I understand how to adjust the needle to the centre of the meter for a correct exposure in manual mode
Short reply :
That is not the answer to a "correct" exposure unless the thing you are metering from happens to average out to a tone (brightness if you like) equivalent to middle grey. If your subject/scene you are metering is predominantly dark you want the meter to be somewhere in the negative portion of the scale. If the subject/scene is bright then you want the meter to be somewhere in the positive portion of the scale. If all you are doing is trying to centre the needle then you are wasting your time. One of the autoexposure modes can do it far quicker than you can.
Long reply :
Camera meters are stupid. They have no idea what your scene contains and whether it should be dark or light. They play safe and assume that overall the brightness of the scene averages out to a middle grey tone. The problem with camera meters is that they measure the light reflected back from a scene, not the light that is illuminating it.
Dark scenes don't reflect back much light. The camera wants to brighten the subject/scene in order to make it appear roughly middle grey. Pale scenes reflect a lot of light. The camera does not understand that the scene should be pale/bright and will try to darken it to achieve a middle grey tone.
If you were to leave the camera in an autoexposure mode (Av, Tv, P, Auto) and leave the compensation set to zero then this is the problem you will see - dark scenes will be overexposed and bright scenes will be underexposed, as the camera strives to match them to middle grey.
If you choose to shoot with manual exposure and keep the needle centred all the time then you are applying no more intelligence than the stupid camera. However, the camera will get that needle centred far quicker than you can and without fumbling about. If it's your sole purpose to centre the needle then don't bother shooting with manual exposure.
So, moving on, the big advantage of shooting with manual exposure is that you can fix your exposure so that it does not jump all over the place. If the light upon the subject/scene is stable then this is a good thing. It doesn't matter whether your subject or scene is dark or light. Once you have the exposure set up correctly for the light illuminating the scene then you can blast away shot after shot without fussing over adjustments to the exposure every time the subject or background alters tonality. The question is - How do you arrive at that correct exposure?
Well this is where you need to apply the intelligence that the camera lacks. You need to consider your metering mode and how it will react to the tones in the scene. Spot metering will take a reading from a very small area of the scene, partial metering will average a larger area and centre weighted average will consider the whole scene, with emphasis on the tones towards the centre of the frame. Evaluative metering will divide the scene up into segments and, with reference to a built in database, will try to guess at the type of scene you have and work out how best to expose for that scenario. This is the closest thing to "intelligence" within the camera. With average scenes that people typically shoot it may do a very good job. With more difficult scenes, with unusual lighting, it may fail miserably.
So, for manual exposure you need to pick your metering mode first and decide how you want the camera to measure the light coming back from the scene. Then you have to decide which are of the scene to meter from and whether that area averages out to middle grey or is actually a bit brighter or darker or a lot brighter or darker. You use the meter needle to guide you, but instead of aiming to centre the meter needle you have to decide how dark or bright your metered area is and where the needle should be positioned in order to reproduce those tones correctly.
When I shoot with manual exposure, which is a lot of the time, I generally (practically always) prefer to use spot metering. This allows me to very precisely meter a specific area of the subject, scene or other reference point and then dial in an exposure to place the meter needle where I think it needs to go. I don't always guess right, but I probably do a better job than the camera would without input from me. Good reference tones included green grass, red brickwork, white paint, dry tarmac and concrete, the palm of your hand - anything that usually has a pretty consistent tone wherever you go. They won't necessarily all require the same position of the meter needle as each other, but if you learn the meter position typically required for each of those things then you can adopt that wherever you go. For example, if I spot meter my outstretched palm, facing back towards the camera, at + 1 1/3 then that is a good starting point from which to move forward. If I meter rich green grass at -2/3 then again that is a good starting point as well. If I find brilliant white, sunlit paintwork, or a swan's feathers in the scene then I might meter that at +3 - the clipping point of my camera - in order to achieve maximum data capture with minimum (zero) loss of important highlight detail.
That's my basic approach to setting exposure initially. In practice I will finesse things depending on the scene, since one size does not quite fit all. If my scene contains nothing important in the highlights then I may well increase the exposure by up to 1 stop (occasionally more) in order to record more detail in the shadow region. This is something I would do when shooting my dark brown dog. There would be nothing important in the highlights to record, but if I tried to maintain sufficient headroom to record those absent highlights I would be wasting dynamic range that could be better used to record details in her dark fur.
Here's an experiment to try on a bright, sunny day, with the sun behind you. Set a manual exposure that corresponds to a "Sunny 16" exposure. An example might be f/8, 200 ISO, 1/800, as confirmed by this incident meter reading taken this afternoon....
With the sunlit subject/scene in front of you and the sun directly behind you try taking a few shots. Zoom in, zoom out, pan a little (up to 45 degrees) left and a little right, a little up and a little down and take a shot each time you change the composition. How do the exposures look? They should all be pretty perfect. Now set your metering mode to the smallest area your camera offers - spot metering if you have it, partial metering if you don't. Now repeat the exercise of panning around and zooming, but instead of taking shots, watch how the meter moves. If you have green grass in front of you see what reading you get. Make a note. Repeat for other common things - concrete, red bricks, your own outstretched palm held up in front of you. Make a note of the meter readings for all these things. These will be good starting points for setting a manual exposure when you come across similar things in other locations and in other lighting intensities. Some of them may even lead to a centred meter needle. Some probably will not.
Here's a recent example of a shot using manual exposure for which I didn't meter a thing. It was a bright, sunny day, clear skies, and a sunlit swan. I dialed in an exposure equivalent to "Sunny 16" and Bingo! I don't think I could have done better if I'd tried. The fabulous thing is I didn't have to try. It was easy....
Knowing the "Sunny 16" rule/guideline and how to juggle the exposure triangle is a very powerful weapon in the battle for perfect exposures in sunny conditions. For me it beats any autoexposure mode 100% of the time. For other lighting scenarios knowing how to meter off common reference targets, or simply having a handle on how bright things should look relative to middle grey, is the way to conquering the exposure challenge. There is nothing to suggest that the meter needle should always be centred.
Here's my dog, shot with an exposure 1 stop brighter than "Sunny 16", because even though a "Sunny 16" exposure is "correct" for the incident light (the exposure an incident light meter would calculate) I know from experience that such an exposure will simply not do for this subject.
So the real skill comes in knowing how to fit the dynamic range of the scene within the dynamic range constraints of your camera, and optimising the exposure to make best use of your camera's capabilities. That means that you need to learn how to read a scene, to determine which bits are most important (highlights, lowlights or mid tones) and make sure the exposure you set will capture the most important tones safely. A basic incident meter reading will optimise the exposure for the mid tones That's often perfectly fine, and a great place to start, but if your interest lies in the highlights, such as with a sunlit swan/egret/wedding dress, or the lowlights, such as with a dark coloured dog/cat/bear/horse then you may need to finesse things to accommodate your most important tones.
