If you're metering through the lens, and therefore trusting the camera's judgement, why bother with manual mode? I mean, if you're just going to accept the lens metering as correct exposure and then dial in some adjustment +/-EV, why not just use one of the PAS modes and miss out a step?
I'm not sure what you mean by the bit in bold. I trust the camera to meter accurately, but the camera has no idea whether I'm aiming it at a pile of coal or a snowdrift. All that the camera knows is how to turn whatever it is metering into a mid tone. It is down to me to know what I am metering and what the camera will make of it. Then I can instruct the camera on what adjustments are required to expose correctly for those conditions. I can do that with EC, and keep doing it every single time I aim the camera at something different, or set that adjustment once in manual mode.
For manual mode to be efficient and effective you don't need
controlled lighting in the sense that you are in charge of it. What you need is
constant lighting that you can rely on to stay the same for a while at least.
If you rely on the automatic modes then every time you point at a new subject, with different reflectivity, you're going to get a different meter reading, or more specifically a different exposure calculated. If you zoom out and get more sky in the shot your exposure will change. Well, as I'm sure I said before, if the light falling on the subject/scene remains constant, why would you want your exposure to change just because something in the scene changed? I wouldn't want a new exposure and I wouldn't want to fight with estimating how much EC to dial in either.
Here's a scenario.... You're shooting portraits of dogs at the end of an obedience training session. Most of the frame will be filled with the dog sitting and facing you. The lighting is constant. It might be overcast; it might be blue skies; we'll assume we're outside for the shoot and the backdrop is grass and trees. Now the dogs will be all colours, shapes and sizes, from skinny white ones to big fat black ones and every shape, size and colour in between. You can choose to shoot in manual mode or automatic. I choose manual. Here's what happens for me....
I meter off the grass and set a manual exposure that so happens to centre the meter needle. That's the end of metering for me. White dogs will appear white. Black dogs will appear black. Grey dogs will appear grey. A small dog, surrounded by grass, will be exposed correctly. A dog that fills the frame will be exposed correctly. I am left free to concentrate on the posing, focusing and timing of the shot. I have no worries about exposure while the light remains unchanged. If it does change then I re-meter off the grass and adjust.
Now let's look at the approach with autoexposure. Quite simply you will have to re-meter and readjust your exposure every time the tonal content of your scene changes. Stick a large black dog in front of the camera and your autoexposure mode will think you're underexposing and fix that for you. Stick a white dog in front of the camera and the camera will try and turn it grey. You'll have to compensate for that, guessing how much compensation to dial in and perhaps have to run off a test shot to check exposure before proceeding. Then you get a small dog - less dog, more grass. Now how much will your exposure be affected? It all seems like a monstrous faff to me.
Of late a lot of my shooting has been of my dog on the run. He is black. At the start of the run he'll be pretty small in the frame and the composition will mostly be grass. By the end of his run, maybe five seconds later he is pretty much filling the frame. How would autoexposure deal with that? Well, the more the dog filled the frame the more "black" the camera would see and the brighter it would try to make the exposure. How nuts is that? The exposure should be identical throughout the sequence. Shooting manual will give me that constant exposure. Leaving things on auto would just be screwy. Here's an example at the end of his run, shot with manual exposure...
So that's dogs. But the same thing applies whatever you are shooting. Black bird, grey bird, white bird - in the same lighting they all want the same exposure to keep white birds white and black birds black. Ditto racing cars, horses, people in clothes, people out of clothes, BIF etc. etc..
In short....
If the light is constant but the scene/subject changes then shoot manual.
If the light is changing but the scene/subject is constant then shoot auto.
If the light is changing and so is the scene/subject then one way or another you will need to keep making adjustments.
If nothing is changing then shoot however you like. Once you're set you're set.
I don't know if that is any clearer or just a rehash of what I wrote earlier. I did not re-read the thread. Hopefully it makes at least a little sense
I don't know whether anything here will help illustrate the point about incident vs reflected light and the fact that your exposure should be the same, regardless of your subject, if the incident light is the same....
http://www.sekonic.com/classroom/classroom_2.asp