Lens Essentials – Quick Review

Lens Basics

• Simple Lens (like a magnifying glass): Just a few parts.
• Compound Lens (most camera lenses): Many parts (6+ elements) to fix image problems.
• What Lenses Do: Gather light and make an image on your camera's sensor or film. You need to control this light!
• Refraction: Light bends when it goes from air into glass. This bending is how lenses focus light.
  • Convex surfaces: Bend light inward to a single focal point.

Focal Length & Angle of View

• Focal Length ($f$): This is how “zoomed in” your lens is. It’s the distance from the lens's back to the image sensor when focused very far away.
• Key Relationship:
  • Longer $f$: Makes things look closer (more magnified), but you see less of the scene (narrower view).
  • Shorter $f$: Makes things look further away (less magnified), but you see more of the scene (wider view).
• Examples (for a 35mm camera, also called 'full-frame'):
  • $17\text{ mm}$: Very wide angle, about $104^{\circ}$ view.
  • $50\text{ mm}$: Standard view, about $47^{\circ}$ view.
  • $300\text{ mm}$: Very zoomed in, about $8^{\circ}$ view.
• Crop Factor (CF):
  • Some cameras have smaller sensors (like APS-C).
  • This makes your lens act like a longer one.
  • Formula: $f_{equiv}=f_{lens}\times\text{CF}$
  • For APS-C, CF is about $1.5 - 1.6$ (so a 50mm lens acts like a 75-80mm lens).

Normal, Long & Short Lenses

• Normal Lens: Its $f$ is roughly the diagonal size of your camera's sensor.
  • Examples: $50\text{ mm}$ for 35mm, $27\text{ mm}$ for APS-C.
• Long Lenses (e.g., $85-300\text{ mm}$):
  • Good for: isolating subjects (like portraits or faraway things).
  • Create: a shallow Depth of Field (blurry background).
  • Are usually: heavier.
  • Need: faster shutter speeds to avoid blur (rule of thumb: $1/f$).
• Telephoto Design: Makes long lenses physically shorter.

Zoom Lenses

• What they are: Lenses with a variable focal length (e.g., $24-105\text{ mm}$). Lets you change your view without changing lenses.
• Trade-offs:
  • Often have a slower maximum aperture (less light gathering).
  • Can be bigger and more expensive.
  • Might have slightly less perfect image quality than fixed lenses.
  • Best quality: zooms with a moderate range and a constant $f\text{-number}$ (like f/2.8).

Special-Purpose Lenses

• Macro: Focuses very close-up for tiny details. Corrects for flat subjects.
• Fisheye: Super wide angle (usually $\ge 180^{\circ}$ view). Makes images look curved (barrel distortion). Has huge depth of field.
• Soft-focus: Adds intentional blur or glow, often used for dreamy portraits.
• Tilt-shift:
  • Shift: Moves the lens to fix converging lines (like tall buildings leaning backward).
  • Tilt: Swings the focus plane to get more of a scene in focus or create miniature effects.
• Mirror (Catadioptric): A long $f$ lens in a small body using mirrors. Has a fixed, small aperture and creates donut-shaped bokeh (blurry highlights).
• Image-stabilized versions: Have floating elements inside to reduce blur from camera shake.

Focusing Techniques

• Manual Focus:
  • How: Rotate the focus ring past the sharp point, then turn back until it's sharp.
  • Portraits: Focus critically on the nearest eye.
  • Diopter adjustment: Adjust your camera's viewfinder to match your eyesight.
• Autofocus (AF) Modes:
  • Center: Focuses only on the center point.
  • Focus-lock: Focuses, then locks it so you can recompose.
  • Wide-area: Focuses across a larger part of the frame.
  • Eye/face detect: Automatically finds and focuses on eyes or faces.
  • Continuous tracking: Keeps focus on a moving subject.
• AF Systems:
  • Active IR: Uses infrared light to measure distance (less common).
  • Passive (Contrast/Phase Detection): Looks for contrast or analyzes light phases.
  • Combined Systems: Many cameras use both for better reliability.
• Follow-focus Practice: Practice keeping moving subjects in focus.

Depth of Field (DOF)

• What it is: The area in your photo that looks acceptably sharp.
• Circle of Confusion: A tiny blurry spot that still looks sharp to our eyes; it defines the limits of DOF.
• DOF Extends: About 1/3 in front of your focus point and 2/3 behind it.
  • Exception: When focusing very close, it's closer to $1/2$ and $1/2$.
• Factors Increasing DOF (more things in focus):
  • Smaller Aperture (higher $f\text{-number}$, e.g., f/16).
  • Shorter Focal Length (wide-angle lens).
  • Greater Subject Distance (focusing on something far away).
  • Smaller Sensor (like a phone camera vs. a full-frame).
• Trade-off: While smaller apertures increase DOF, closing it too much can cause diffraction softening (the whole image becomes less sharp).
• Bokeh: The quality or look of the blurry parts of your image.
  • Depends on: your lens's aperture blades (diaphragm) and its design.

DOF Control

• Aperture First: This is your main tool!
  • Stop down (use a higher $f\text{-number}$ like f/11) to make more of the image sharp.
  • Open up (use a lower $f\text{-number}$ like f/2.8) to make the background blurry.
• Zone Focusing: Pre-set your lens to keep things sharp between specific near and far distances. Good for quick shooting.
• Hyperfocal Distance ($H$):
  • How: Focus your lens so that the infinity symbol ($\infty$) on the lens lines up with your far DOF mark.
  • Result: Everything from $H/2$ (half the hyperfocal distance) to infinity ($\infty$) will be sharp.
  • Formula: $H=\dfrac{f^{2}}{N\,c}+f$ (where $c$ is the circle of confusion, $N$ is the f-number).
• Focus Stacking: Take multiple photos focusing on different parts of a scene, then combine them later in software. Great for extreme sharpness in macro or landscapes.

Perspective

• Main Idea: Perspective (how objects look relative to each other) is set by your camera's distance to the subject, NOT by your lens choice.
  • Close + Short Lens: Makes near objects look hugely bigger than far ones (called "wide-angle distortion" or exaggeration).
  • Far + Long Lens: Makes distances look compressed and objects seem closer together (the "telephoto effect").
• How to Control: Move your camera position to change perspective. Then, pick the right lens to get the framing you want.

Buying Lenses

• Starting Out: Get a normal lens (like a 50mm) or a moderate zoom (e.g., $24-70\text{ mm}$).
• Later Additions: Consider lenses that are about half your normal lens's $f$ (e.g., 24-28mm) and double your normal lens's $f$ (e.g., 85-100mm).
• Extra-fast Lenses ($\le f/1.4$): Very expensive. Modern cameras with good high ISO performance often make f/2.8 lenses perfectly fine.
• Variable-aperture Zooms: Be careful if you shoot in low light, as the lens gets 'slower' as you zoom in.
• Used Lenses: Can be a great deal! Always check the glass, how the focus/zoom rings feel, and if the aperture blades move smoothly.
• Protection: Always use a matched lens hood. A UV or skylight filter can offer cheap front protection.

Care & Maintenance

• General: Keep your gear dry, clean, and within normal temperatures.
• Avoid: Sand, salt, and extreme heat.
• Cleaning Lenses:
  1. Blow off loose dust (with a blower bulb).
  2. Use a soft brush.
  3. If needed, use a lens tissue with a single drop of lens cleaner.
  • NEVER put fluid directly on the lens.
• Sensor Cleaning: Use your camera's clean mode first. A dedicated air bulb is usually enough. Only use swabs if you've been trained.
• Storage: Store with caps on, in a low humidity environment. Remove batteries for long-term storage.
• Optimal Sharpness: Lenses are usually sharpest 1-2 stops down from their widest aperture (e.g., an f/2.8 lens might be sharpest at f/4 or f/5.6).
  • Beyond that, diffraction will start making your images less sharp.

Key Formulae & Rules of Thumb

• f-number: Describes your lens's aperture size.
  • Formula: $N=\dfrac{f}{D}$ (where $N$ is the f-number, $f$ is focal length, and $D$ is aperture diameter).
• Reciprocal Rule for Hand-holding: Helps you avoid blurry photos when not using a tripod.
  • You need a shutter speed of at least $1/f$ (e.g., 1/50th sec for a 50mm lens).
  • For crop sensors: Use $1/(f\times\text{CF})$ (e.g., for a 50mm lens on an APS-C camera with a 1.5x crop, you'd need 1/75th sec).