It's a cold hard fact, but in case you didn't know it - photography is not 'real'. We don't capture reality as it is with a camera. You might wonder what I'm on about, but let me explain further. Firstly, a camera sees in 2D, whereas we see in 3D. But also, our perception is a lot more different than a camera sees because we have far greater 'dynamic range'. We are able to register detail in shadows and highlights that a film or digital camera can only dream about.
ISO 50, f4 = average reading of 1/4 second, 5 stop dynamic range where shadows are at 2s and highlights are at 20th of a second
This point is mainly the reason for why we often get images back from the lab, that looked nothing like how we perceived the scene at the time. It takes skill and patience to be able to get to a point where you are confident you are going to capture what you saw, and you may need to use neutral density graduated filter to squeeze the the entire range of tones from highlights right down to shadow onto your film/sensor.
So I thought I'd explain a bit more about dynamic range.
Our eye is capable of seeing over a range of 24 f-stops. But a digital sensor or film can see roughly 3 to 6 f-stops depending on the medium (negative film has a higher latitide and can often record over a wider range than slide film can). So it's very common to want to record a scene that the camera simply cannot handle. You are either going to have an image with underexposed ground, or over exposed sky, or a bit of both.
Now, your camera simply meters everything and works out an average of 18% grey. For instance, if you point your camera at a black wall, the meter will make an exposure which will make the black wall grey. The same is conversely true for a white wall; your meter will make a white wall grey. This is why you need to add or subtract exposure compensation for situations when you know that the overall scene is too bright or dark.
So how do you get the right exposure?
Well the truth is that there is no such thing. It is purely about what you consider important to record, and what you are willing to sacrifice, but there are times when using an ND graduated filter will allow you to squeeze an entire scene which has a dynamic range greater than your film/sensor can record, and here's how I do it.
I have a rather nice little Sekonic meter which allows me to take many spot readings around my scene and from that I can see the entire dynamic range that the scene contains. For example, I will take a spot reading of the darkest part of the scene and then a spot reading of the highlights in the scene, and from that I will perhaps see that there is a 10 stop difference between these extremes. I know that my Fuji Velvia film won't handle this and that I really need to get the scene down to around 6 stops of a difference. If I put on a 3 stop ND graduated filter to compress the sky (highlights) down by 3 stops, I've reduced the entire dynamic range from 10 to 7 stops. Which is a bit more manageable, but I also have to determine what in the scene is going to appear 18% grey. Remember that when I meter the dark areas of the scene, the meter is telling me what the exposure would be if I wanted the dark areas to be grey. And the same is true when I meter the highlights. So I need to decide where in the middle of the range I want to be my mid tone. This is the bit that is subjective, but if I find for example some rocks in the scene that I think are 18% grey, I will decide that is my exposure point. I will then set this exposure manually (to stop the camera's meter from changing the exposure when I add on the grad), and take the shot.
With an SLR, it's a lot easier than this - sure it's nice to have a meter that shows you the entire range so you can figure out what difference in exposure latitude you need to make, but often, the camera will work out a nice average for you - if you have already placed a grad on the scene anway. So it's much more of a point and shoot approach to it, and if you are using a DSLR, you can check the exposure to see if you over did the grad - often for example, I tend to find that 3 stop grad is too much and 2 stops is more appropriate.