Focal length for the common man: "portrait distance"01 Feb 2010
I remember that before I started photography on a serious level, I had some understanding of shutter speed, but none of aperture and focal length. Even when I read what they meant, I still couldn´t ‘picture’ it, had no feeling for the numbers. Let´s leave ‘aperture’ for another time and just concentrate for now on the concept of focal length.
First of all, the focal length of a lens is not the same as the actual physical length of the lens. Yes, 200mm and 300mm lenses (telephoto lenses) tend to be longer, but they´re not exactly 200mm and 300mm long. For instance, the Sigma 55-200mm F4-5.6 DC HSM is 85mm (3.3″) long, while the 70-200mm F2.8 II EX DG lens is 184mm (7.2″). Same maximal focal length, but more than twice as long.
So what is focal length? I could explain that it is “the distance from the center of the lens to the principal foci (or focal points) of the lens“, but that wouldn´t make it more comprehensible, would it? Well, I read through the theory, with tangens of the viewing angle and stuff, and I think I understand it (I´m an engineer, I actually like trigoniometry). A 200mm lens gives a viewing angle of 12° on the diagonal. Still not clear? That´s when I thought: let´s invent something more tangible: the portrait distance.
Say you need a surface of about 72cm x 48cm (28″ x 18″) to make a portrait of a person (not just a headshot, but with some torso on it too).
Portrait distance: to make a portrait in upright (‘portrait’) position, you need a distance of 20 times the focal distance.
For a digital full frame camera (like the Canon 5D or the Nikon D700) this goes as follows:
- a 50mm `portrait´ lens => you need +- 20 x 50mm or 1 meter to make a portrait.
- an 85mm `portrait´ lens => you need +- 20 x 85mm or 1.7 meter to make a portrait.
- a 200mm `tele´ lens => you need +- 20 x 200mm or 4 meter to make a portrait.
For a cheaper dSLR camera like the Canon 50D, Nikon D90 or lower, you need to take into account the crop factor of 1.6:
- a 50mm `portrait´ lens is equivalent to a 80mm => you need +- 20 x 80mm or 1.6 meter to make a portrait.
This is not a law, but a rough approximation, a heuristic. If you go closer, you will still have a portrait, but the face will cover more than a quarter of the surface. If you go further away, you get more of the body in the shot. When you go shopping for lenses, certainly when you´re a photographer of people, it can be nice to keep this in mind.
And this scales linearly, so say you need 2,5 times that height: 180cm x 120 cm because 1,80cm is the average height of a person: you will find this at 20 x 2,5 = 50 times the focal distance.
Person distance: to fit a person completely in the picture (in ‘portrait’ orientation), you need 50 times the focal distance.
20 x focal length for a portrait, 50 x focal length for a person.
if we use the common 3:2 aspect ratio for photography.
30 x focal length for a portrait, 75 x focal length for a person.
So if you´re a paparazzo with a Canon 1D MkIV, and you want to shoot a couple kissing at 120 meters/130 yards? You need a 12000 mm/20 = 600mm lens. 5600 grams and even more euros, but you´ll get that kiss!