What Does AE Actually 'A'? – Understanding Exposure Through Aperture
Correct exposure makes images appear more realistic and comfortable, while exposure control is the first and most critical step affecting subsequent image processing. The three common parameters for controlling exposure are exposure time, sensitivity (ISO), and aperture. As one of the three primary exposure controls, aperture receives limited attention in smartphones due to size constraints. However, recent advancements in awareness and technology—coupled with aperture’s impact on depth of field—have enabled variable apertures in flagship main cameras. These three controls now coexist in mobile imaging systems to manage exposure and depth of field.

Aperture – An optical opening inside the lens formed by overlapping blades.
First, consider EV (Exposure Value), constructed from aperture and shutter speed. For the same CMOS sensor, F5.6 at 1/125s yields the same EV as F2 at 1/1000s, F2.8 at 1/500s, F4 at 1/250s, or F8 at 1/60s. Despite differing characteristics, all produce identical image brightness. The calculation formula is:

Larger apertures allow more light into the lens; smaller apertures admit less light. Thus, aperture size directly affects the camera’s light intake and ultimately its exposure.
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Example: If aperture changes from f/5.6 to f/4 while shutter speed remains constant, double the light reaches the image sensor. However, excess light causes overexposure. To prevent this, halve either shutter speed or ISO.

Identical ISO
ISO 200 | f/16 | 1/250s
ISO 200 | f/11 | 1/500s
ISO 200 | f/8 | 1/1000s
ISO 200 | f/5.6 | 1/2000s

To widen the aperture without changing shutter speed, reduce ISO sensitivity to maintain light volume. A one-stop aperture increase equals halving ISO sensitivity. Under strong light with fast shutter speeds, camera limits may be reached. If neither ISO nor shutter speed can sufficiently reduce light, use an ND filter (Neutral Density filter).

Identical Shutter Speed
1/4000s | f/8 | ISO 800
1/4000s | f/5.6 | ISO 400
1/4000s | f/4 | ISO 200
1/4000s | f/2.8 | ISO 100

Sunny to Cloudy Guide
The "sunny f/16 rule" references clear weather but adapts to overcast or dusk conditions. Key settings vary by lighting and shadow detail:

Sunny with snow/sand, sharp-edged shadows: f/22 | ISO 100 | 1/125s

Sunny, distinct shadows: f/16 | ISO 100 | 1/125s

Slight overcast, soft-edged shadows: f/11 | ISO 100 | 1/125s

Overcast, faint shadows: f/8 | ISO 100 | 1/125s

Heavy overcast, no shadows: f/5.6 | ISO 100 | 1/125s

Sunset, no shadows: f/4 | ISO 100 | 1/125s

Based on the above, we can derive approximately correct exposure parameters corresponding to ambient light. However, on mobile phones, numerous other factors affect parameter settings. For instance: To maximize depth of field, we require a smaller aperture. To meet algorithmic compositing needs or reduce motion blur, we need the shortest possible exposure time. To achieve better signal-to-noise ratio, we require lower ISO

These demands necessitate a test device capable of precise luminance control, enabling comparative testing of shutter speeds to obtain optimal exposure parameters.

The MLB-LTM light box is precisely a bilateral comparable light box covering motion blur and numerous other tests.

MLB-LTM (Bilateral HDR + Motion)
Many image quality factors can be optimally tested using test charts, but some tests (e.g., exposure, motion blur-related tests) can be better evaluated using real-world scenes. The real-scene test light box contains various real-scene test objects with colors and texture details for accurate subjective visual analysis and comparison, enabling image quality assessment through internal scene comparisons.
Building upon the static real-scene light box, it adds two transmissive light boxes. These can be paired with transmissive test charts to create and test high-contrast scenes and HDR (High Dynamic Range) tests. Added motion rail functionality incorporates a laterally moving portrait and a 360° rotating turntable, proving highly useful for testing motion artifacts, and evaluating blur in photos and videos. Through the top light source, two HDR transmissive light boxes, and motion targets, this integrated light box accommodates multifaceted testing possibilities, enriches testing methodologies, and enhances testing efficiency.

Test Items:
• Resolution • Near-IR Sensitivity • Moiré • Color Reproduction
• Motion Blur • Texture Loss • Light Detail • Distortion
• Natural Colors • HDR • Sharpening
• Shadow Detail • Skin Tone Rendering • Low-Contrast Detail

Key Advantages:

The MLB-LTM features independent zone lighting, with bilateral areas utilizing dual light sources (high-brightness + low-brightness) of identical specifications. Its color temperature is continuously adjustable from 2300K to 10000K, enabling not only subjective comparisons of multiple scenes under identical lighting but also subjective evaluations across multiple scenes under varied illumination. This provides a convenient comparative environment for different exposure parameters.
Additionally, both left-right independent zones incorporate motion rail functionality, adding two turntables capable of lateral movement and 360° rotation. The motion system enables adjustment of movement and rotational speeds. For testing rolling shutter across different devices or firmware versions under identical conditions, motion artifacts (including linear and rotational motion), and evaluating photo/video blur, it proves highly useful. The optional HDR module facilitates high-contrast testing and supports interchangeable multiple test charts.

For variable aperture testing, this eliminates extensive data collection. Users can rapidly preview results of different parameters in lab environments and compare effects of identical parameters under varying conditions.
Concurrently, Windows-compatible GUI and API support enable efficient programmable configuration, significantly reducing shooting workload.

This concludes our current discussion. Note that numerous other AE factors affect imaging outcomes—e.g., scenarios requiring "exposure compensation for bright/dark subjects" or intentional under/overexposure for artistic effects. We welcome your insights in the comments!

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