Lens specifications are typically a series of numbers and symbols that tell you about the lens's capabilities. Here's a breakdown of how to interpret them:
Understanding the Basics
Generally, lens specs are presented in this form: [Focal Length(s)] [Aperture]. For example: 18-55mm f/3.5-5.6 or 50mm f/1.4.
Focal Length
- Range (e.g., 18-55mm): This indicates a zoom lens with a range of focal lengths. The first number (18mm) is the widest angle, and the second number (55mm) is the most zoomed-in focal length.
- According to reference [2], the numbers indicate "the range of available focal lengths in millimeters."
- Single Number (e.g., 50mm): This indicates a prime lens (fixed focal length), meaning it does not zoom. It can also mean a lens is at a specific focal length within a zoom lens.
Aperture (Lens Speed)
- f-number (e.g., f/3.5-5.6 or f/1.4): This refers to the aperture, often called "lens speed." It represents the maximum amount of light the lens can let in.
- The f/number is usually listed after the letter 'f' and a slash, per reference [3].
- Variable Aperture (e.g., f/3.5-5.6): Found on many zoom lenses, this means the maximum aperture changes as you zoom. f/3.5 is the maximum aperture at the widest angle (e.g., 18mm), while f/5.6 is the maximum at the most zoomed-in focal length (e.g., 55mm).
- Fixed Aperture (e.g., f/1.4): Often found on prime lenses, this means the maximum aperture remains the same regardless of the focal length. This usually signifies that it's a much faster and more expensive lens than one with variable aperture.
- In the case of 1.4/50 designation (or something similar) per reference [1], the order of the focal length and the aperture is switched where aperture is mentioned first and then the focal length.
Reading the Specs
Here's how to put it all together:
| Specification | Meaning | Example |
|---|---|---|
| 18-55mm f/3.5-5.6 | A zoom lens with a focal length range from 18mm to 55mm and a variable aperture of f/3.5 to f/5.6. | Common kit lens |
| 50mm f/1.8 | A prime lens with a fixed focal length of 50mm and a maximum aperture of f/1.8. | Portrait lens |
| 70-200mm f/2.8 | A zoom lens with a focal length range from 70mm to 200mm and a constant aperture of f/2.8. | Telephoto lens |
| 1.4/50 | A prime lens with a fixed focal length of 50mm and a maximum aperture of f/1.4. | Fast Portrait lens |
Key Takeaways
- Focal length (mm): Determines the field of view (how much you see).
- Aperture (f-number): Affects the depth of field (how much is in focus), and low-light performance; a lower f-number means a wider aperture, more light, shallower depth of field, and is considered a faster lens.
- Zoom lenses offer a range of focal lengths, while prime lenses have a fixed focal length.
- Knowing how to read lens specs helps you understand what kind of images the lens is capable of capturing, its field of view and low light capability.
