There is a lot of differentiation being stated between artificial studio lighting and natural lighting. However, while the physical facts of each lead to certain qualities and situations that become commonly associated with each, in the end light is light, and metering theory is the same for both.
No matter where you are, your light can either be direct (e.g. sun or lamp on face), reflected (e.g. bounce back of light from the environment and sky, bounce umbrella, or bounce card), diffused direct (overcast day or softbox), or diffused reflected (not super common in nature, but the flattest light you can get, and also the hardest to work with IMO; examples would be a bounce umbrella shot through diffusion - or a "beauty dish" when used with the diffusion over the reflector - or reflected sunlight from the environment and sky coming through curtains).
In almost every single photographic situation, you are going to have at least two of these types of light in the picture, illuminating different parts of the subject with different qualities and different intensities. (Exceptions would be things like a single light studio setup with steps taken to eliminate all bounce, or shooting someone illuminated by a "stream" of light coming into a totally dark building that provides no reflection of the light whatsoever.) There are almost always a main light and a fill light, whether you are outdoors or not. A subject shot on an overcast day is primarily illuminated by direct diffused light from the clouds – the world's largest softbox. But it is also being illuminated by reflected light from everything around it: ground, sky, buildings, trees, etc. A subject shot in open shade is primarily being lit by reflected sky light. But it is also being lit by that sky light bouncing off of the ground or other nearby objects. A subject shot in hard, direct sun is not only being illuminated by the sun, but by reflected light from the surroundings and the sky. It is just the same in studio, only it is more obvious to the untrained eye; the fact that multiple sources are illuminating the subject is painfully obvious in the studio. But this doesn't mean that the same thing is not going on in nature.
So, the statement that metering the main light is only useful in studio does not make sense. There are multiple sources of illumination, usually of some difference in intensity, in nearly every shot. You just have to realize it even though it is harder to do than in studio.
The other thing is that I have heard it stated that an incident meter averages what the camera sees. But the meter is completely independent of the camera; that is why you are using it and why it is such a great tool! An incident meter averages what it is pointed at, not what the camera sees. There are 180 degrees around its dome; you choose how to best use those 180 degrees depending on the situation.
Don't get me wrong; it does average. But only what it is pointed at; it doesn't have to be used to average main and fill all the time. Do you want to average all the light coming from the main source? Do you want to average all the light coming from a secondary source? Or do you want to split the difference? The answer is that it depends. Incident meters are simply used to measure light falling onto the meter's dome, wherever you choose to point it, not just to meter the light falling on a plane parallel to the film plane. Aside from making sure that what you are metering is visible in the frame (for purposes of practicing common sense) this has nothing to do with that your camera sees.
Not wanting to average the whole picture based on the light falling on a subject plane that runs parallel to the film is part of the reason why we use incident meters. Yet many feel that doing just this is going to benefit them in every situation. It just doesn't make sense to take a tool capable of such precise and creative control and use it in such a loose and broad manner as a matter of course. It kind of negates a lot of the purpose of using the tool in the first place. It's like getting a Toyota Prius and driving it like a maniac just because it won't cost you that much in gas. You get that car to use as little gas as possible, not to cheapen the costs of your aggressive driving. Doing the latter is to waste the capabilities of the car, and to make things worse for yourself.
Athirill, I certainly agree with you! What I meant, in response to the Original Poster, is that even if the reading is the same, if the subject is pure black or pure white it might fall, on film, in a zone of the characteristic curve where there is no much detail. So the reading is the same, but you might want to "adjust" it so that your subject is rendered with satisfactory texture even though with "less than pure" black or white.
Originally Posted by Athiril
An extreme case is the man with golden teeth shown above. The incident meter reading is "right" but the teeth, being very reflective, can fall outside of the film "good" range of densities. (That case is a bit different because we have a specular reflection in that image, but you get my point).
For not extremely white or not extremely black subjects you can trust the incident reading "as is"*.
* And I do continue thinking that 2F/2F is basically right on how to use the incident light meter.
Today being a sunny day here in Rome I went to my balcony with the two incident light meters I own: a Gossen Sixtino II and a Gossen Multisix. They were both set for incident light reading.
With each of them, I did three readings: 1) straight toward the sun; 2) in a direction which was around 50° away from the sun (lateral lighting); 3) pointing the device opposite to the sun (my body projecting a shadow on it).
Case 1): 1/125 @ f/11 + 0,6EV for both devices. Perfect concordance.
Case 2): 1/125 @ f/8 the Multisix, 1/125 @ f/4 the Sixtino;
Case 3): 1/125 @ f/4 both instruments, perfect concordance.
I draw some conclusions from this test:
-) The Sixtino is "useless" in incident light unless pointed toward the sun (light source). The sliding thing in front of the cell does not intercept light falling on it from the sides. It works more like a disk than like a sphere, this is not explained in the user manual.
-) Using the Sixtino as per instruction manual is in a few words a waste of film; That's something I noticed as soon as I bought it. The "incident" light reading was more of an "indecent" light reading. But using it to base exposure as 2F/2F suggests works and gives results that are consistent with what I would obtain with any reflective light meter (or the sunny 16 rule if you prefer).
-) The Multisix gives 1,6 EV of difference in the "canonical" use and the "un-canonical" use. That's way too much to be ignored when using slides and probably also when using colour negatives. The sphere of this light meter does not "equalize" the light coming from 2/3 and the light coming from the front. The reading is different and so the direction of use is determinant in the reading. This was not obvious to me before this test. I thought that the task of the sphere was to internally bounce the light so that a lateral light and a frontal light don't give very different readings. But that's not the way it is.
The case may be that other incident light meters have a more "equalising" sphere though.
PS I'll begin using those bloody things in my daily practice for a few rolls, as the use of the spot reflective light-meter is a bit slow and cumbersome at times and incident light metering can definitely be practical for the photography I practice. I'm glad that the Sixtino II can be used for incident reading anyway, provided I ignore the instruction manual. This can be useful info for some other photographer, I hope.
EDIT: I would like to stress that if I were to make a self-portrait of myself in position 2, using the Multisix as indicated by the common wisdom would very likely lead to overexposure of the sunny side of my face. Using the Multisix as suggested by 2F/2F would lead to a very predictable 1/125 @ f/14 or so, which would likely place the sunny parts and the shadowy side of the portrait just right on the film. I encourage anyone to perform the same test and report experiences and considerations.
EDIT II: valued above obtained with light meters set at 100 ISO, just for clarity.