Native ISO

Definition

Base ISO (also called Native ISO) is the ISO sensitivity setting at which a camera sensor operates optimally, without electronic amplification or gain reduction. At Base ISO, the sensor achieves:

• Maximum Signal-to-Noise Ratio (SNR)
• Maximum Dynamic Range
• Minimum Noise
• Best Color Fidelity

Below Base ISO: Electronic attenuation (loss of highlights/details)
Above Base ISO: Electronic amplification (increased noise)

Physical Principle

How ISO Works

Photon to Signal Conversion:

┌─────────────────────────────────┐
│ 1. Light Hits Sensor │ Uniform for all ISOs
├─────────────────────────────────┤
│ 2. Photodiode → Electrons │ Uniform for all ISOs
├─────────────────────────────────┤
│ 3. Electronic Amplification │ HERE ISO makes difference
│ (Gain Application) │
├─────────────────────────────────┤
│ 4. Analog-to-Digital Conversion │ Varying SNR
├─────────────────────────────────┤
│ 5. Output Signal (Low/High Gain)│ Result of ISO
└─────────────────────────────────┘

Gain Mechanism

Base ISO (e.g., ISO 160):
 Sensor Gain: 0dB (no amplification)
 
 Signal: 100 Units
 Noise: 5 Units
 SNR = 100/5 = 20:1 (Optimal)

Above Base ISO (e.g., ISO 320, +1 Stop):
 Sensor Gain: +6dB (Factor 2)
 
 Signal: 100 × 2 = 200 Units
 Noise: 5 × 2 = 10 Units (Noise is amplified!)
 SNR = 200/10 = 20:1 (Same)
 
 However: Noise is now visible (10 instead of 5)

Below Base ISO (e.g., ISO 80, -1 Stop):
 Sensor Gain: -6dB (Factor 0.5)
 
 Signal: 100 × 0.5 = 50 Units
 Noise: 5 × 0.5 = 2.5 Units
 SNR = 50/2.5 = 20:1 (Same)
 
 However: Signal loses detail (50 instead of 100)

Technical Specifications

Base ISO in Modern Cameras

ISO vs. Light Level

Lighting Requirements at Base ISO:

ARRI Alexa Mini (ISO 160):
 Daytime Exterior (Sunny): Perfect
 Interior Day (Windows): Ideal
 Interior Night (Minimal Light): Underexposed
 
 → Needs ~2000+ Lux for normal exposure

Sony FX30 (ISO 100):
 Daytime Exterior (Sunny): Perfect
 Interior Day (Windows): Ideal
 Interior Night (Minimal Light): Underexposed
 
 → Needs ~1500-2000 Lux for normal exposure
 → 1.5x more sensitive than ARRI (only 1 stop!)

RED Komodo (ISO 320):
 Daytime Exterior (Sunny): Perfect
 Interior Day (Windows): Ideal
 Interior Night (Minimal Light): Better
 
 → Needs ~3000-4000 Lux for normal exposure
 → 2x less sensitive than ARRI
 → For Night Scenes, ISO must be increased

Base ISO in Practice

Lighting Planning

Base ISO determines the minimum lighting budget:

Scene: Interior Drama (classically lit)

ARRI Alexa Mini (ISO 160 Base):
 Target Lux: ~500-800 Lux for Key Light
 Fill Light: ~100-200 Lux (1:3 Ratio)
 Equipment: Standard Fresnels / Soft Lights
 Budget: Moderate
 
Sony FX30 (ISO 100 Base):
 Target Lux: ~300-500 Lux for Key Light
 Fill Light: ~75-150 Lux (1:3 Ratio)
 Equipment: Less power needed
 Budget: Cheaper
 
RED Komodo (ISO 320 Base):
 Target Lux: ~1000-1500 Lux for Key Light
 Fill Light: ~200-300 Lux (1:4 Ratio)
 Equipment: Stronger lights needed
 Budget: More expensive

Exposure Strategy

Golden Rule: "Always shoot on Base ISO if possible"

Why?
 1. Maximum Dynamic Range
 2. Minimum Noise
 3. Maximum Color Information
 4. Minimal Grading Noise Handling

Only deviate if:
 - Not enough light → Increase ISO
 - Too much light → Use ND filter (do not decrease ISO!)
 - Low-light scene → Increase ISO (acceptable)

ND Filter vs. ISO Reduction

Scenario: Bright Day Exterior (Overexposed at Base ISO)

Problem:
 ARRI Alexa Base ISO: 160
 Midday Sunlight: Too bright, would overexpose
 
Solution 1: ND Filter (CORRECT)
 - Apply ND4 Filter (2 Stops)
 - ISO remains 160 (Base)
 - Signal: Unchanged, only light reduced
 - Result: Optimal, best image quality
 
Solution 2: Lower ISO (INCORRECT)
 - ISO 160 → ISO 40 (4 Stops)
 - No filter
 - Signal: Electronically attenuated
 - Result: Highlights/details are lost
 
Rule of Thumb: Always use ND filters instead of ISO reduction

ISO Ranges

Low ISO (Below Base)

Electronically attenuated, loses detail:

ARRI Alexa: ISO 160 Base
 ISO 80: -1 Stop electronically reduced
 ✗ Highlights clipping possible
 ✗ Signal details lost
 ✓ However: Noise floor is lowered
 
 ISO 40: -2 Stops reduced
 ✗ Significant detail loss
 ✗ Not recommended

Native/Base ISO (Optimal)

ARRI Alexa: ISO 160 (Optimal)
 ✓ Maximum SNR
 ✓ Maximum DR
 ✓ Minimum Noise
 ✓ Best Colors

Sony FX30: ISO 100 (Optimal)
 ✓ Maximum SNR
 ✓ Maximum DR
 ✓ Minimum Noise
 ✓ Best Colors

High ISO (Above Base)

Electronically amplified, more noise:

ARRI Alexa: ISO 160 Base
 ISO 320: +1 Stop electronic amplification
 ✓ 1 Stop more light sensitivity
 ≈ Noise increases minimally (almost imperceptible)
 ✓ Practical for low light
 
 ISO 640: +2 Stops
 ≈ Visible noise
 ≈ Color details slightly degraded
 ~ Still acceptable for night scenes
 
 ISO 1280: +3 Stops
 ✗ Significant noise
 ✗ Colors degraded
 ✗ Only for emergency low light

Base ISO and Sensor Size

Interestingly: Larger sensors have higher Base ISOs

Reason: Pixel size and Photoelectric Conversion Efficiency

ARRI Alexa Mini (Super35, 5.3µm Pixel):
 Base ISO: 160
 Reason: 5.3µm pixels are large = many photons per pixel

Sony FX30 (APS-C, ~2.4µm Pixel):
 Base ISO: 100
 Reason: Smaller pixels = fewer photons
 Physically less efficient
 However: Electronics are very good, thus similar SNR

RED Monstro (Large Format, ~4.9µm Pixel):
 Base ISO: 320
 Reason: 4.9µm pixels, but highly optimized electronics

Rule of Thumb:
 Larger pixels → Higher Base ISO (Rule confirmed)
 However: Modern electronics can compensate for differences

Extended ISO and Dual Native ISO

Extended ISO

ARRI Alexa 35:
 Base ISO: 160
 Extended ISO Options:
 - 40: -2 Stops (Highlights Priority)
 - 80: -1 Stop
 - 160: Base
 - 320: +1 Stop
 - 640: +2 Stops
 
Usage:
 - 40 or 80: Very bright light (snow, water)
 - 320 or 640: Very low light

However: Outside of Base ISO, quality is compromised

Dual Native ISO (Special Case)

Some cameras have two Native ISOs:

Sony FX30:
 Native ISO 1: 100 (Standard)
 Native ISO 2: 3200 (Dual Native!)
 
Meaning:
 - ISO 100-1600: Normal noise behavior
 - ISO 3200: Second sweet spot (optimal electronics)
 - ISO 1600-3200: Transition range (higher noise)
 - ISO 3200+: Normal high-ISO noise
 
Practically:
 For low light: Use ISO 3200 (not 1600)
 Noise profiles are cleaner at 3200

See Also

• Dual Native ISO – Extended Base ISO Approach
• ISO Sensitivity – Fundamentals of ISO
• Sensor Technology – Physics of ISO
• Dynamic Range – Influenced by Base ISO
• ND Filter – Alternative to ISO Change
• Exposure Metering – Practical Use of Base ISO