How are Image Sensors Measured?

Image sensors are measured by several key specifications that define their physical characteristics, performance, and image quality capabilities. Beyond just their physical dimensions, these measurements encompass factors like resolution, pixel size, light sensitivity, and dynamic range, all of which contribute to the final image output.

1. Sensor Size (Physical Dimensions)

The most fundamental way image sensors are measured is by their physical dimensions. Sensor size is typically measured using the length and width of the rectangular sensor in millimeters (mm). This measurement dictates the sensor's surface area, which directly impacts its light-gathering capability and the field of view for a given focal length lens.

Camera manufacturers select specific sensor sizes to optimize for various photographic needs, influencing the design of compatible lenses. Common sensor sizes include:

• Full-Frame: Roughly 36mm x 24mm, mirroring traditional 35mm film. Offers excellent low-light performance and shallow depth of field.
• APS-C (Advanced Photo System type-C): Smaller than full-frame, typically around 23.6mm x 15.7mm (Nikon/Sony) or 22.2mm x 14.8mm (Canon). These sensors introduce a "crop factor" (e.g., 1.5x or 1.6x) which effectively magnifies the lens's focal length.
• Micro Four Thirds (M4/3): Approximately 17.3mm x 13mm, with a 2.0x crop factor. Balances portability with good image quality.
• 1-inch type: Around 13.2mm x 8.8mm, often found in high-end compact cameras.
• Smaller Sensors: Used in smartphones and basic point-and-shoot cameras, these can be significantly smaller (e.g., 1/2.3-inch type, 1/3-inch type).

Understanding sensor size is crucial because it influences:

• Low-Light Performance: Larger sensors generally have larger pixels (or more pixels for the same resolution), allowing them to gather more light and produce less noise in dim conditions.
• Depth of Field: Larger sensors make it easier to achieve a shallower depth of field (more background blur).
• Lens Compatibility: Lenses are often designed for specific sensor sizes.
• Camera Size and Cost: Cameras with larger sensors tend to be bulkier and more expensive.

2. Resolution (Megapixels)

Resolution is measured in megapixels (MP), which refers to the total number of individual photosensitive elements (pixels) on the sensor. A megapixel is one million pixels. For example, a 24 MP sensor contains 24 million pixels.

• Calculation: Resolution is calculated by multiplying the number of horizontal pixels by the number of vertical pixels. For instance, a sensor with 6000 pixels horizontally and 4000 pixels vertically has 24,000,000 pixels, or 24 MP.
• Impact: Higher resolution allows for larger prints with finer detail and provides more flexibility for cropping images without losing quality. However, more megapixels don't automatically mean better image quality, especially if pixel size is sacrificed.

3. Pixel Size (Pixel Pitch)

Pixel size, also known as pixel pitch, is the physical dimension of an individual pixel on the sensor, typically measured in micrometers (µm).

• Significance: Larger individual pixels can capture more light photons, leading to a higher signal-to-noise ratio and better low-light performance. This is why a 12 MP full-frame sensor might outperform a 24 MP APS-C sensor in dim light, as its individual pixels are significantly larger.
• Trade-off: Packing more pixels onto a smaller sensor means individual pixels must be smaller, which can reduce their light-gathering efficiency and increase noise, especially at higher ISO settings.

4. Dynamic Range

Dynamic range measures the sensor's ability to capture detail across the brightest and darkest parts of a scene simultaneously. It is typically expressed in stops of light.

• Importance: A wider dynamic range means the sensor can preserve more detail in both highlights (e.g., bright skies) and shadows (e.g., dark foregrounds) without them becoming completely white or black.
• Measurement: While not a direct "size" measurement, it's a critical performance metric for image sensors. Labs use specialized equipment to test and quantify a sensor's dynamic range. Learn more about dynamic range on B&H Photo.

5. ISO Sensitivity

While not a direct physical measurement, ISO sensitivity reflects how well a sensor performs in low light. It's a standardized scale that indicates the sensor's sensitivity to light.

• Performance Metric: Higher ISO values allow the camera to capture images in dimmer conditions, but often at the cost of increased digital noise. Good ISO performance means a sensor can produce clean images at relatively high ISO settings.
• Testing: Sensor ISO performance is evaluated through rigorous testing, measuring noise levels and detail retention at various ISO settings.

6. Quantum Efficiency (QE)

Quantum Efficiency (QE) measures how efficiently a sensor converts incoming photons (light particles) into electrons (electrical signals). It's expressed as a percentage.

• Significance: A higher QE means more of the light hitting the sensor is successfully converted into an electrical signal, leading to more efficient light capture and better image quality, especially in low light. High-end scientific and astrophotography cameras often boast very high QE.

7. Signal-to-Noise Ratio (SNR)

Signal-to-Noise Ratio (SNR) compares the level of the desired signal (light captured) to the level of unwanted background noise. It's often expressed in decibels (dB).

• Impact: A higher SNR indicates a cleaner image with less digital noise. Factors like pixel size, sensor temperature, and readout speed can influence SNR.

8. Frame Rate (Frames Per Second - FPS)

For video capture and continuous shooting, the frame rate (measured in frames per second (FPS)) indicates how many individual images the sensor can capture and process within one second.

• Application: High FPS is crucial for capturing fast-moving subjects (sports, wildlife) and for slow-motion video.

Summary of Image Sensor Measurements