Understanding lens mm, or millimeters, essentially means understanding the focal length of a lens and how it affects the image you capture. The focal length is expressed in millimeters (mm) units.
What is Focal Length?
The focal length is defined as the distance from the center of the lens to the image focal point (which is the surface of the image sensor). This measurement determines the lens's angle of view and how much it magnifies the subject.
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A short focal length (e.g., 16mm, 24mm) means a wider angle of view, capturing more of the scene. These are known as wide-angle lenses.
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A long focal length (e.g., 200mm, 400mm) means a narrower angle of view, magnifying the subject and making it appear closer. These are known as telephoto lenses.
Focal Length and its Impact
Here's a table summarizing how different focal lengths affect your images:
| Focal Length | Angle of View | Magnification | Common Uses |
|---|---|---|---|
| Wide-Angle (16-35mm) | Wide | Low | Landscapes, architecture, interiors |
| Standard (50mm) | Normal | Normal | General photography, portraits |
| Telephoto (70-200mm) | Narrow | High | Portraits, sports, wildlife |
| Super Telephoto (300mm+) | Very Narrow | Very High | Wildlife, sports, surveillance |
Practical Implications
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Choosing the right lens: Knowing the mm specification allows you to select the appropriate lens for your desired shot. If you want to capture a vast landscape, a wide-angle lens (low mm) is suitable. For bringing distant subjects closer, a telephoto lens (high mm) is needed.
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Understanding perspective: Focal length influences the perspective of the image. Wide-angle lenses can exaggerate the distance between objects, while telephoto lenses compress perspective, making objects appear closer together than they are.
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Crop Factor: Sensor size affects the effective focal length. For example, a 50mm lens on a crop sensor camera (like many DSLRs and mirrorless cameras) will have a narrower field of view than a 50mm lens on a full-frame camera. This is because the crop sensor only uses the central portion of the image projected by the lens. You will have to multiply a crop factor to the actual focal length to get the effective focal length for the cropped sensor.
Examples:
- 24mm lens: Excellent for landscapes, allowing you to capture a wide vista.
- 50mm lens: Often called the "nifty fifty," it provides a field of view similar to human vision and is great for general photography and portraits.
- 200mm lens: Useful for capturing portraits with a blurred background or for shooting sports from a distance.
- 400mm lens: Ideal for wildlife photography, allowing you to capture animals without getting too close.
