1. Understanding Focal Length:
* Focal length is measured in millimeters (mm) and determines the field of view and perspective compression. It's the *most* important factor.
* Classic Portrait Range (50mm - 135mm on a full-frame camera): This range offers a good balance between perspective compression, working distance, and background blur.
* 50mm: A versatile "standard" lens. Good for environmental portraits, showing more of the surroundings. Can feel a bit wide for very close-up portraits, potentially leading to slight distortion (though usually minor). Often very affordable.
* 85mm: Considered by many to be the *ideal* portrait lens. Excellent perspective compression (flattering!), a comfortable working distance, and beautiful background blur (bokeh). Popular choice among professionals.
* 100mm - 135mm: Compresses features even more and throws backgrounds further out of focus. Requires more working distance. Great for tighter headshots and when you want to minimize distracting backgrounds. 135mm can be tricky indoors due to the longer distance needed.
* Shorter Focal Lengths (e.g., 35mm): Can work for portraits, especially environmental portraits where you want to show the subject within their surroundings. Requires careful composition to avoid distortion, particularly when close to the subject. More challenging for achieving significant background blur.
* Longer Focal Lengths (e.g., 200mm+): Compresses features even further and provides even more background blur. Requires a *lot* of working distance, often making them impractical for indoor portraits. Excellent for tight headshots when you can't get close to your subject.
2. Sensor Size Matters (Crop Factor):
* The focal length's effect is different on cameras with different sensor sizes. A crop sensor camera (APS-C, Micro Four Thirds) has a smaller sensor than a full-frame camera. This "crops" the image, making the lens appear to have a longer focal length.
* Crop Factor (usually 1.5x or 1.6x for APS-C): Multiply the lens's focal length by the crop factor to get the equivalent focal length on a full-frame camera.
* Example: A 50mm lens on an APS-C camera with a 1.6x crop factor is equivalent to an 80mm lens on a full-frame camera (50mm * 1.6 = 80mm).
* Important for Choosing the Right Lens: If you have a crop sensor camera, you'll need a shorter focal length lens to achieve a similar effect to a longer lens on a full-frame camera.
* Good Crop Sensor Portrait Options: 35mm (equivalent to ~50mm on full-frame), 50mm (equivalent to ~75mm - 80mm on full-frame)
3. Aperture (f-stop):
* Aperture controls how much light enters the lens and affects the depth of field (the area that's in focus). A *wider* aperture (lower f-number like f/1.4, f/1.8, f/2.8) lets in more light and creates a shallower depth of field, resulting in blurred backgrounds (bokeh).
* Wide Aperture Benefits:
* Beautiful Background Blur (Bokeh): Separates the subject from the background, drawing attention to them.
* Low Light Performance: Allows you to shoot in darker conditions without increasing ISO or slowing down the shutter speed.
* Creamy Highlights: Wide apertures can create beautiful, soft highlights.
* Wide Aperture Considerations:
* Shallower Depth of Field: Can be difficult to nail focus perfectly, especially with fast-moving subjects or when shooting wide open.
* Lens Cost: Lenses with wider apertures are typically more expensive.
* Portrait Aperture Recommendations:
* f/2.8: A good balance between sharpness, background blur, and cost. A very popular choice.
* f/1.8 - f/2: Even more background blur, great for separating the subject. Often more affordable than f/1.4 lenses.
* f/1.4 or Wider: The ultimate in background blur and low-light performance. More expensive and requires careful focusing. Can be challenging to use effectively, but the results can be stunning.
* f/4 - f/5.6: Can work for group portraits or environmental portraits where you want more of the background in focus.
4. Image Quality:
* Sharpness: A key factor. Read reviews to see how sharp the lens is, especially at wider apertures. Many lenses are sharpest when stopped down a bit (e.g., f/2.8 on a lens that goes to f/1.4).
* Contrast: The lens's ability to render tones accurately.
* Color Rendition: How the lens renders colors. Some lenses have a warmer or cooler color cast.
* Distortion: Wider lenses can exhibit distortion (straight lines appearing curved). Portrait lenses generally have minimal distortion.
* Chromatic Aberration (CA): Color fringing, especially visible in high-contrast areas. Better lenses have better CA control.
* Vignetting: Darkening of the corners of the image. Can be corrected in post-processing.
5. Autofocus (AF) Performance:
* Speed and Accuracy: Important, especially when shooting moving subjects. Look for lenses with fast and accurate AF systems. Modern autofocus systems are very good, even in less expensive lenses.
* Quietness: Important if you plan to shoot video.
6. Build Quality:
* Durability: Consider the materials used and how well the lens is constructed. A metal lens barrel is typically more durable than a plastic one.
* Weather Sealing: Protects the lens from dust and moisture, allowing you to shoot in a wider range of conditions.
7. Price:
* Set a Budget: Lenses can range from a few hundred dollars to several thousand. Determine your budget before you start looking.
* Consider Used Lenses: Buying used can save you money. Check the lens carefully for scratches, fungus, and other damage.
* Prioritize Quality Over Brand Loyalty: Don't automatically assume that a lens from a specific brand is the best. Read reviews and compare lenses from different manufacturers. Third-party lenses (e.g., Sigma, Tamron, Rokinon) often offer excellent value.
8. Features:
* Image Stabilization (IS/VR): Helps reduce camera shake, allowing you to shoot at slower shutter speeds. Useful for handheld shooting, especially in low light. Not essential for portraiture if you are shooting at reasonable shutter speeds, or using flash.
* Aperture Ring: Allows for manual aperture control directly on the lens, favored by some photographers, particularly for video.
* Focus Limiter: Allows you to limit the focusing range, which can speed up autofocus performance.
* Manual Focus Override: Allows you to manually fine-tune the focus after the autofocus has locked on.
9. Testing and Research:
* Read Reviews: Read reviews from reputable sources (e.g., DPReview, Photography Life, Ken Rockwell) to get an idea of the lens's performance.
* Look at Sample Images: See how the lens renders images in real-world scenarios. Flickr and 500px are good sources for finding sample images.
* Rent Before You Buy: If possible, rent the lens before you buy it to see if it's a good fit for your shooting style. BorrowLenses and LensRentals are popular rental services.
* Try Before You Buy (if possible): Go to a camera store and try out different lenses on your camera body.
Summary Checklist:
* Camera Type: Full-frame, APS-C, or Micro Four Thirds?
* Focal Length: 50mm, 85mm, 100-135mm (on full-frame or equivalent)? Consider your shooting space and desired compression.
* Aperture: f/2.8, f/1.8, f/1.4, or wider? How much background blur do you want? How important is low-light performance?
* Image Quality: Sharpness, contrast, color rendition, distortion, chromatic aberration.
* Autofocus: Speed and accuracy.
* Build Quality: Durability and weather sealing.
* Price: Set a budget and stick to it.
* Features: Image stabilization, aperture ring, focus limiter.
* Research: Read reviews, look at sample images, rent if possible.
Ultimately, the "perfect" portrait lens is subjective and depends on your individual needs and preferences. Experimentation is key! Good luck!