Professional portrait photographers may favour top-money lenses such as the Canon EF 85mm f/1.2 or Nikon 85mm f/1.4G. But for those of us who don't want to use this type of lens on a daily, money-earning basis, the respective prices of £1,700/US$2,000/AU$2,300 and £1,200/US$1,450/AU$1,800 put them beyond sensible reach.
At the other end of the scale, lenses such as the Canon EF 50mm f/1.8 cost a mere £80/US$120/AU$130, but come up short in terms of features, performance and build quality. Thankfully, there's plenty of choice to be had in the middle ground, with a range of prime (fixed length) lenses that are affordable but capable. So what makes a good portrait lens?
The first thing to consider is focal length. If you're using a camera with an APS-C format image sensor, a 50mm lens will give an effective focal length of around 75mm to 80mm. This is very close to the 85mm focal length that's considered ideal for portraiture. It enables half-length portraits to be taken from a comfortable distance of around 3m, so you can direct your subject without crowding in and making them feel awkward.
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Use an 85mm lens on an APS-C camera, and you'll be able to take head-and-shoulders portraits from about the same distance. One particular advantage of using an 85mm lens for portraiture is that the short telephoto focal length has the effect of slightly compressing any prominent facial features (think noses and chins) for a bit of added flattery.
Generally, prime lenses offer superior image quality to zoom lenses, which makes the optics on test instantly appealing. Their killer feature, however, is a wide maximum aperture that's usually between f/1.4 and f/1.8. This enables a tight depth of field, so you can blur fussy backgrounds and make the person you're shooting really stand out in an image.
The lens's 'bokeh' is all-important. This is the quality of defocused areas within the image, and the aim is to produce a smooth and creamy-looking blur effect. When shooting at anything other than a wide-open aperture, one thing that helps with this is for the lens to feature a well-rounded diaphragm.
Wider apertures also help if you're taking indoor or twilight portraits and you want to make the most of ambient lighting effects without using flash. The key benefit is that you can use faster shutter speeds to enable handheld shooting, as well as freezing any slight movement in the person you're photographing, without having to greatly increase your camera's sensitivity (ISO) setting.
That said, boosting the ISO is less of an issue than it was just a few years ago, since most current cameras offer very good image quality at high sensitivity settings.
At their widest available apertures, very fast lenses often suffer from a significant drop in image sharpness. That's not always a bad thing, since it can give a soft, dreamy look to portraits. It's a good anti-wrinkle feature, for reducing the signs of premature ageing. The only real downside is that the eyes may not be as sharp as you'd like them. You may also need to use a Neutral Density filter in bright lighting conditions, to avoid exceeding your camera's fastest shutter speed.
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Portrait lenses aren't just about large apertures. So-called environmental portraits are perennially popular, featuring people in their home or working environment.
When shooting, say, a craftsman in his workshop, you may want to use a small aperture to give a larger depth of field, giving clarity to the surroundings. Sharpness at small apertures can therefore also be an important consideration. As always, maximum sharpness is usually achieved at mid-range aperture settings of around f/8.
Medium apertures are useful when using studio flash lighting as well as for general portraiture. Bear in mind that if you use a very wide aperture for a head-and-shoulders portrait, the person's ears may be slightly defocused when you're focusing on their eyes.
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In most portraiture, sharpness is crucial in the central region of the frame. It's a frequently used composition trick to have the person off-centre in a portrait image but, even so, sharpness at the extreme edges and corners is usually unimportant. The exception is when taking group portraits, where people on the periphery may be quite close to the corners of the frame.
It's tempting to use a wide-angle lens for group shots, for the sake of convenience, but the problem with this is that the people near the sides of the image will end up with stretched heads. It's much better practice to stand further back if possible, and to use a lens with a more standard focal length.
Fast autofocus isn't as crucial as it is for action sports or wildlife photography, but it can still be a factor. Nobody wants to miss a fleeting expression or classic moment because they're waiting for autofocus to crawl into place. Quietness of autofocus is also a bonus for shooting candid portraiture.