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). See some examples below:
Well, the distance between the camera and the person you’re making the portrait of, will be +- 20 times the focal length.
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.
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‘. When you go closer, you 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. But if 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: in order to fit a person in the picture when you hold the camera in upright (‘portrait’) position, you need 50 times the focal distance.
20 for a portrait, 50 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!