Exposure is central to photography and a lot has been thought and written about. I find it often misleading.
Medium Grey Exposure
Our eyes adjust to light and darkness. So we perceive most scenes as more or less medium bright or medium grey in photography speak. Basic light meters measure and show how much a camera needs to be adjusted to achieve exactly this. Very often a good Guess for a pleasing picture. Usually the centre of the image is the most important part and the edges less so light meters usually place higher importance on the centre and lead to better results.
Today cameras can compare scenes with a database of saved pictures to get an idea what the photographer wants to shoot to get better results. However I found that for night photography this is often misleading and the old centre weighted exposure mode is better suited than Matrix Metering Mode of my Nikon and similar for the Olympus.
Optimised Exposure by Using the Zone System or Exposure to the Right
Ansel Adams developed the zone system to achieve maximum brilliance of his pictures. Black and white film can capture a higher dynamic range than we can see at a give an moment or than a print can deliver. BW film can also be manipulated in the darkroom. The idea of the zone system is to measure the scene with a spot meter and expose rather bright. During development of negative and print the photographer can then compress the dynamic range and get more details in highlights and shadows.
Bruce Barnbaum‘s book The Art of Photography has a detailed introduction to the zone system
Exposure to the Right is an adaptation for digital cameras. New digital sensors can also capture a very high dynamic range even for colour. We can use use live histograms to place the highlights on the maximum of the sensor. If we capture in RAW data format we can then process the file on our computer and compress the dynamic range and get again amazing details in the highlights and shadows.
In reality this only achieves much at very low ISO and there are practical limitations on adjusting aperture (best quality around f8, depth of field needed) and even more on shutter speed (movement in the scene, no Tripod). And many scenes just don’t have a big photodynamic range and all this is not needed. Nevertheless understanding the principles and applying the ideas behind this is really important to achieve technically good pictures.
In the end the Metering system of the camera just suggests us roughly how to achieve a pleasing exposure and the zone system helps us to get details. Both are helpful guide points but neither answers us how to expose to make a great picture, tell the story we experienced and the emotions we felt.
In fact exposure is an artistic decision.
Should this picture be medium grey, very bright, very dark? Maybe I do not want to show details in a certain area of the picture or want to guide the viewers attention to certain point by exposing it middle bright. Such questions a photographer has to answer by choosing an exposure.
And there are no absolute rules or step by step instructions to make this decision only helpful guidelines, hints and techniques and tools.
Many photographers approach exposure so nerdish and assume that there is something like a „correct“, precisely defined exposure which we must search. This is the thinking which often holds photography back. True art is not precise and involves bold decisions and photographers must develop such skills as well if we want to be artists.