Can you recommend a good portrait lens? I have Nikon D90 and would like to buy such a lens, preferably a Sigma or Nikon. I already own the two kit lenses but want something that is better for portraiture.
Before I provide a recommendation, Roberto, let’s consider what is meant by a “portrait lens.” That would be one that allows for a tight head-and-shoulders portrait without the need to move too close to the subject, in order to provide pleasing perspective. In 35-mm film photography, 85 mm, 105 mm and 135 mm lenses were considered to be ideal for indoor use. Photographers generally wanted a wide aperture lens—such as f/1.4—in order to produce shallow depth of field: a narrow range of acceptably sharp focus to blur away a cluttered background when not shooting in a studio.
Your digital SLR camera produces a “focal length magnification” factor of 1.5x. (Actually, this is “field of view crop” but in practical terms, the focal length of any lens seems to be 1.5x longer.) Consequently, an ideal portrait lens would be something with a focal length of around 50 mm to 85 mm. All of those are available with many Nikon or Sigma zoom lenses with a maximum aperture of f/2.8. Because of the wide aperture, rugged construction and large size, they are pricey as well as heavy.
Note too that you can find single focal length macro lenses in suitable focal lengths, the Sigma 70 mm f/2.8 EX DG Macro and the 50 mm f/2.8 EX DG Macro or two from Nikon with the very fast AF-S/Silent Wave (ultrasonic) autofocus motor: the AF-S Micro NIKKOR 60 mm f/2.8G ED and the AF-S DX Micro NIKKOR 85 mm f/3.5G ED VR (the latter with a VR image stabilizer.) All of these lenses can focus extremely close for frame-filling photos of a small blossom for instance, but they also provide suitable focal lengths for portraiture. While they’re large, heavy and relatively expensive any of them would be very useful if you also enjoy nature photography.
In a conventional single focal length lens, I would recommend one of the following, with an even wider maximum aperture: Sigma’s 50 mm f/1.4 EX DG HSM or 85 mm f/1.4 EX DG HSM or the AF NIKKOR 50 mm f/1.8D, AF-S NIKKOR 50 mm or f/1.4G AF-S NIKKOR 50 mm f/1.8G or AF-S NIKKOR 85 mm f/1.8G . Frankly, one of these five lenses would be the best bet if you’re primarily interested in portraiture. It will be smaller/lighter than a Macro lens and (especially if f/1.4) it will offer a much wider maximum aperture for more obvious background blurring for a more pleasing image.