Camera Lenses and Their Functions

Convex Lens
- Also called a positive lens.
- Shape: Bulges/thick in the center, resembling a lentil.
- Function: Converges parallel light rays to a focal point beyond the lens.
- Image Characteristics: Magnifies objects (e.g., a fly appears bigger).
Concave Lens
- Also referred to as a negative lens.
- Shape: Curves inward at outer surfaces.
- Function: Diverges parallel light rays, causing them to spread out.
- Image Characteristics: Reduces the size of objects (e.g., a fly appears smaller).

Normal Lens (Standard Lens)
- Image resembles what the human eye sees.
- Angle of view: 45 to 75 degrees.
- Focal length: Approximately $35 - 70 ext{ mm}$ .
Wide Angle Lens
- Focal length significantly shorter than a normal lens, typically $< 50 ext{ mm}$ (often $28 ext{ mm}$ ).
- Useful in architectural, landscape photography, where space is limited to move away from the subject.
Telephoto Lens
- Provides close-up images of distant subjects.
- Covers a smaller field of view, typically ranging from $60 - 1000 ext{ mm}$ in focal length.
- Produces a flat composition where distant objects appear larger than near objects.

Macro Lens
- Designed for shooting very close to subjects (a few inches away).
- Origin of the name: Derived from Greek word meaning "to enlarge".
Zoom Lens
- Allows quick adjustments between wide and narrow fields of vision without moving closer or further from the subject.
- Parfocal lens: Maintains focus when changing focal lengths.

Lenses cannot produce perfect images; they introduce various types of distortions or aberrations.
Minimizing aberrations is essential for improving image quality.

Spherical Aberration
- Occurs when rays of light passing through various parts of the lens focus at different points, leading to a blurry image.
Chromatic Aberration
- Different colors of light are focused at different points, resulting in color fringes around objects and blurred images.
Astigmatism
- Failure to focus horizontal and vertical lines in the same plane, causing images to appear distorted, particularly at the image edges.
Coma
- Occurs when light hits the lens obliquely, leading to images resembling a comet with a tail.
Curvature of Field
- Flat objects appear curved due to varied distance from the lens's axis.
Distortion
- The inability to maintain uniform enlargement across the image; can appear as a pincushion or barrel distortion.
Flare/Optical Flare
- Caused by double reflections from lens surfaces, which results in unwanted haze or light streaks in images.
Mechanical Flare
- Bright spots caused by stray light from worn out parts of the lens.
Light Loss
- Coatings on lenses can reduce flares but also cause a decrease in light transmission.
Stray Light
- Can be managed by using proper lens shades to block unwanted light.

Simple Meniscus Lens
- Composed of a single piece of glass; suffers from inherent defects.
Rapid Rectilinear Lens
- Combines two achromatic lenses; corrects some defects but not astigmatism.
Anastigmatic Lens
- Free from astigmatism and other defects like spherical aberrations.
Achromatic Lens
- Partly corrected for chromatic and spherical aberration; focuses two wavelengths in the same plane.
Process Lens
- Corrected for astigmatism and provides excellent image definition.
Fixed Focus Lens
- Used in fixed focus cameras; short focal length with a greater depth of field.

Relative Aperture (ƒ-number system): Influences light passing, depth of field, and sharpness due to lens defects.
Depth of Field: Distance within an image that appears acceptably sharp.
Hyperfocal Distance: Optimal focusing distance to maximize depth of field, keeping foreground and background in focus simultaneously.

Focusing Scale: Marker on the lens barrel indicating the distance to focus.
Range Finders: Measure angles of light convergence to achieve focus, including split image and coincident image types.
Ground Glass: Used in still photography to check sharpness via direct observation of the image.
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