1. Focal Length:
* General Guidelines:
* 50mm: Often considered a good starting point for portraits, especially on full-frame cameras. It offers a perspective close to human vision. Good for head-and-shoulders shots and environmental portraits.
* 85mm: A classic portrait focal length. Provides excellent subject isolation and flattering compression. Ideal for headshots, tight head-and-shoulders portraits, and upper body shots.
* 100mm-135mm: Excellent for tighter headshots and portraits where you want to be further away from the subject. They offer even more compression, making the background appear closer and often smoother.
* 70-200mm (zoom): A versatile zoom lens that covers a range of portrait-friendly focal lengths. Offers flexibility in framing and composition. Great for outdoor portraits.
* 35mm: Wider focal length, suitable for environmental portraits that show the subject in their surroundings. Less flattering for tight headshots due to distortion.
* Crop Sensor vs. Full-Frame:
* Crop Sensor (APS-C, Micro Four Thirds): You need to consider the crop factor when choosing a focal length. Multiply the focal length by the crop factor to get the equivalent focal length on a full-frame camera. For example:
* 50mm on a 1.5x crop sensor camera = 75mm equivalent
* 50mm on a 1.6x crop sensor camera = 80mm equivalent
* 25mm on a 2x (Micro Four Thirds) crop sensor camera = 50mm equivalent
* Key Considerations for Focal Length:
* Compression: Longer focal lengths compress the image, making features appear closer together and generally more flattering. Wider focal lengths can distort features, especially the nose.
* Distance to Subject: Longer focal lengths require you to stand further away from the subject, which can be helpful for making them feel less self-conscious.
* Background Isolation (Bokeh): Longer focal lengths, combined with wider apertures, generally produce more shallow depth of field and blurrier backgrounds (better bokeh).
* Environmental vs. Close-Up: Think about whether you want to capture the environment around your subject or focus solely on their face.
2. Aperture:
* Wider Aperture (e.g., f/1.4, f/1.8, f/2.8):
* Pros:
* Shallow Depth of Field: Creates beautiful bokeh (blurred background), isolating the subject.
* Low Light Performance: Allows more light to enter the lens, enabling you to shoot in darker conditions without increasing ISO.
* Subject Isolation: Helps separate the subject from the background, drawing attention to them.
* Cons:
* Shallower Focus Plane: Can be more challenging to get both eyes perfectly in focus. Requires precise focusing.
* Cost: Lenses with very wide apertures tend to be more expensive.
* Chromatic Aberration: Some wide-aperture lenses can exhibit chromatic aberration (color fringing).
* Narrower Aperture (e.g., f/4, f/5.6):
* Pros:
* Deeper Depth of Field: More of the image will be in focus.
* Easier to Focus: Less critical focus accuracy needed.
* Cheaper: Lenses with narrower maximum apertures are generally more affordable.
* Cons:
* Less Background Blur: Doesn't isolate the subject as much.
* Less Low Light Performance: Requires higher ISO or slower shutter speeds in low light.
* The Sweet Spot: Many lenses are sharpest a few stops down from their widest aperture (e.g., f/2.8 or f/4 on an f/1.8 lens).
3. Image Quality:
* Sharpness: A sharp lens captures fine details clearly. Look for reviews that test sharpness at different apertures.
* Distortion: Ideally, a portrait lens should have minimal distortion. Wider lenses can have more noticeable distortion.
* Chromatic Aberration: Avoid lenses with excessive chromatic aberration (color fringing), especially in high-contrast areas.
* Vignetting: Some lenses cause vignetting (darkening of the corners). This can be corrected in post-processing, but it's best to choose a lens with minimal vignetting if possible.
* Bokeh: Evaluate the quality of the bokeh (background blur). Look for smooth, pleasing bokeh with no harsh edges or "onion ring" effect.
4. Autofocus Performance:
* Speed and Accuracy: A fast and accurate autofocus system is essential for capturing sharp portraits, especially when shooting moving subjects.
* Silent Autofocus: Can be important if you're shooting video.
* Focusing Modes: Consider the focusing modes offered by the lens and how they work with your camera's autofocus system (e.g., continuous autofocus, eye-detection autofocus).
5. Build Quality and Handling:
* Durability: Choose a lens that's well-built and can withstand the rigors of professional use.
* Size and Weight: Consider how the lens will balance on your camera body and whether it will be comfortable to carry around.
* Manual Focus Ring: A smooth, well-damped manual focus ring is important for fine-tuning focus.
* Weather Sealing: If you shoot in challenging conditions, look for a lens with weather sealing to protect it from dust and moisture.
6. Budget:
* Portrait lenses range in price from a few hundred dollars to several thousand. Set a budget and stick to it.
* Consider buying used lenses to save money.
* Don't feel like you need the most expensive lens. A good lens in the right hands can produce stunning results.
Specific Lens Recommendations (General Guidelines - Check Compatibility with your Camera Brand and Sensor Size):
* Budget-Friendly Options:
* 50mm f/1.8: A classic "nifty fifty" that's affordable and offers excellent image quality.
* 85mm f/1.8: A step up from the 50mm, providing better subject isolation and compression.
* Mid-Range Options:
* 50mm f/1.4: Offers slightly better image quality and low-light performance than the f/1.8.
* 85mm f/1.4: A popular choice for professional portrait photographers.
* 70-200mm f/2.8: A versatile zoom lens that covers a range of portrait-friendly focal lengths. (Often more expensive)
* High-End Options:
* Specialty prime lenses (e.g., 135mm f/2): Known for exceptional image quality, bokeh, and sharpness. Often carry a high price tag.
Tips for Choosing:
* Rent Before You Buy: Rent a lens before you buy it to see if it meets your needs and suits your shooting style.
* Read Reviews: Read reviews from reputable sources (e.g., DPReview, Imaging Resource, LensRentals).
* Look at Sample Images: Examine sample images taken with the lens to get an idea of its image quality and bokeh.
* Consider Your Shooting Style: Think about the types of portraits you want to create and choose a lens that's well-suited for those types of shots.
* Test in Real-World Conditions: Test the lens in the type of lighting and environments where you typically shoot.
* Focus on the Art, Not the Gear: Remember that the most important factor in creating great portraits is your skill as a photographer, not the lens you use.
In summary: The "perfect" portrait lens is a balance of focal length, aperture, image quality, autofocus performance, build quality, and budget. By carefully considering these factors and testing different lenses, you can find the perfect lens for your needs and shooting style. Good luck!