Portrait lenses are quite simply, lenses that are considered suitable for portraits. But what makes them suitable?
The EXIF data on the RAW image of this photo tells me that I photographed this person with a 105 mm lens with a Nikon D70, which is an APS-C sensor camera, so that the full frame focal length equivalent of the lens is 157mm.
The full data is that I shot at an aperture of f4.0, a shutter speed of 1/320 second, and at ISO 200.
Photographers would describe a lens with 105 mm focal length as a short telephoto lens, meaning it is built for taking shots where the subject is at a medium distance of a few metres from the camera. Of course you can shoot subjects that are further away, but it will take a bigger subject to fill the frame.
105 mm is not a long telephoto. Photographers at an Olympic event or shooting wildlife might shoot with a 500mm or 600mm lens.
Long lenses have a narrow angle of view. Wide angle lenses have a wide angle of view. Imagine rolling up a newspaper and looking down the tube. That’s a narrow angle of view.
There are online calculators that will tell you what side to side distance a lens will cover at different distances based on the angle of view of a lens of that focal length.
If I am photographing a person then I can shoot with a 35mm lens on a crop sensor camera for an environmental portrait of their head and waist. I can shoot a head shot with a 50mm lens on a crop frame camera. Or I can move further back and shoot with a longer lens – let’s say anything from 85mm to 135mm.
And each time I move further away, the shape of the person’s head will appear to change. That is because the camera to subject distance affects how the subject looks. When the camera is close to the subject it pulls the central part of the subject forward and bends the outer parts of the subject further back. My wife calls short focal length lenses ‘sheep’ lenses because they accentuate the nose like on the long face of a sheep. A 28mm lens would be a sheep lens.
As the distance from the camera to the subject increases, the subject flattens and widens. And there is a point – a camera to subject distance and focal length combination – that is most appealing to the eye. From this we know what qualities portrait lenses should have that make them suitable for portraits.
One thing to remember is that even a 50mm lens, with which I photographed this sheep, will act like a sheep lens if it is very close to the subject. And especially so when the subject is a sheep.