Copyright Michael Karbo, Denmark, Europe.

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    Chapter 18. Spot metering and AE Lock

    Spot metering is a rather specialised and ”manual” way of measuring light, which is very easy to use when you know how. It is precise; we know what the result will be because the light is measured in a small spot we have chosen ourselves. Spot metering is even more practicable when we combine it with the exposure lock AE Lock. This is a particular little button, which locks the diaphragm and the shutter speed in accordance with a fixed light metering.

    Spot measuring

    Spot measuring is found in most cameras, and it is selected by either pressing a button or via the menu system.

    The camera changes the light metering every time this button is pressed.

    When the spot metering is activated, the light is measured in a little precisely defined field in the middle of the image. This field is typically about 1/32 of the whole of the area and is seen as a frame in the viewfinder:

    Figur 71. The frame in the middle of the viewfinder shows us in which field the light metering is taking place.

    Spot metering is used to determine the lighting based on a little part of the subject. You select the spot, which is to be exposed correctly.

    If you, for example, are taking a photograph of the building against the light, then it will often be underexposed if you use automatic light metering. This also applies to people, who are standing with their backs to the sun. If it is impossible to move either the people or the photographer, then the solution is to make a spot metering.

    Figur 72. Spot measurement measures the light inside an area, which is seen as a frame on the LCD screen. Here the frame is placed in that part of the subject, which is in shadow. This gives the result that while the sky is overexposed, the building is exposed correctly.

    Spot on the autofocus points

    Spot metering usually measures a field in the middle of an image. This is a bit impracticable if you want to measure the light in another area than the center. Using the exposure lock can do this; we will come to this in a little while.

    But there also is a variant of spot metering called Spot AF area. It is still a very limited area in the image, which is measured. But instead of placing the sensitive spot in the middle of the motive, you let it follow the autofocus points.

    A camera’s autofocus function can, you see, automatically find the areas it should focus on. And with Spot AF area, the same points are used for light metering. Spot AF area is regarded in some cameras as a separate measuring method. In other cameras it functions as a setting option for spot metering, which is activated via the menu system.

    Figur 73. Spot metering here is attached to either a spot in the middle of the image or to the variable focus points, an autofocus uses. (Minolta A1)

    Light metering and exposure lock

    Many cameras have an exposure lock, which is usually operated by a little button on the back of the camera body. The function is called AE Lock (Auto Exposure Lock); and a camera often has several types of locks, where different settings can be frozen.

    The exposure lock is used to select a certain set of exposure values and freeze them. In practice, a light meter makes a spot metering or a center-weight metering often together with an AE lock. It is done like this:

    1.    Activate the function spot metering (or center-weighted).

    2.    Move the camera so that the little spot frame hits exactly the part of the subject, which is to be light-measured.

    3.    Press the release halfway down, so that the camera’s image computer can calculate the exposure from the light in the spot frame.

    4.    Activate the exposure lock by pressing the AE-Lock. With some cameras you have to hold the button down. The image on the LCD screen will display a little star or the letters EL (for Exposure Locked), which tells you, that the exposure values are locked.

    5.    Move the camera so that the image is composed properly – independent now of the position of the spot frame.

    6.    Press the release completely down. The exposure is taken with the exposure values that are locked with the AE lock.

    7.    The lock is automatically unlocked when the exposure has been taken.

    This method works really well with cameras, which have spot measure or center-weighted and have a button for an AE lock.

    The advantage being that manual light metering has been made of the part of the subject, which is correctly lighted. The exposure values are locked and then used with another subject. The light meter is used for one precise metering and is then temporarily suspended.

    Figur 74. The button for the AE lock should preferably be placed where it is easily available when taking exposures. This one fits a right-handed thumb very well (Fujifilm S7000).

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