1. Understanding Focal Length and Perspective:
* Focal Length (measured in mm): This is the primary factor determining how much of the scene the lens captures and how it renders perspective.
* Typical Portrait Focal Lengths:
* 35mm: Good for environmental portraits, showing the subject in their surroundings. Can introduce some distortion if you get too close.
* 50mm: Versatile, close to the human eye's perspective. Great for head-and-shoulder shots and wider portraits. Can be used in many situations
* 85mm: A portrait workhorse. Excellent for headshots and upper-body portraits. Flattering perspective, good subject isolation.
* 100-135mm: Offers even more compression, blurring the background further and creating a very flattering perspective. Ideal for tight headshots and avoiding distortion.
* 200mm+: Useful for shooting from a distance, offering extreme compression and background blur (bokeh). Can be challenging to use in tight spaces.
* Perspective Compression: Longer focal lengths compress the distance between objects in the foreground and background, making them appear closer together. This is flattering for portraits because it minimizes the appearance of noses and other facial features that might be accentuated by wider lenses.
2. Aperture (f-stop):
* Definition: Aperture is the size of the lens opening, controlling the amount of light entering the camera and the depth of field (the area in focus).
* Importance for Portraits: A wide aperture (e.g., f/1.2, f/1.4, f/1.8, f/2.8) is crucial for portraits because:
* Shallow Depth of Field: Creates a blurred background (bokeh), isolating the subject and drawing attention to them.
* Low-Light Performance: Allows you to shoot in darker environments without raising the ISO too high, minimizing noise.
* Considerations:
* Cost: Wider apertures are generally more expensive.
* Sharpness: Some lenses may not be at their sharpest wide open. Test the lens at different apertures to find its "sweet spot."
* Focusing: Shallower depth of field requires precise focusing on the subject's eyes.
3. Camera Sensor Size (Crop Factor):
* Full-Frame Cameras: The sensor size is the same as a 35mm film frame. The focal length printed on the lens is the *actual* focal length.
* Crop Sensor Cameras (APS-C, Micro Four Thirds): The sensor is smaller than a full-frame sensor. This effectively *crops* the image, making the lens appear to have a longer focal length.
* APS-C: Multiply the lens focal length by 1.5 (Nikon, Sony, Pentax) or 1.6 (Canon) to get the equivalent full-frame focal length.
* Micro Four Thirds: Multiply the lens focal length by 2 to get the equivalent full-frame focal length.
* Example: A 50mm lens on an APS-C camera with a 1.5x crop factor will have an effective focal length of 75mm (50mm x 1.5).
* Implication for Portrait Lens Choice: If you have a crop sensor camera, you'll need to choose a lens with a shorter focal length to achieve the same field of view as a full-frame camera. For example, a 35mm lens on an APS-C camera might be a good equivalent to a 50mm lens on a full-frame camera for portraits.
4. Autofocus vs. Manual Focus:
* Autofocus (AF): Most modern lenses have autofocus. Fast and accurate autofocus is essential for capturing sharp portraits, especially when shooting with wide apertures.
* Manual Focus (MF): While autofocus is generally preferred, some photographers prefer manual focus for precise control, especially in challenging lighting situations or when using older lenses.
* Hybrid AF/MF: Many lenses allow you to switch between autofocus and manual focus.
5. Lens Construction and Image Quality:
* Sharpness: A sharp lens is crucial for capturing detailed portraits. Look at lens reviews and image samples to assess sharpness across the frame, especially at different apertures.
* Chromatic Aberration (CA): Also known as color fringing, CA can appear as purple or green fringes around high-contrast edges. Good lenses minimize CA.
* Distortion: Some lenses can introduce distortion, which can make straight lines appear curved. This is less of a concern with portrait lenses than with wide-angle lenses.
* Vignetting: The darkening of the corners of an image. Some vignetting can be desirable for portraits, as it draws attention to the subject.
* Build Quality: A well-built lens will be more durable and resistant to the elements.
6. Budget:
* Portrait lenses range in price from a few hundred dollars to several thousand.
* Prioritize Aperture and Sharpness: Focus your budget on lenses with wide apertures and excellent sharpness.
* Consider Used Lenses: Buying used lenses can save you money while still getting a high-quality optic.
* Kit Lenses: While kit lenses are often not ideal for portraits, they can be a starting point. Experiment with the zoom range and aperture settings to see what focal lengths and depths of field you prefer. Then, invest in a dedicated portrait lens.
7. Lens Brands and Models:
* Canon:
* EF 50mm f/1.8 STM (affordable and excellent value)
* EF 50mm f/1.4 USM
* EF 85mm f/1.8 USM
* EF 85mm f/1.4L IS USM (higher end, image stabilization)
* EF 135mm f/2L USM (excellent compression and bokeh)
* Nikon:
* AF-S Nikkor 50mm f/1.8G (affordable and excellent value)
* AF-S Nikkor 50mm f/1.4G
* AF-S Nikkor 85mm f/1.8G
* AF-S Nikkor 85mm f/1.4G
* Sony (E-Mount):
* Sony FE 50mm f/1.8
* Sony FE 50mm f/1.4 GM
* Sony FE 85mm f/1.8
* Sony FE 85mm f/1.4 GM
* Sony FE 135mm f/1.8 GM
* Third-Party Brands (Sigma, Tamron, Samyang): These brands offer excellent alternatives, often at a lower price point.
* Sigma 50mm f/1.4 DG HSM Art
* Sigma 85mm f/1.4 DG HSM Art
* Tamron SP 85mm f/1.8 Di VC USD
* Samyang (Rokinon) 85mm f/1.4 (manual focus)
8. Try Before You Buy:
* Rent Lenses: Renting lenses allows you to test them out in real-world situations before committing to a purchase. Websites like LensRentals.com and BorrowLenses.com offer lens rentals.
* Visit a Camera Store: Handle the lenses in person, mount them on your camera, and take some test shots (if possible).
* Read Reviews and Watch Videos: Research online reviews and watch videos to learn about the lens's performance and characteristics.
Summary: A Step-by-Step Decision Process
1. Determine your camera's sensor size (full-frame or crop sensor).
2. Choose your desired focal length range based on the type of portraits you want to shoot (environmental, headshots, etc.). Consider the crop factor of your camera.
3. Determine your budget.
4. Prioritize a wide aperture (f/2.8 or wider).
5. Research specific lens models within your budget and focal length range, paying attention to sharpness, autofocus performance, and build quality.
6. Read reviews and watch videos.
7. Rent or try out the lens before buying, if possible.
By following these steps, you'll be well on your way to selecting the perfect portrait lens for your needs and creating stunning, professional-looking portraits. Good luck!