1. Understanding Focal Length:
* The Classic Range: 50mm - 135mm: This is generally considered the sweet spot for portraiture. These focal lengths offer a good balance between flattering compression and a manageable working distance.
* Shorter Focal Lengths (e.g., 35mm): Wider lenses can be used for environmental portraits, showing more of the subject's surroundings. However, be cautious of distortion, especially when the subject is close to the edges of the frame. Can make the subject look smaller relative to the environment.
* Longer Focal Lengths (e.g., 200mm+): Telephoto lenses offer extreme compression, blurring the background dramatically and isolating the subject. Can lead to a less personal and more detached feel. Also, they require more working distance.
* Crop Sensor vs. Full Frame: Keep in mind the crop factor of your camera. A 50mm lens on a crop sensor camera (e.g., APS-C with a 1.5x crop) will have a field of view equivalent to a 75mm lens on a full-frame camera. Multiply the focal length by the crop factor to get the equivalent full-frame focal length.
General Guidelines Based on Focal Length:
* 35mm - 50mm: Environmental portraits, showing the subject in their context.
* 50mm - 85mm: Versatile, good for headshots to half-body portraits. Often considered the ideal range.
* 85mm - 135mm: Classic portrait range, flattering perspective, beautiful bokeh.
* 135mm - 200mm+: Dramatic background blur, isolates the subject, requires more distance.
2. Aperture (f-stop):
* Fast Apertures (e.g., f/1.2, f/1.4, f/1.8, f/2.8): These are highly desirable for portrait lenses.
* Shallow Depth of Field: Creates beautiful background blur (bokeh), isolating the subject.
* Low-Light Performance: Allows you to shoot in darker conditions without raising ISO too high.
* More Light Gathering: Helps with faster shutter speeds to freeze motion.
* Slower Apertures (e.g., f/4, f/5.6): Still usable for portraits, but offer less background blur and require more light. Often found on zoom lenses or kit lenses.
3. Image Quality and Sharpness:
* Sharpness: A sharp lens is crucial for capturing details. Look for reviews that discuss sharpness at various apertures, especially wide open.
* Aberrations:
* Chromatic Aberration (CA): Color fringing (purple/green edges) around high-contrast areas. Good lenses minimize CA.
* Vignetting: Darkening of the corners of the image, especially at wider apertures. Some vignetting can be desirable for aesthetic reasons.
* Distortion: Straight lines appearing curved. Prime lenses generally have less distortion than zoom lenses.
* Contrast and Color Rendition: Consider the overall look of the lens. Does it produce images with rich colors and good contrast, or a more muted, vintage look?
4. Autofocus (AF) Performance:
* Speed and Accuracy: Important, especially if you're photographing moving subjects or shooting in challenging lighting conditions.
* Focusing Modes: Look for lenses with various focusing modes (e.g., single-point AF, continuous AF, face detection).
* Manual Focus (MF): Even with good autofocus, the ability to manually focus is useful for fine-tuning or specific creative effects.
5. Budget:
* Entry-Level Options: 50mm f/1.8 lenses are very affordable and offer excellent value. Consider also kit lenses within the 50-85mm range.
* Mid-Range Options: 35mm f/2, 50mm f/1.4, 85mm f/1.8 lenses offer a good balance of performance and price.
* High-End Options: 35mm f/1.4, 50mm f/1.2, 85mm f/1.4 lenses are top-of-the-line and deliver exceptional image quality, but come with a higher price tag.
* Used Market: Consider buying used lenses to save money.
6. Other Considerations:
* Build Quality: A well-built lens will be more durable and withstand more wear and tear. Consider the materials used (e.g., metal vs. plastic).
* Size and Weight: A large, heavy lens can be tiring to use for extended periods.
* Image Stabilization (IS/VR): Helpful for shooting handheld in low light, especially with longer focal lengths.
* Zoom vs. Prime:
* Prime Lenses (fixed focal length): Generally sharper, faster aperture, more compact, and often less expensive than zoom lenses. Force you to "zoom with your feet," which can improve your composition.
* Zoom Lenses: More versatile, allowing you to change focal lengths without changing lenses. Convenient for situations where you can't physically move closer or further from your subject.
7. Testing and Research:
* Read Reviews: Check reputable websites and magazines for lens reviews that discuss sharpness, aberrations, autofocus performance, and overall image quality.
* Look at Sample Images: Examine sample images taken with the lens to see how it renders colors, bokeh, and details. Look on Flickr, 500px, or lens review sites.
* Rent a Lens: If possible, rent a lens before buying it to try it out with your own camera and shooting style. This is the best way to determine if it's the right fit for you.
* Test in Store: If possible, visit a camera store and try out different lenses on your camera.
In summary, to choose the perfect portrait lens:
1. Decide on your desired focal length range based on the type of portraits you want to create (environmental, headshots, etc.).
2. Prioritize a fast aperture (e.g., f/1.8 or faster) for shallow depth of field and low-light performance.
3. Research and read reviews to find lenses with good sharpness, minimal aberrations, and reliable autofocus.
4. Consider your budget and look at both new and used options.
5. If possible, rent or test lenses before buying to ensure they meet your needs and preferences.
Ultimately, the best portrait lens is the one that helps you achieve your creative vision and produce images that you love. Don't be afraid to experiment and find what works best for you. Good luck!