1. Focal Length: This is arguably the most crucial factor. Portrait lenses typically fall within these ranges:
* 85mm (full-frame equivalent): This is a classic portrait focal length. It offers a flattering perspective, compressing the background nicely and creating pleasing bokeh (background blur). It's a great all-around choice for many situations. On crop-sensor cameras, the equivalent focal length will be longer (e.g., a 50mm on an APS-C camera will approximate an 85mm on a full-frame).
* 50mm (full-frame equivalent): A slightly wider perspective than 85mm, the 50mm is more versatile. It's suitable for portraits, but you might need to be further away from your subject. The background compression isn't as dramatic as with an 85mm. On crop-sensor cameras, this is a popular choice for portraits.
* 135mm (full-frame equivalent) and beyond (e.g., 200mm): These longer telephoto lenses offer even greater background compression and subject isolation. They're excellent for environmental portraits where you want to keep a more substantial portion of the background visible but still have a shallow depth of field. They require more distance from your subject.
2. Aperture: This determines how much light reaches the sensor and influences the depth of field. For portraits, you generally want a wide maximum aperture (low f-number, e.g., f/1.4, f/1.8, f/2). A wider aperture allows for:
* Shallow depth of field: This blurs the background, drawing focus to the subject and creating a pleasing aesthetic.
* Better low-light performance: More light reaches the sensor, allowing for faster shutter speeds and less noise in dimly lit environments.
However, wider apertures are generally more expensive. An f/2.8 lens is a good compromise between cost and performance.
3. Image Stabilization (IS) or Vibration Reduction (VR): This feature helps to reduce camera shake, especially useful when shooting in low light or with longer focal lengths. It's a worthwhile investment if your budget allows.
4. Autofocus: A fast and accurate autofocus system is essential for capturing sharp portraits, especially when shooting moving subjects. Look for lenses with features like:
* Ultrasonic Motor (USM) or similar: These provide quiet and fast autofocus.
* Multiple autofocus points: Ensures accurate focusing even with complex backgrounds.
5. Build Quality and Weather Sealing: Consider the lens's build quality and whether it's weather-sealed. Durable construction is important if you plan to use the lens frequently in various conditions.
6. Budget: Portrait lenses can range significantly in price. Set a budget before you start your search and prioritize the features that are most important to you.
Lens Recommendations (Examples - specific models depend on your camera system):
* Budget-friendly: 50mm f/1.8 or similar. Excellent value for the price.
* Mid-range: 85mm f/1.8 or f/2. A good balance between cost and performance.
* High-end: 85mm f/1.4 or 135mm f/2. Exceptional image quality and low-light performance, but significantly more expensive.
In summary:
Start by considering your focal length preference (85mm is a popular starting point), then prioritize a wide maximum aperture for shallow depth of field and low-light capabilities. Finally, weigh the other factors (IS/VR, autofocus, build quality) against your budget to find the perfect portrait lens for your needs. Reading reviews from other photographers can also be very helpful.