(based on an I @ M (Andrew) post)
Your DSLR camera will have a in-built light meter that will show up in the viewfinder.
The actual meter will only show up in your viewfinder at specific times depending on the mode the camera is in.
In Manual mode, M on the control dial, the meter lines will show in the viewfinder all the time.
In any of the other modes, Av(A), Tv(S) or P on the control dial, the meter lines will show in the viewfinder ONLY
- when the meter detects an under or over exposure situation that the camera cannot correct automatically.
- when the exposure compensation button is pressed and the camera is set to either + or - exposure compensation (Ev) to the automatically metered scene.
The view finder typically looks like
And the meting part in detail like (This is a Nikon display with the plus exposure on the left)
Some cameras have a display on the main screen (This is a Pentax display with plus exposure on the right)
The images below showing the meter lines as you should see them in your viewfinder.
The first image shows the meter and graduations indicating a correctly exposed reading.
Second image shows an under exposed picture ( the 3 little lines on the - side of the meter ),
the more lines there are showing, the more underexposed it will be.
The shutter button initially needs to be half pressed to "wake up" the camera and meter,
once awake the meter is analysing the scene. Depending on how your camera menu is set,
half pressing the shutter button again will "lock" the exposure setting at that place and
pressing the shutter button that bit more will then activate the shutter and take the picture.
Further, your camera can be set for (usually) three modes of metering as shown below.
1) Multi Segment
2) Centre Weighted
3) Spot Metering
While learning we recommend either Multi Segment or Centre Weighted metering.
Spot metering requires some further understanding to obtain correct exposures.
||This setting is the default metering mode for many DSLRs. It collects exposure data from many points on the screen (usually not shown in the viewfinder) and uses sophisticated software algorithms to decide which points to use in calculating the correct exposure.
||Any scenes that don't require the special treatment provided by the other two modes. In other words, you'll use multipoint metering most of the time at first.
|| This system looks at the entire frame, but tends to emphasise the portion of the image in the centre, assigning a centre weighting determined by the camera manufacturer, but is usually about 70-80% for the centre and 20-30% for the rest of the image.
||Best used for scenes in which the most important subjects are in the centre of the frame. Of course you can use your AE-Lock to metre then re-compose. Useful for portraits or close-ups of flowers and want to centred subjects. Centre weighting zeroes in on those subjects and isn't influenced by very bright or very dark areas outside the centre.
||This setting makes its exposure recommendations based only on a centre spot shown in the viewfinder which might measure 5mm to 12mm. Light outside the spot is ignored. Your DSLR might allow you to choose the size of the centre spot.
||Subjects that don't dominate the frame, and which are surrounded by areas of misleading brightness or darkness.
For more detailed information and your own research please refer to:
Please read you camera manual to find how to set Ev or metering modes.