Notes: Lenses, DSLRs, Exposure, and Metering

Lenses: structure, materials, and how they affect image

• Lenses contain many panes of glass inside, not just an outer pane; internal glass elements contribute to weight.
• Some elements are plastic; nicer, heavier lenses tend to be glass.
• Internal elements can be adjusted (extended/shortened, spread/compressed) to change how the image looks through the lens.
• The arrangement of glass elements affects focus and image quality, and heavier lenses reflect this complexity.

DSLR anatomy and light path: mirror, viewfinder, and sensor

• Light travels through the lens and strikes a mirror inside the camera body at about a 45-degree angle.

• The mirror reflects light upward to the optical viewfinder (through a pentaprism housing in DSLRs).

• When you remove the lens, you can look inside the camera to see the mechanism; the red dots on the lens and camera body align for mounting.

• You push a lens release button and rotate the lens to detach; alignment dots ensure proper mount orientation.

• A button near the lens mount is often used to release and rotate the lens; different brands use slightly different nomenclature, but the function is the same.

• When you take a picture, the reflex mechanism is involved: the light path is redirected by the mirror to the viewfinder, then the mirror pops up (exposing the sensor) and the shutter behind it opens to expose the sensor.

• The momentary darkening of the viewfinder during capture is caused by the mirror flipping out of the way.

• The mirror movement and shutter action together generate the characteristic sound of a DSLR; the mirror movement—not the shutter—produces the audible click.

• This reflex action is what gives a DSLR its name: Digital Single Lens Reflex (DSLR).

How a DSLR differs from a twin-lens reflex (TLR) and from mirrorless cameras

• TLRs: older cameras with two lenses—one for viewing through the viewfinder and a second for exposing film; not a single-lens reflex system.
• DSLR vs Mirrorless:
  • DSLRs use a reflex mirror; the viewfinder is an optical path when the mirror is in position. The view through the viewfinder may not perfectly match the final image exposure due to separate optical paths and live view behavior.
  • Mirrorless cameras do not have a reflex mirror; what you see on the viewfinder or LCD screen is essentially what will be recorded.
  • In DSLRs, when taking a shot, the lens initially opens to its maximum aperture for brightness in the viewfinder; after the shutter fires, the lens stops down to the chosen aperture, affecting brightness and depth of field in the captured image.
  • Mirrorless cameras can shoot more quietly (some models silent shutter) because there is no mirror flip or mechanical shutter sound in every mode; some bodies still have audible feedback depending on settings.
• Practical implications: view through the viewfinder vs live view accuracy, depth of field representation, and potential for silent shooting with mirrorless systems.

Lenses, mounts, and basic operation in practice

• To attach a lens: align red dot on lens with red dot on camera body; gently press lens-release button and rotate until it locks.

• Inside the body, you can see the mirror and the housing above it; the mirrors and housing are part of the optical path in DSLRs.

• The viewfinder shows what you’d see through the lens when the mirror is down; with the camera off, you don’t see through it.

• Understanding the big feature: the mirrorless era reduces mechanical complexity for view accuracy and often enables silent shooting; DSLRs emphasize the optical view through the viewfinder and the reflex mechanism.

Basic camera body controls: getting started with the mode dial

• Most cameras have a top-mode dial with several settings including:

  • M: Manual mode (full manual control of aperture and shutter speed)
  • P: Program mode (auto-exposure with some manual tweakability, like shifting the pairing of aperture/shutter)
  • A or AV: Aperture Priority (you set the aperture; camera selects shutter speed)
  • S or TV: Shutter Priority (you set the shutter; camera selects the aperture)
  • Green box or Auto: Fully automatic mode (camera controls all settings)
  • Scene icons (e.g., running person for action/sports, flower for macro/close-up, mountain for landscapes, etc.) for auto scene modes.

• Some cameras have a separate icon or mode like a point-and-shoot mode (P) which is similar to green auto but allows some manual shifts.

• In class, the instructor emphasizes starting in manual (M) to learn the core relationships between aperture and shutter speed, and to later explore other modes.

• When adjusting in manual mode, you’ll often need to know how to adjust apertures vs shutter speeds:

  • A typical camera may require you to press a dedicated button (often labeled with an “A” symbol, a star, or “Aperture”) while turning a dial to adjust the aperture.
  • Some cameras have two dials (front/back) to adjust aperture and shutter independently; others require a button press to toggle which parameter you’re adjusting.

The exposure triangle: aperture, shutter speed, ISO

• Core concepts:
  • Aperture: the size of the opening in the lens through which light passes; controlled by the lens’s iris leaves inside the barrel.
  • Shutter speed: how long the sensor is exposed to light; the time the shutter stays open.
  • ISO: sensor sensitivity to light; higher ISO means more sensitivity (more noise/grain at higher levels).
• The goal is the right exposure: brightness depends on aperture, shutter speed, and ISO.
• In manual, you control aperture and shutter speed; ISO is often left at a fixed value to simplify class practice (the instructor suggests ISO 400 for this course).
• Key relationships:
  • Increasing aperture size (lower f-number, e.g., f/2.8) increases brightness; decreasing aperture size (higher f-number, e.g., f/16) decreases brightness.
  • Increasing shutter time (slower shutter, e.g., 1/60 s vs 1/2000 s) increases brightness; decreasing shutter time reduces brightness.
  • Increasing ISO increases brightness by making the sensor more sensitive to light, but also increases noise.
• The instructor’s practical simplifications:
  • Set ISO to a fixed value (e.g., 400) for most learning; do not rely on Auto ISO for the moment.
  • Focus on mastering two numbers: aperture and shutter speed; ISO is kept constant to simplify learning.
• Example explained in class:
  • Changing from f/8 to f/16 is a two-stop reduction in light; to compensate, you can change shutter from 1/400 to 1/100 (two stops brighter) or raise ISO accordingly.
  • Corresponding LaTeX-style note: if you decrease the aperture by 2 stops and increase shutter by 2 stops, exposure stays the same: ext{Aperture: } f/8
    ightarrow f/16 ext{ is } -2 ext{ stops}; ext{Shutter: } 1/400
    ightarrow 1/100 ext{ is } +2 ext{ stops}. which yields the same exposure.
• Light metering overview:
  • The camera has a light meter display (meter) that shows exposure as a scale with a center point (the middle) and a plus/minus side indicating brighter or darker than middle gray.
  • The meter reading can be off the chart; you aim to bring it toward the middle for a “proper exposure.”
  • Meter readings depend on what you’re pointing at; 50% gray is the mid-point brightness standard used by metering systems.

Depth of field and why aperture matters

• Depth of field (DOF) is how much of the scene is in sharp focus from foreground to background.
• DOF depends on aperture, subject distance, and lens focal length:
  • Smaller aperture (larger f-number, e.g., f/22) increases DOF (more of the scene is in focus).
  • Larger aperture (smaller f-number, e.g., f/2.8) reduces DOF (less of the scene is in focus; background becomes blurrier).
  • Closer subject distance reduces DOF; farther subject distance increases DOF.
• Practical examples from the lecture:
  • A leaf close to the camera often shows shallow DOF (background blurred).
  • A distant scene (mountains) can be in sharp focus from near to far (maximum DOF) when the setup is appropriate.
  • A bug photographed very close typically yields a shallow DOF regardless of aperture due to proximity.
• The “pinhole” analogy (not recommended as a real practice): small openings increase DOF because light is constrained, but this is a crude simplification; do not attempt DIY pinhole methods for driving realism.
• Tilt-shift lenses (briefly mentioned): allow DOF to be controlled at an angle, producing illusions of perspective or plane-of-focus shifts (tilt-shift effect).
• Practical takeaway: for most street photography (5–20 feet away), DOF is strongly influenced by aperture; close-up work often requires smaller DOF control; large distances give more DOF by default.

Shutter speed: freezing action vs motion blur

• Shutter speed is a time value indicating how long the sensor is exposed to light; expressed as fractions of a second or seconds in longer exposures:
  • Fast shutter speeds (e.g., $1/1000, 1/2000, 1/4000$) freeze motion and require more light or higher ISO to compensate.
  • Slow shutter speeds (e.g., $1/30, 1/15, 1/2, 30 ext{ s}$) blur motion and require more light or higher ISO to compensate; movement during exposure becomes visible as blur.
• Common practical milestones:
  • 1/1000 to 1/4000 second: freezes fast motion (e.g., hummingbird wings).
  • Around 1/60 to 1/125 second: captures general motion with some blur; handshake/camera shake becomes noticeable if not stable.
  • 30 seconds and Bulb mode: long exposures for light painting or night scenes; Bulb keeps the shutter open as long as you hold the shutter release.
• Important operational tip: to avoid camera shake, avoid going below around 1/60 s when hand-holding; many recommend not going below 1/60–1/125 s depending on stability; the instructor emphasizes a practical threshold of around 1/60 s (60) to minimize blur due to hand shake.
• If you do need longer exposure than a few seconds, use Bulb mode (shutter open while held) or a remote release to avoid camera shake.
• If you use long exposures for moving subjects, the motion blur indicates subject movement; lack of motion indicates freezing of motion at fast shutter speeds.
• Practical tip: for handheld shots, aim to shoot at or faster than 1/60 s; for moving subjects or low-light conditions, increase ISO or open aperture to maintain exposure without motion blur.

The role of ISO and practical guidance for learners

• ISO stands for International Standards Organization; it measures sensor sensitivity to light.
• Higher ISO makes the sensor more sensitive, allowing you to shoot in lower light, but introduces more noise/grain in the image; lower ISO yields cleaner images but requires more light.
• In the course, the instructor suggests:
  • Set ISO to a fixed 400 for class practice; do not leave it on Auto (initially).
  • If you research and find your particular camera performs as well at ISO 800 as ISO 400, you may set to 800, but for most students 400 is a safe benchmark.
  • Note: cell phones use different night-shooting processing (multi-frame blending) to reduce noise, not directly comparable to DSLR/DSLR performance in low light.
• Potential observation: higher ISO values (e.g., 3200, 6400, 12800, 25600) can enable faster shutter speeds in lower light but at the cost of visible grain.
• Practical rule of thumb for this course: keep ISO at 400 unless there is a compelling reason to adjust (e.g., very low light or fast action). The focus remains on mastering aperture and shutter speed.

Light metering: reading light and metering for exposure

• Your camera includes a light meter display in the viewfinder or on the LCD/back screen, which shows whether the current exposure is too bright or too dark.
• The meter uses a scale with a central reference (the middle), and a movable indicator that shifts toward + (bright) or – (dark) depending on the scene.
• If the meter is off the chart, the exposure will be too bright or too dark; adjust aperture and shutter speed to bring the meter toward the middle for a proper exposure.
• Metering technique in class:
  • Point the camera at a scene (e.g., your foot, a light fixture, a table, or the carpet) and read the meter.
  • Adjust aperture and shutter speed so the meter sits toward the middle, ideally around the 50% gray target.
  • Use this reading to guide exposure decisions rather than relying solely on automatic modes.
• Half-press and locking concept (for some cameras and some workflows):
  • Press the shutter button halfway down to activate autofocus and meter the exposure; this locks in the metered exposure value and focus momentarily for recomposition.
  • In some point-and-shoots, pressing halfway down locks exposure and focus; you can reframe and then press the button fully to capture.
  • The meter can be read while blocking or re-framing the scene; the shutter half-press helps to meter a scene for one exposure, which you can then adjust by re-meting on a different element.
• Practical use: metering off a bright or dark target will bias the exposure toward that target; using the meter to target 50% gray makes it easier to balance background and subject lighting; then you recompose and shoot.

Putting it all together: practical workflow for the learning phase

• Exercise: meter, set exposure, and shoot using manual mode to understand how aperture, shutter speed, and ISO impact the image:
  • Start with shutter speed around 1/60 s and aperture around the widest setting of your lens (e.g., 2.8 or 3.5 depending on your lens).
  • Move around the room with the light meter reading, keeping the meter near the middle; adjust aperture and shutter to compensate so the meter stays near middle.
  • Use a light fixture or bright surface to test bright exposures and a shadowed surface to test darker exposures; compare results.
• Practical example from the lecture:
  • If you meter for a bright surface (e.g., a light fixture) with a certain exposure, and you then photograph a darker object, you may get a blown-out bright area or a silhouette; you can compensate by adjusting exposures or by metering for a middle-gray subject and then re-metering to capture details in the light area.
• Conceptual takeaway: you can use the light meter’s middle position as a target for a proper exposure, then adapt the scene by changing composition or lighting while staying within the exposure range.
• Final guidance: during the course, practice with manual exposure in daylight or bright interior lighting to build intuition; inside poor light, expect more noise and potential blur; street shooting in daylight helps you learn DOF and shutter choices more predictably.

Quick responses to common questions raised in the session

• What does the “sound” of a camera mean? The sound is caused by the moving mirror in a DSLR, not the shutter.
• Why do some cameras let you shoot silently? Mirrorless and some modern DSLRs offer silent shutter modes, often by using electronic shutters or quieter mechanical systems.
• What is the biggest advantage of mirrorless cameras mentioned in the lecture? The view you see in the viewfinder or on the screen is exactly what is being recorded, providing a direct preview of exposure; silent shooting is another advantage.
• What is the difference between a 50% gray target and a real scene? Metering aims to map the average brightness to a middle gray; different targets in the scene will reflect light differently, so metering helps you determine an appropriate exposure, not the literal brightness of every object.
• What is the practical role of “scene modes” (portrait, action, macro, etc.)? They provide quick presets to optimize exposure, contrast, and autofocus behavior for common scenarios; they automate some decisions to help beginners get better results faster.
• How does depth of field relate to subject distance? The closer you are to the subject, the shallower the depth of field tends to be; farther distances increase depth of field. This is in addition to the aperture setting.

Summary points you should remember

• Lenses are complex: multiple glass elements affect weight, clarity, and focus.
• In a DSLR, light path involves a 45-degree mirror to the viewfinder; when taking a shot, the mirror flips up and the sensor is exposed; the mirror movement creates the characteristic camera sound.
• Mirrorless cameras remove the reflex mechanism, giving a true-to-recorded-view experience and often silent operation.
• The three pillars of exposure are aperture, shutter speed, and ISO; learning to balance them with a fixed ISO (e.g., 400) helps beginners understand exposure without being overwhelmed by sensor noise.
• Aperture controls brightness and depth of field; smaller numbers (f/2.8, f/4) give brighter images and shallower DOF; larger numbers (f/11, f/16) give less brightness and deeper DOF.
• Shutter speed controls how long the sensor collects light; fast speeds freeze motion; slow speeds create motion blur; very slow speeds require a tripod or stabilization.
• ISO adjusts sensor sensitivity; higher ISO enables shooting in low light but adds noise; start with ISO 400 for learning.
• Light metering helps you judge exposure; use the middle to aim for 50% gray; half-press the shutter to meter and AF in many cameras; adjust aperture/shutter to keep the meter in the middle.
• Practical approach: practice with manual mode outside in daylight to develop intuition about how aperture and shutter speed affect exposure and depth of field; use the light meter as a guide and recompose after metering when necessary.
• Real-world contexts include silent shooting for candid or sensitive moments and using appropriate modes for portraits, action, or macro work.