First of all, there's no such thing as a 'Perfect' exposure; See
Exposure - Exposed!
An exposure's an exposure, it may be darker or brighter, and depending on what you prefer, better or worse; but there's no scientifically calculated 'perfect'.
Which begs the question of metering, and most cameras meter for something it guesses you'll like... and will usually try and force 'settings' to get that for you.. but meters themselves are imperfect, and can easily be fooled.
Comment about starting by metering... well that is what we usually always do anyway, regardless of the kind of photography we are doing; and normally it's pretty transparent; camera has an inbuilt 'Through The Lens' reflected light meter, and that is usually 'coupled' to the cameras settings, and the electrickery in its circuits will usually make actual shutter speed, aperture and ISO settings for you based on what it meters.
Using a hand held accessory meter, you can usually take what is known as an 'incident' reading; measuring the level of ambient light falling on your subject, rather than the actual light being reflected off it. In some 'tricky' situations this can give you a better idea of what settings to use, and avoid you or your camera making settings likely to grossly over or under expose, because the subject is very light or very dark, and reflecting a lot more or a lot less light that you need to make an exposure in which it looks 'normal'.
Spot metering, is a way of doing the same thing with reflected light meter readings; in camera or with a hand held meter. Basically taking a reflected light meter reading from a tiny tiny angle of view, so you point it at your scene and you take readings of 'just' the bright 'highlights' and then at your dark low lights and find the 'range' of brightness across your scene, and then make an 'average' for the scene as a whole, somewhere in the middle. Which, is essentially what the camera would do in it's various metering schemes; whether that's 'evaluative' or 'matrix' or 'center weighted'.. basically all those systems are doing is taking a different number of sport readings from different parts of the scene in the lens and then doing a different set of sums to average them to get an exposure reading for the scene as a whole.
Doing it manually, your camera probably has maybe 5 stops of dynamic range; if your highlights and low lights are further apart than that you may have to decide whether to skew the exposure one way or the other, and let the brightest highlights blow, or whether to loose detail in the shadows. Or you might only be concerned not to blow high-lights, so measure those only via spot reading, and then 'back off' to the center of your cameras range
But, whether you are using in camera metering modes or schemes or a hand held meter, of whatever level of sophistication and necessary manual calculation; its the same thing. Measuring light levels, then applying some discretion to turn that into apropriate camera settings. Some metering schemes are more or less appropriate to different subjects in different situations, but the bottom line is that they are almost always fallible and the camera doesn't know what you want, only what a programmer has told it you probably want....
Last night we had the 'super-moon', I don't do astrophotography, but my G/F called me because she wanted to take a photo of it and was struggling; problem was that her camera, with the kit lens was looking at a lot of sky, that was rather dark, with just a tiny bright blob in the middle.. so it was trying to average the whole scene, and force a long exposure to make the black sky about 18% grey....The subject, however, is in bright direct sunlight... it's not night time on the moon! So, I told her to switch her camera onto manual, and use the f16 sunny rule.... she set f16 and ISO100 and then used a shutter of 1/100th.. and guess what? sky came out black, And the moon suddenly had shaddow detail rather than being a bright blob...
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Now, that is old fashioned metering by eye & experience, but that's what a lot of it comes down to.
In another couple of threads, commenting on milky waterfall photo's; and folk being determined they have to have big stoppa ND's to get them; similar issue with 'long exposure' arises; the desired milking is being generated by the highlights reflecting off the water streaking across the frame during exposure. When you meter, and doesn't matter much what method you use, the metering you get in that situation is for the average of the scene as it exists in real time... but longer your exposure, more of the highlights you let streak across the frame, and so during that period you effectively capture umpety times the number of highlights as they move... and so skewing the exposure towards over exposure, as what you are capturing didn't exist in real time. In that instance, you can use whatever metering method you like, but without recognizing this anomaly and adjusting settings from what metering tries to suggest or force on you, you will always tend to get this skewing towards over exposure, big stoppa or no big stoppa!
It's a matter of know how and knowing where and when your cameras meter is fallible, and when to apply some compensation... and then how much!
And its all very circumstance dependent, and not just for long exposure photography.
