How to choose the correct exposure
Choosing the correct exposure requires first deciding which camera mode and meter mode you want to use. First you should set the camera mode. For almost all circumstances, aperture mode is the proper choice, since aperture determines the depth of field. Choose the proper aperture for the desired depth of field. Next you need to choose the meter mode. I generally use only 2 modes on my Canon camera; evaluative (matrix for Nikon users) or spot metering. Evaluative mode is a good choice when you plan to process the final output from a single image. Spot meter mode is a good choice for taking a series of HDR images, but I also use it for single image photos. Finally, be sure to enable the highlight alert and/or have the histogram visible on your LCD. On my camera the histogram display is small and hard to read, so I generally show the image full screen and enable highlight alert (aka “blinkies”) which flash to show overexposed areas.
Evaluative mode is the simplest method and is probably the most commonly used. When using evaluative mode, the camera software analyzes the tonalities in the scene and chooses a shutter speed which tends to center tonalities around the midtone stop on the histogram while avoiding clipping the highlights. The goal is to produce a good JPEG image, but may or may not be the best exposure depending on the scene and how you want it to look. Therefore, you may need to use your camera’s exposure compensation feature (read your camera manual) to adjust the brightness. With the camera set to aperture mode and the metering mode set to evaluative/matrix, compose the scene in the viewfinder or LCD screen. Take the picture and observe the LCD screen for evidence of over exposure. It would be unusual for a camera set to evaluative/matrix mode to seriously over expose an image, but if you see highlight clipping of important areas, then reduce the exposure compensation by ½ stop or more and reshoot. Remember that it does not hurt to overexpose otherwise featureless bright highlights such as bright reflections off metal or water for example. While evaluative mode is designed to produce a good JPEG image, JPEG shooters might try taking extra exposures ½ or 1 stop longer or shorter to see which looks best, particularly in a high contrast scene. If taking RAW images, you may try increasing the exposure time as long as there is no evidence of important highlight clipping. There is no reason to decrease the exposure, which you can always do later using image processing software. Remember you are trying to expose to the right.
My own preference for taking static images is to use spot meter mode and to take the picture with the camera set to manual mode. Spot meter mode uses a tiny portion of the sensor to read the light, and sets the exposure time to give the subject a midtone value. With this method, first set the camera to aperture mode, choose the aperture, and set the meter mode to spot meter. Aim the center point in the view finder at an object in the image which you see as being a midtone in the final processed image and read the shutter speed by pressing the shutter half way. Some examples of midtones in nature include grey (e.g. beech tree) bark in average light, medium green grass or leaves (before they turn summer-dark), medium grey rocks, or a blue sky away from the sun and the horizon. With typical bright sky/darker foreground scenes, I read off a foreground object to be sure of getting good detail in the shadows. Now change to manual mode and set the aperture and shutter speeds to the ones you just determined in aperture mode. Compose and take the picture. If this is a low contrast scene and you see no highlight warning, you may choose to take longer exposures by ½ to 1 stop and reshoot to see if this improves image quality when you process the images (it will make more difference if there is shadow detail, than if there is not). With high contrast images which typically include darker foreground and brighter sky, the sky will almost always be overexposed on the first exposure. If you see highlight warning then increase the shutter speed by 1 stop and reshoot. Repeat this until you see no highlight warning in the brightest areas. If you only have to make a 1 stop increase in the shutter speed to capture the entire image without overexposure, then that image can probably be processed alone. If you have to make multiple increases in shutter speed to obtain an image with no highlight clipping, this image may be underexposed in the shadows. In this case, the image may be best processed by combining all the images using an HDR program. If you plan this to be an HDR image from the beginning, then you may choose to take your first reading in a shadow area, but expect in this case to need to combine several images in HDR. See also the guide for The Zone System.
Advantages of using evaluative/matrix mode
The main advantage is simplicity. The first shot will usually be at least reasonable. However, in high contrast situations this may not properly expose the shadows or highlights. Wildlife photographers will use this mode because there is usually very limited time to capture a good image, and you need a good exposure on the first shot. Many professional landscape photographers use this mode because it is simple, and if the exposure isn’t right, you simply adjust the exposure compensation. My personal issue with this (and it’s just me) is that changing exposure compensation on my Canon camera requires two separate button pushes, and it doesn’t change back to zero automatically. I’m always forgetting to reset exposure compensation and getting the wrong exposure on my next shot.
Advantages of spot meter/manual mode
The main advantage is being sure to get good shadow exposure, since you can choose to take your first meter reading in the darker part of the scene. This is particularly important with high contrast scenes. It is also useful when your main subject is very different in tonality from the rest of the scene (think black bear on a snowfield) and you want to be sure the main subject is exposed properly. It also generates a good series of images for HDR which is often needed for high contrast scenes, particularly if you take your first reading in the darker area of the scene. In manual mode, when I need to change shutter speed quickly in changing light I only need to turn one dial. I have my dial set to ½ stop increments and can do this by feel without looking. I also like the idea of being in manual mode and knowing exactly what my aperture and shutter speed are with each shot.