Equivalent Aperture: How Crop Factor Affects Bokeh and Light Gathering

Many phone manufacturers advertise "F1.8 main camera" as if it were comparable to a full-frame F1.8 lens. It's not even close.

Why You Can't Compare Aperture Directly

The F-number definition is straightforward:

F = focal length / effective aperture diameter

The problem: F-number only uses physical focal length, but different sensor sizes require drastically different focal lengths to achieve the same field of view. Smaller sensor → shorter focal length → smaller aperture diameter — even at the same F-number.

Example:

• Full-frame 50mm F1.8: aperture diameter = 50 / 1.8 ≈ 27.8mm
• 1/1.3" phone main 8.6mm F1.8: aperture diameter = 8.6 / 1.8 ≈ 4.8mm

Same F1.8 label, nearly 6x difference in light-gathering area. This is why phone low-light quality can't match full-frame cameras.

The Equivalent Aperture Formula

Equivalent Aperture = Physical Aperture × Crop Factor

Crop Factor = Full-frame diagonal (43.27mm) / Sensor actual diagonal.

PhoneCameraData uses a dual-threshold rule:

• Sensor denominator ≤ 2.0 (larger sensors) → diagonal = 16mm / denominator
• Sensor denominator > 2.0 (smaller sensors) → diagonal = 18mm / denominator

Example Calculation

Xiaomi 17 Ultra main camera (LYT-900, 1/0.98"):

• Sensor diagonal ≈ 16 / 0.98 ≈ 16.33mm
• Crop factor = 43.27mm / 16.33mm ≈ 2.65
• Physical aperture F1.6 → equivalent aperture ≈ F4.2

So this flagship main camera has bokeh and low-light capability roughly equivalent to full-frame F4.2.

Practical Implications

Bokeh (Depth of Field): Equivalent aperture directly predicts background blur. An equivalent F4.0 phone main camera produces blur similar to full-frame F4.0 — noticeable but nowhere near an F1.4 full-frame lens.

Signal-to-Noise (Low Light): At the same shutter speed, total light gathered depends on aperture diameter. Phones with lower equivalent F-numbers perform better in low light.

Try the Simulator

Drag the aperture and crop factor sliders to see how equivalent aperture and bokeh blur change in real-time.

Open the "Aperture Bokeh Simulator" in Related Tools on the right.