Presentation on theme: "Chapter 3 The Lens. Pinhole lens Light, hitting a solid barrier with a very small hole, admits straight rays of light that make an image when it hits."— Presentation transcript:
Chapter 3 The Lens
Pinhole lens Light, hitting a solid barrier with a very small hole, admits straight rays of light that make an image when it hits a flat surface on the other side of the hole, like film. Without a prism, the image is flipped upside down. See page 38.
Using a Pinhole Camera
What does a glass lens do? Admits more light and gives a sharper image. Modern lenses are based on the CONVEX lens. They are thicker at the middle than a concave lens.
Canon 500mm F4 L
Refracting light Light passing through glass causes the light to refract or BEND. The light will bend to strike the film at the focal point. See diagrams on page 39.
Refracting light to the film
What lenses do Refract light Collect light rays Focus light rays onto light sensitive material
How lenses differ The most important way lenses differ is their focal length. Focal length is the distance between the rear nodal point(rear of lens) and the focal plane, or the FILM.
Angle of view Focal length controls the angle of view A longer focal length increases the magnification, but decreases the angle of view. See page 41 for examples. 17mm lens has very wide view but small magnification 500mm lens smaller angle of view, but increased magnification.
Your 35mm camera What is a normal lens? 50mm (about 2 inches)
What is normal? A normal lens (regardless of film size) will Approximate what the human eye sees Differs by the size of the film (35mm, 120, or 4x5) Is about the same as the diagonal measurement of the film.
What lens to use If I were photographing a wild, maybe dangerous, animal with a 35mm camera what lens would work best? A long lens is useful when you cannot get close to the subject, so 200
A long lens Greater image magnification but narrower angle of view Relatively shallower depth of field than a short lens. f/4 on a short lens, subject is close will have a deeper DOF than f/4 on a long lens. Must be focused accurately They are heavier, bulkier, and more expensive
Difficult to shoot handheld. A 105mm lens needs shutter of 125 A 200mm lens needs shutter of 250 (a full stop) Or you need a tripod
Medium length An 85 to 105mm lens Good for portraits Less distortion – see page 44 Shorter shutter speeds Shoot an 85mm at 125 handheld. Thats a normal daylight shutter speed.
Short or wide angle focal length From fisheye (8-15mm) to 35mm lens More depth of field. Good in fast moving situations (35mm) Dont need to stop and refocus each shot
Sharpness of the print CAN be affected by Shutter speed f/stop Focal length of the lens Film format (35, 120, 4x5) doesnt matter
Which lens If I needed to photograph a single room in a house, which lens would be more appropriate? Telephoto – 200mm Long – 180mm Short- 35mm Normal-50mm
Wide angle lens Can show both real and apparent distortions Straight lines near the edge can become curved See a fisheye lens example
More on the lens Everything else being equal: a longer focal length lens, like 150mm, 200mm, even 400mm, will cause a shallower depth of field.
Focusing your lens With manual focus you select the part of the scene that you want to be sharpest. The closer you are to the subject, the more important it is to focus critically. Focus a little beyond, then come back, more than once if necessary
Plane of Critical Focus
Within every depth of field there is a PCF. The further an object is from that PCF, the less sharp its focus will be. PCF is NOT the Depth of Field! It is INSIDE the depth of field. Reducing the aperture(from f/11 to f/16) will increase the DOF but will not increase the PCF 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 appears dotted until it is focused, or a split image that appears offset until focused. Rangefinder cameras have a split image (2 images) that are out of focus until the 2 images are superimposed over each other.
Auto focus Can use dots on the screen to focus on many areas or 1 single dot. Can be fooled by objects moving in front of your shot Low light defeats auto focus.
Focus Screen – Auto Camera
Distance Scale on the lens
How to control DOF 1.Aperture 2.Focal length 3.Distance from subject
Aperture Shallow needs large aperture opening (small aperture value like 2.8). Deep need small aperture opening (large aperture value like 16).
Focal Length Length like 24mm at f/2.8 will give greater DOF Length like 200mm at f/2.8 will give a smaller DOF All other adjustments are the same at both focal lengths.
Distance from Subject Physical distance from subject will affect the DOF Closer to subject the shallower the DOF Same focal length, aperture & shutter speed Further away from subject=more DOF Closer to subject = shallower DOF
Hyperfocal Distance Maximize DOF. Aperture f/8, f/11, f/16, f/22. Subject is X feet away. That is the hyperfocal distance. DOF extends from ½ the distance to the hyperfocal point to infinity. If subject (therefore hyperfocal distance) is 10 feet away, they DOF starts at 5 feet and extends to infinity.
Use Smartphone Download DOF app for your phone. DOFMaster online Example: Format is 35mm film Focal length of lens is 50mm F stop is 11 Subject distance is…..
Calculation 50mm lens;f/11;6 feet DOF 4.83 ft to 7.91 ft for total of 3.08 feet 50mm lens; f/11; 16 feet 9.67 ft to 46.4 feet for total of 36.8 feet
Misc. Lens Focus is most critical when using a short lens (below 50mm). Clean your lens with soft air, specially designed tissue for lens cleaning Wipe in a circular motion Never use a glass cleaner or spray. Small particles of dust can scratch your lens.
More Misc. Perspective in photography: compares elements in the foreground to those in the middle and background.Perspective in photography: compares elements in the foreground to those in the middle and background. This is how we judge depth in a photo. Perspective changes when the photographer moves close to or further from the subject, NOT by the lens.
Review Using f/2.8 means that Less? OR More? Of a scene will be sharp, near to far? Using f/16 means that Less? More? Of a scene will be sharp, near to far?