the subject, so 200close to the subject, so 200

A long lens

Greater image magnification but narrower Greater image magnification but narrower angle of viewangle of view

Relatively shallower depth of field than a short Relatively shallower depth of field than a short lens. f/4 on a short lens, subject is close will lens. f/4 on a short lens, subject is close will have a deeper DOF than f/4 on a long lens.have a deeper DOF than f/4 on a long lens.

Must be focused accuratelyMust be focused accurately

They are heavier, bulkier, and more expensiveThey are heavier, bulkier, and more expensive

Difficult to shoot handheld. Difficult to shoot handheld.

A 105mm lens needs shutter of 125A 105mm lens needs shutter of 125

A 200mm lens needs shutter of 250 (a full stop)A 200mm lens needs shutter of 250 (a full stop)

Or you need a tripodOr you need a tripod

Medium length

An 85 to 105mm lensAn 85 to 105mm lens

Good for portraitsGood for portraits

Less distortion – see page 44Less distortion – see page 44

Shorter shutter speedsShorter shutter speeds

Shoot an 85mm at 125 handheld. That’s a Shoot an 85mm at 125 handheld. That’s a normal daylight shutter speed.normal daylight shutter speed.

Short or wide angle focal length

From “fisheye” (8-15mm) to 35mm lensFrom “fisheye” (8-15mm) to 35mm lens

More depth of field.More depth of field.

Good in fast moving situations (35mm)Good in fast moving situations (35mm)

Don’t need to stop and refocus each shotDon’t need to stop and refocus each shot

Sharpness of the print

CAN be affected byCAN be affected by

Shutter speedShutter speed

f/stopf/stop

Focal length of the lensFocal length of the lens

Film format (35, 120, 4x5) doesn’t Film format (35, 120, 4x5) doesn’t mattermatter

Which lens

If I needed to photograph a single room If I needed to photograph a single room in a house, which lens would be more in a house, which lens would be more appropriate?appropriate?

Telephoto – 200mmTelephoto – 200mm

– Long – 180mmLong – 180mm

– Short- 35mmShort- 35mm

– Normal-50mmNormal-50mm

Wide angle lens

• Can show both real and apparent distortionsCan show both real and apparent distortions

– Straight lines near the edge can become curvedStraight lines near the edge can become curved

• See a fisheye lens exampleSee a fisheye lens example

Fisheye example

More on the lens

• Everything else being equal: a Everything else being equal: a longer longer focal length lensfocal length lens, like 150mm, 200mm, , like 150mm, 200mm, even 400mm, will cause a even 400mm, will cause a shallower shallower depth of field.depth of field.

Focusing your lens

• With manual focus you select the part of With manual focus you select the part of the scene that you want to be sharpest.the scene that you want to be sharpest.

• The closer you are to the subject, the The closer you are to the subject, the more important it is to focus critically.more important it is to focus critically.

• Focus a little beyond, then come back, Focus a little beyond, then come back, more than once if necessarymore than once if necessary

Plane of Critical Focus

Plane of Critical Focus

• Within Within everyevery depth of field there is a PCF. depth of field there is a PCF.

• The further an object is from that PCF, the less The further an object is from that PCF, the less sharp its focus will be.sharp its focus will be.

• PCF is NOT the Depth of Field! It is INSIDE the PCF is NOT the Depth of Field! It is INSIDE the depth of field. depth of field.

• Reducing the aperture(from f/11 to f/16) will Reducing the aperture(from f/11 to f/16) will increase the DOF but will increase the DOF but will not increase not increase the PCFthe PCF

• In a portrait the PCF are the eyes.In a portrait the PCF are the eyes.

Where is the PCF?

Plane of critical focus

Types of focus

• You may have a microprism circle that You may have a microprism circle that appears dotted until it is focused, or a appears dotted until it is focused, or a split image that appears offset until split image that appears offset until focused. focused.

• Rangefinder cameras have a split image Rangefinder cameras have a split image (2 images) that are out of focus until the (2 images) that are out of focus until the 2 images are superimposed over each 2 images are superimposed over each other.other.

Auto focus

• Can use “dots” on the screen to focus on Can use “dots” on the screen to focus on many areas or 1 single dot.many areas or 1 single dot.

• Can be fooled by objects moving in front Can be fooled by objects moving in front of your shotof your shot

• Low light defeats auto focus.Low light defeats auto focus.

Focus Screen – Auto Camera

Distance Scale on the lens

How to control DOF

1.1. ApertureAperture

2.2. Focal length Focal length

3.3. Distance from subjectDistance from subject

Aperture

– Shallow needs large aperture opening (small Shallow needs large aperture opening (small aperture value like 2.8).aperture value like 2.8).

– Deep need small aperture opening (large Deep need small aperture opening (large aperture value like 16).aperture value like 16).

Focal Length

• Length like 24mm at f/2.8 will give greater Length like 24mm at f/2.8 will give greater DOFDOF

• Length like 200mm at f/2.8 will give a smaller Length like 200mm at f/2.8 will give a smaller DOFDOF

– All other adjustments are the same at both focal All other adjustments are the same at both focal lengths. lengths.

Distance from Subject

• Physical distance from subject will affect the Physical distance from subject will affect the DOFDOF

• Closer to subject the shallower the DOFCloser to subject the shallower the DOF

• Same focal length, aperture & shutter speedSame focal length, aperture & shutter speed

– Further away from subject=more DOFFurther away from subject=more DOF

– Closer to subject = shallower DOFCloser to subject = shallower DOF

Hyperfocal Distance

• Maximize DOF. Aperture f/8, f/11, f/16, f/22.Maximize DOF. Aperture f/8, f/11, f/16, f/22.

• Subject is X feet away. That is the hyperfocal Subject is X feet away. That is the hyperfocal distance.distance.

• DOF extends from ½ the distance to the hyperfocal DOF extends from ½ the distance to the hyperfocal point to infinity.point to infinity.

• If subject (therefore hyperfocal distance) is 10 feet If subject (therefore hyperfocal distance) is 10 feet away, they DOF starts at 5 feet and extends to away, they DOF starts at 5 feet and extends to infinity.infinity.

Use Smartphone

• Download DOF app for your phone. DOFMaster Download DOF app for your phone. DOFMaster onlineonline

• Example: Format is 35mm filmExample: Format is 35mm film

– Focal length of lens is 50mmFocal length of lens is 50mm

– F stop is 11F stop is 11

– Subject distance is…..Subject distance is…..

Calculation

• 50mm lens;f/11;6 feet50mm lens;f/11;6 feet

– DOF 4.83 ft to 7.91 ft for total of 3.08 feetDOF 4.83 ft to 7.91 ft for total of 3.08 feet

• 50mm lens; f/11; 16 feet50mm lens; f/11; 16 feet

– 9.67 ft to 46.4 feet for total of 36.8 feet9.67 ft to 46.4 feet for total of 36.8 feet

Misc. Lens

• Focus is most critical when using a short Focus is most critical when using a short lens (below 50mm).lens (below 50mm).

• Clean your lens with soft air, specially Clean your lens with soft air, specially designed tissue for lens cleaningdesigned tissue for lens cleaning

– Wipe in a circular motionWipe in a circular motion

• Never use a “glass” cleaner or spray. Small Never use a “glass” cleaner or spray. Small particles of dust can scratch your lens.particles of dust can scratch your lens.

More Misc.

• ““Perspective” in photography: compares Perspective” in photography: compares elements in the foreground to those in elements in the foreground to those in the middle and background.the middle and background.

• This is how we judge “depth” in a photo.This is how we judge “depth” in a photo.

• Perspective changes when the Perspective changes when the photographer moves close to or further photographer moves close to or further from the subject, NOT by the lens.from the subject, NOT by the lens.

Review

• Using f/2.8 means that Less? OR More? Of a Using f/2.8 means that Less? OR More? Of a scene will be sharp, near to far?scene will be sharp, near to far?

• Using f/16 means that Less? More? Of a Using f/16 means that Less? More? Of a scene will be sharp, near to far?scene will be sharp, near to far?

Questions?