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The Basics of Lens Focal Length in Camera Modules
Created on 11.11
In today’s tech-driven world, camera modules are everywhere—from smartphones and security cameras to automotive dashcams and medical devices. But what makes one camera module capture wide landscapes while another zooms in on distant details? The answer lies in a critical parameter: lens focal length. For anyone involved in designing, purchasing, or using camera-equipped devices, understanding focal length is key to unlocking a module’s full potential. This guide breaks down the fundamentals of focal length in camera modules, explaining how it works, its impact on images, and how to choose the right focal length for your needs.
What Is Focal Length, Exactly?
At its core, focal length is a measurement that describes how a lens bends light to form an image on a camera module’s image sensor. Technically, it’s the distance (measured in millimeters, mm) between the lens’s optical center and the image sensor when the subject is in sharp focus (for subjects at an infinite distance, like the sky or distant mountains).
To simplify: Think of focal length as the “zoom power” of a lens—but it’s not just about magnification. It also determines how much of a scene the camera can capture (its field of view) and how objects appear relative to each other.
A common point of confusion is equivalent focal length, especially for small camera modules (like those in smartphones). Since camera modules use different-sized image sensors, a “5mm lens” on a tiny smartphone sensor won’t produce the same field of view as a 5mm lens on a full-frame DSLR sensor. To make comparisons easier, manufacturers often reference “35mm equivalent focal length”—translating the actual focal length to what it would be on a traditional 35mm film camera (the industry standard for perspective). For example, a 12mm lens on a smartphone module might have a 35mm equivalent of 24mm, which is a wide-angle view.
The Core Impacts of Focal Length on Camera Module Performance
Focal length isn’t just a number—it directly shapes the images your camera module captures. Here are the three most important effects to understand:
1. Field of View (How Much You Can See)
The field of view (FOV) is the area of a scene that the camera module can record. Focal length and FOV have an inverse relationship:
• Short focal lengths (e.g., 8mm–24mm) = Wide FOV: These lenses capture more of the scene. Think of a smartphone’s ultra-wide camera (often 12mm–16mm actual focal length) that fits an entire room or landscape in one shot. They’re ideal for indoor spaces, group photos, or scenic views—common in home security cameras (to monitor large rooms) or automotive surround-view modules (to see the car’s full perimeter).
• Long focal lengths (e.g., 50mm–200mm) = Narrow FOV: These lenses “zoom in” on a small portion of the scene. A 100mm lens on a security camera, for example, can focus on a distant doorway or license plate without capturing unnecessary background. Long focal lengths are also used in telephoto smartphone cameras (like the 52mm equivalent lenses in many flagship phones) for portrait shots or distant subjects.
2. Magnification (How Big Subjects Appear)
Focal length directly affects how large a subject appears in the final image. A longer focal length magnifies subjects more:
• A 24mm lens (wide-angle) will make a person 10 feet away appear small in the frame.
• A 85mm lens (telephoto) will make the same person fill more of the image, even from the same distance.
This is why wildlife photographers use long focal lengths (200mm+) to capture animals without getting too close—and why smartphone manufacturers add telephoto modules for “zoom” capabilities (though some use digital zoom, which degrades quality, vs. optical zoom via longer focal lengths).
3. Depth of Field (How Much Is in Focus)
Depth of field (DOF) is the range of distances in a scene that appear sharp. Focal length plays a big role here:
• Long focal lengths = Shallow DOF: Only a small portion of the image is in focus, with the background and foreground blurred (bokeh effect). This is perfect for portrait photography—using a 50mm+ lens on a camera module to blur a messy background and focus on a person’s face.
• Short focal lengths = Deep DOF: Most of the scene is in focus, from near to far. A 16mm wide-angle lens on a security camera, for example, will keep both a door (2 feet away) and a window (10 feet away) sharp—critical for monitoring details across a space.
Note: DOF is also affected by aperture (the lens’s “opening”) and distance to the subject, but focal length is the easiest parameter to adjust for DOF in pre-designed camera modules.
Common Focal Length Types for Camera Modules
Camera modules are designed for specific use cases, so their lenses come in standard focal length ranges. Here’s how to categorize them:
1. Ultra-Wide-Angle Lenses (Actual Focal Length: 8mm–16mm)
• 35mm Equivalent: 16mm–24mm
• Best For: Capturing large areas. Used in smartphone ultra-wide cameras (to fit more of a landscape), automotive front-facing modules (for broad road coverage), and indoor security cameras (to monitor entire rooms).
• Pros: Wide FOV, deep DOF.
• Cons: May cause slight distortion at the edges of the image (barrel distortion), which is often corrected via software in modern modules.
2. Wide-Angle Lenses (Actual Focal Length: 18mm–35mm)
• 35mm Equivalent: 28mm–50mm
• Best For: Everyday photography. The 35mm equivalent (28mm–35mm) is popular in smartphone main cameras (e.g., iPhone’s 26mm equivalent main lens) because it mimics the field of view of the human eye—natural for photos of people, food, or daily moments. They’re also used in outdoor security cameras to balance coverage and detail.
• Pros: Natural perspective, minimal distortion, versatile.
3. Standard (Normal) Lenses (Actual Focal Length: 35mm–50mm)
• 35mm Equivalent: 50mm–75mm
• Best For: Balanced perspective. These lenses are common in action cameras (like GoPros) and mid-range smartphone main cameras—they don’t stretch or compress the scene, making them ideal for everyday shots.
4. Telephoto Lenses (Actual Focal Length: 50mm–200mm+)
• 35mm Equivalent: 85mm–300mm+
• Best For: Zoom and detail. Used in smartphone telephoto modules (e.g., 52mm or 100mm equivalent) for portrait shots or zooming in on distant objects. They’re also critical in security cameras for license plate recognition or long-range monitoring (e.g., 100mm+ lenses for parking lots).
• Pros: High magnification, shallow DOF.
• Cons: Larger size (challenging for slim smartphone modules) and narrower FOV.
How to Choose the Right Focal Length for Your Camera Module
Selecting a focal length depends on your device’s purpose, the environment it will be used in, and your imaging goals. Follow these steps:
Step 1: Define the Use Case
Start with what the camera module needs to accomplish:
• Smartphone: Do you need a main camera (versatile wide-angle), ultra-wide (landscapes/groups), or telephoto (portraits/zoom)? Most flagships now use multi-module setups (e.g., 12mm ultra-wide + 26mm main + 52mm telephoto) to cover all needs.
• Security Camera: Indoor use (wide-angle, 12mm–18mm) or outdoor long-range (telephoto, 50mm–100mm)? A store might use a 16mm lens to monitor aisles, while a parking lot needs a 85mm lens to read license plates.
• Automotive: Surround-view (ultra-wide, 8mm–12mm) to see blind spots, or front-facing (wide-angle, 18mm–24mm) for collision detection.
Step 2: Consider the Image Sensor Size
As mentioned earlier, sensor size affects equivalent focal length. A small sensor (like the 1/2.3-inch sensors in most smartphones) will have a more “zoomed-in” equivalent focal length than a large sensor (like full-frame) with the same actual focal length. For example:
• A 12mm lens on a 1/2.3-inch sensor = ~24mm 35mm equivalent (wide-angle).
• A 12mm lens on a full-frame sensor = ~12mm 35mm equivalent (ultra-wide).
Always check the 35mm equivalent focal length when comparing modules—this ensures you’re comparing apples to apples.
Step 3: Prioritize Key Imaging Goals
Do you need deep DOF (e.g., security cameras) or shallow DOF (e.g., portrait cameras)? Do you need to capture wide scenes (e.g., automotive surround-view) or zoom in (e.g., wildlife cameras)?
• Prioritize wide FOV + deep DOF → Choose short focal lengths (8mm–24mm).
• Prioritize magnification + shallow DOF → Choose long focal lengths (50mm+).
Common Myths About Focal Length in Camera Modules
Let’s debunk two widespread misconceptions to avoid mistakes:
Myth 1: “Longer Focal Length = Better Image Quality”
Focal length does not determine image quality. A 200mm telephoto lens can produce blurry images if it has poor optical design (e.g., cheap glass), while a 16mm wide-angle lens can capture sharp photos with high-quality optics. Image quality depends on factors like lens coating (to reduce glare), sensor resolution, and software processing—not just focal length.
Myth 2: “You Can’t Get Wide-Angle Shots with a Telephoto Module”
While telephoto lenses have narrow FOV, some camera modules use software (like panoramic stitching) to simulate wide-angle shots. However, this is not the same as an optical wide-angle lens—stitched images may have inconsistencies in color or sharpness, and they can’t capture the same depth of field. For true wide-angle results, always choose an optical wide-angle lens (short focal length).
Real-World Examples of Focal Length in Action
To tie it all together, here are three common use cases and how focal length impacts their performance:
1. Smartphone Camera Modules
The iPhone 15 Pro, for example, uses three rear camera modules with distinct focal lengths:
• Ultra-wide: 13mm actual (24mm equivalent) → Captures landscapes and group photos.
• Main: 24mm actual (26mm equivalent) → Everyday shots, balanced perspective.
• Telephoto: 48mm actual (52mm equivalent) → Portraits (shallow DOF) and 2x optical zoom.
This setup covers most user needs by combining short, medium, and long focal lengths.
2. Security Camera Modules
A home indoor security camera might use a 16mm actual lens (28mm equivalent) → Wide FOV to monitor a living room. A commercial outdoor camera could use a 100mm actual lens (150mm equivalent) → Narrow FOV to zoom in on a parking lot entrance and read license plates.
3. Automotive Camera Modules
A car’s rearview camera often uses an 8mm actual lens (16mm equivalent) → Ultra-wide FOV to see pedestrians and obstacles behind the vehicle. A front-facing ADAS (Advanced Driver Assistance Systems) camera might use a 20mm actual lens (28mm equivalent) → Wide enough to monitor the road ahead, deep enough DOF to keep both near and far objects sharp.
Conclusion
Lens focal length is the backbone of how a camera module captures the world. It determines how much you can see (field of view), how big subjects appear (magnification), and what’s in focus (depth of field). By understanding the basics—short focal lengths for wide scenes, long focal lengths for zoom and detail—and aligning them with your use case (smartphone, security, automotive), you can choose or design a camera module that delivers the images you need.
Whether you’re a consumer shopping for a smartphone, an engineer designing a security system, or a developer building automotive cameras, never overlook the power of focal length. It’s not just a number—it’s the key to turning a camera module into a tool that solves real problems.
For more tips on camera module design or help selecting the right focal length for your project, feel free to reach out to our team of imaging experts!
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