Demystifying Computational Photography: How iPhone Slow Shutter Works
A deep dive into the technology that allows handheld long exposures without hardware filters.
If you've ever tried to take a long exposure photo with a traditional camera in broad daylight, you know the struggle: the image comes out completely white (overexposed). To fix this, photographers use dark glass known as Neutral Density (ND) filters to block light. So, how does your iPhone act like a slow shutter camera without these physical filters? The answer lies in Computational Photography.
The Problem with Physics
A physical shutter staying open for 5 seconds lets in massive amounts of light. Without a way to restrict that light (like a small aperture or ND filter), the sensor gets overwhelmed instantly. Smartphone lenses have fixed apertures (usually around f/1.8), which are wide open and let in tons of light. This makes traditional long exposure impossible in daylight.
The Solution: Image Stacking
Instead of taking one long continuous exposure, apps like Iris Flow take a rapid burst of standard photos—sometimes hundreds of them in a few seconds.
- Capture: The app shoots a fast sequence of images (e.g., 30 frames per second).
- Alignment: It analyzes each frame for camera shake. If your hand moved slightly, the software shifts the images to align them perfectly. This is why you can shoot handheld!
- Blending: The app then mathematically averages the pixel values across all the frames.
- Stationary pixels (like a rock) stay the same in every frame, so they remain sharp.
- Moving pixels (like water) change position in every frame. When averaged, they create a smooth blur.
AI Stabilization
Modern iPhones have powerful Neural Engines. Iris Flow utilizes this hardware to perform alignment in real-time. It "sees" the scene, identifies the stable background, and locks onto it. This digital stabilization is often far superior to optical image stabilization (OIS) alone for long exposures.
Benefits of the Computational Approach
- No ND Filters Needed: You don't need to carry fragile glass filters.
- Live Preview: You can see the long exposure effect building up in real-time on your screen.
- Handheld Freedom: The intelligent alignment corrects for minor jitters, freeing you from carrying a tripod.
This technology has democratized a technique that used to be exclusive to serious landscape photographers. Now, the power to capture time is in your pocket.