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AGR-CSI53-02 · Lesson 2 · 13 clock hours · demo

Camera Fundamentals: Exposure, Focus & Depth of Field

Objective. Students will adjust aperture, shutter speed, and ISO on a DSLR to produce a correctly exposed, sharp image of a small evidence item under three different lighting conditions.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Why Photography Matters at a Crime Scene

Photographs are permanent, legally admissible records. A missed shot or a blurry image cannot be re-taken once the scene is released. Courts, attorneys, and juries rely on your photographs to understand spatial relationships, condition of evidence, and context. Because of this, technical accuracy is not optional — it is a professional and legal obligation.

The Exposure Triangle

Every photograph is controlled by three settings that work together. Change one and you must compensate with at least one of the others to maintain correct exposure.

Aperture is the opening inside the lens, measured in f-stops (f/2.8, f/8, f/16, and so on). A lower f-number means a wider opening and more light reaching the sensor. A higher f-number means a narrower opening and less light. Aperture also controls depth of field — the range of distance that appears sharp in the image. A wide aperture (f/2.8) produces shallow depth of field, keeping only the subject sharp while the background blurs. A narrow aperture (f/16) produces deep depth of field, keeping near and far objects sharp simultaneously. For close-up evidence photography, narrow apertures (f/8 to f/16) are generally preferred so the entire evidence item — a shell casing, a tool mark, a fingerprint card — is in sharp focus from edge to edge.

Shutter speed is how long the sensor is exposed to light, expressed in fractions of a second (1/60, 1/250, 1/1000). A slow shutter speed (1/30) admits more light but risks motion blur if the camera or subject moves. A fast shutter speed (1/500) freezes motion but admits less light. When photographing static evidence on a tripod, slower shutter speeds are acceptable. When hand-holding the camera, a general rule is to keep shutter speed at or above 1/(focal length in mm) — for example, at 50 mm, use at least 1/50 s to reduce camera shake.

ISO is the sensor's sensitivity to light. A low ISO (100–200) produces clean images with minimal noise and is preferred whenever light allows. A high ISO (1600–6400) brightens the image in dim conditions but introduces digital noise (grain), which can obscure fine detail in a fingerprint or fiber. Use the lowest ISO that still yields correct exposure.

Think of the three settings as a connected system: if you narrow the aperture to gain depth of field, you can compensate by slowing the shutter speed, raising the ISO, or adding supplemental light.

Depth of Field in Evidence Photography

For small evidence items — a ruler-sized placard, a spent cartridge, a bite mark — you need the entire item sharp. Position the camera so the film/sensor plane is as parallel to the evidence surface as possible. This maximizes the portion of the item that falls within the plane of sharp focus. Use a narrow aperture (f/8 to f/16). Place the camera on a tripod or evidence stand to eliminate camera shake when using slower shutter speeds. A cable release or the camera's self-timer prevents vibration when the shutter fires.

Focus mode matters too. Use single-point autofocus aimed at the center of the evidence, or switch to manual focus and use live-view magnification to confirm critical sharpness. Never trust autofocus blindly in low-contrast scenes.

Reading the Histogram

The histogram is a graph displayed on the camera's LCD that shows the distribution of tones from pure black (left) on to pure white (right). A well-exposed image has the bulk of the histogram in the mid-tones, with no large spike crushed against the far left (underexposed, lost shadow detail) or far right (overexposed, blown highlights). For forensic photography, blown highlights on an evidence item are unacceptable — detail in a label, a serial number, or a wound margin may be lost. Adjust exposure until the histogram sits comfortably in the mid-range.

Adapting to Three Lighting Conditions

Daylight (bright): Natural light is often harsh and directional. Use a low ISO (100), a narrow aperture for depth of field (f/8–f/11), and a shutter speed that brings the histogram into the mid-tones. A diffuser or reflector can soften harsh shadows that could obscure detail.

Fluorescent (dim, indoor): Fluorescent light is relatively flat but dimmer than daylight. You may need to open the aperture slightly, slow the shutter speed (use a tripod), raise the ISO moderately (400–800), or add an external flash. Set white balance to Fluorescent or shoot in RAW format to correct color cast in post-processing.

Tungsten (warm, lower output): Tungsten sources are dim and produce an orange color cast. A Custom or Tungsten white balance setting corrects the cast in-camera. Expect to use a wider aperture, slower shutter speed, or higher ISO — balance these carefully to retain depth of field and avoid noise.

Safety at the Photography Station

Key terms9

Exposure TriangleThe interdependent relationship among aperture, shutter speed, and ISO that together control how much light reaches the camera sensor and the overall brightness of the image.
Aperture (f-stop)The adjustable opening in the lens through which light passes, expressed as an f-number; a lower f-number means a larger opening and more light.
Shutter SpeedThe length of time the camera's shutter remains open, controlling how long the sensor is exposed to light; expressed in fractions of a second.
ISOA numerical rating of the camera sensor's sensitivity to light; higher values brighten images in low light but introduce digital noise that can obscure fine evidence detail.
Depth of FieldThe range of distances in a scene that appear acceptably sharp in the photograph; controlled primarily by aperture, focal length, and camera-to-subject distance.
HistogramA graph on the camera LCD showing the distribution of tonal values from black to white; used to confirm correct exposure without relying on the LCD brightness alone.
White BalanceA camera setting that adjusts color rendition to compensate for the color temperature of a light source, ensuring whites appear neutral rather than orange (tungsten) or green (fluorescent).
Digital NoiseRandom variation in pixel brightness and color that appears as grain at high ISO settings, potentially obscuring fine detail in evidence items such as fingerprints or fiber patterns.
Plane of FocusThe flat plane, parallel to the camera sensor, within which subjects are rendered sharpest; aligning the evidence surface with this plane maximizes in-focus detail across the item.

Worked examples4

1. You are photographing a ruler-sized evidence placard indoors under fluorescent light. Your initial settings are f/4, 1/60 s, ISO 800, and the histogram shows the bulk of tones shifted toward the left (underexposed). How can you correct exposure while keeping depth of field as deep as possible for the placard?
  1. Step 1 — Identify the goal: you need more light reaching the sensor, but you also want a narrow aperture (high f-number) for deep depth of field. Widening the aperture would help exposure but hurt depth of field, so try the other two controls first.
  2. Step 2 — Raise ISO: double it from 800 to 1600. Each doubling of ISO is one exposure stop brighter. Check whether the histogram shifts to the mid-tones. If the scene is still underexposed, consider ISO 3200, but monitor noise.
  3. Step 3 — Slow the shutter speed: if noise at high ISO is unacceptable, lower ISO back toward 400 and place the camera on a tripod, then slow the shutter speed from 1/60 s to 1/30 s (one stop brighter) or 1/15 s (two stops brighter). Motion blur is not a concern with a static evidence item on a tripod.
  4. Step 4 — Add supplemental light: if ISO and shutter adjustments still do not bring the histogram to the mid-tones, introduce an external flash or move a light stand closer to the placard.
  5. Step 5 — Confirm: after each adjustment, review the histogram. The bulk of tones should sit in the mid-range with no clipping at either edge.
Answer: Correct the underexposure by raising ISO (e.g., to 1600) and/or slowing the shutter speed with the camera on a tripod, keeping aperture at f/8 or narrower for depth of field, and adding supplemental light if needed — confirmed by a histogram centered in the mid-tones.
2. You are hand-holding a camera with a 50 mm lens to photograph evidence on a table. What is the slowest shutter speed you should use before camera shake becomes a significant risk of blur?
  1. Step 1 — Apply the reciprocal rule: the minimum safe hand-held shutter speed (in seconds) is approximately 1 divided by the focal length in millimeters.
  2. Step 2 — Insert the focal length: 1 ÷ 50 mm = 1/50 s.
  3. Step 3 — Select the nearest available shutter speed: most DSLRs offer 1/60 s as the closest standard value at or above 1/50 s.
Answer: Use at least 1/60 s when hand-holding a 50 mm lens to minimize the risk of camera-shake blur.
3. A student photographs an evidence placard at f/16, 1/125 s, ISO 100 in daylight and gets a correct exposure. She then moves the station indoors under tungsten light — approximately four stops dimmer. She wants to keep f/16 for depth of field and ISO 100 for low noise. How many stops must she adjust the shutter speed, and what is her new shutter speed?
  1. Step 1 — Identify the exposure deficit: moving four stops darker means the sensor receives 1/16 of the original light (each stop halves the light; four stops = 2⁴ = 16 times less light).
  2. Step 2 — Compensate entirely with shutter speed: to recover four stops of brightness by slowing the shutter, double the exposure time four times. Start at 1/125 s: → 1/60 s (1 stop) → 1/30 s (2 stops) → 1/15 s (3 stops) → 1/8 s (4 stops).
  3. Step 3 — Evaluate practicality: 1/8 s hand-held guarantees motion blur. A tripod is essential. Also confirm the histogram after the adjustment.
Answer: She must slow the shutter speed four stops, from 1/125 s to approximately 1/8 s, and must use a tripod to prevent camera shake at that slow speed.
4. A student's image shows the near edge of an evidence ruler sharp but the far edge blurry. The camera was held at a 30-degree angle to the ruler surface. What two adjustments will most effectively bring the entire ruler into sharp focus?
  1. Step 1 — Address the camera angle: the primary problem is that the sensor plane is not parallel to the evidence surface. Reposition the camera directly above (or directly in front of) the ruler so the sensor plane and the ruler surface are parallel. This places the entire ruler within the same plane of focus.
  2. Step 2 — Narrow the aperture: increase the f-number (e.g., from f/4 to f/11 or f/16) to increase depth of field, providing a buffer for any remaining slight angle variation.
  3. Step 3 — Compensate for the narrower aperture by raising ISO or slowing shutter speed (using a tripod) to maintain correct exposure.
Answer: Reposition the camera so the sensor plane is parallel to the ruler surface, and narrow the aperture to f/11 or f/16 to maximize depth of field — then compensate exposure with ISO or shutter speed adjustments.
In class: Instructor demonstrates the exposure triangle on a DSLR projected live via HDMI to a classroom screen, then adjusts settings for a bright outdoor scene, a dim indoor scene, and a high-contrast backlit scene while narrating each decision. · Students rotate through three lighting stations (daylight, fluorescent, tungsten) and photograph the same ruler-sized evidence placard, recording their chosen settings on a camera settings log sheet. · Paired critique: each student selects their best and worst image from each station, uploads to a shared class folder, and presents a one-minute verbal explanation of what setting change would fix the worst image.
Materials: DSLR cameras (one per two students); 50 mm and 18–55 mm lenses; external flash units; light stands with daylight, fluorescent, and tungsten bulbs; evidence placards; HDMI cable and projector; camera settings log sheet.
Check: Instructor reviews three submitted images (one per lighting condition) against an exposure rubric: histogram within mid-tones, subject in focus across full depth, no motion blur — all three images must pass to receive credit.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Crime Scene Photography and Documentation
1. A forensic photographer is shooting a small shell casing indoors and needs the entire length of the casing in sharp focus. Which aperture setting best achieves this?
Crime Scene Photography and Documentation
2. The histogram of a crime scene photograph shows a large spike crushed against the right edge of the graph. What does this indicate?
Crime Scene Photography and Documentation
3. Which combination of settings would best minimize digital noise while photographing evidence in bright daylight?
Crime Scene Photography and Documentation
4. A student is hand-holding a DSLR with a 50 mm lens to photograph evidence. To reduce camera-shake blur, the shutter speed should generally be at least:
Crime Scene Photography and Documentation
5. When photographing evidence under tungsten lighting without adjusting white balance, the image will most likely appear:
Crime Scene Photography and Documentation
6. An evidence placard photograph shows the near edge sharp but the far edge blurry. The most likely primary cause is:
Safety and Scene Management
7. During a scene photography exercise, a student's light stand with a tungsten bulb has been on for 20 minutes and must be repositioned. What is the correct safety action?
Crime Scene Photography and Documentation
8. A forensic photographer must shoot the same evidence item under three different lighting conditions and submit images that meet a mid-tone histogram standard. If the first image (daylight, ISO 100, f/11, 1/125 s) is correctly exposed, and the indoor fluorescent station is approximately two stops dimmer, which adjustment — keeping ISO 100 and f/11 unchanged — would restore correct exposure?
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