Exposure, Metering, and your viewfinder

Brain Coral

In workshops and classes I often meet photographers who haven’t really worked out what their camera is doing regarding exposures, and don’t know how to take advantage of the information their cameras provide. Many times these photographers have heard of exposure modes and metering modes, but sometimes they’re just missing that “aha” moment of everything clicking together and making sense.

Maybe these notes will be a useful resource that students can refer back to.

Note that while my own cameras are mostly Canon, I am familiar with and do use most other brands of camera. I’ve tried to make this discussion as general as possible, although I have had to point out some differences in behaviour between brands.

Metering modes

The underlying process used by most of the metering modes is based on the assumption that most scenes average out to “middle grey”. The camera will use the brightness in the area it measures to decide on the right exposure.

For many scenes this assumption works, although it will render snowscapes and desert scenes as quite grey, and a black cat could be shown a lot brighter than it actually was.

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Here the camera was fooled by the white snow (this is a crop from a larger frame) and made the image on the left very dark.

F1_56FE F1_56FE
Here the camera was fooled by the dark mud-covered elephant and tried to take a much-brighter photo than I needed.

So the cameras allow you to set Exposure Compensation to a positive value (“make the scene brighter than the metered value”) or negative (darker).

But rather than just averaging the whole picture, the cameras can use different calculation modes. Different camera makes use different names for these, but there are a range of metering modes usually available:

Partial (Canon partial_mode)

Uses just the centre of the image.
If the centre area is over the subject you’re interested in, you probably don’t care about the rest.
But if important parts of the image are outside that metered circle the camera will ignore them.

Center-weighted average (Canon average_mode, Nikon nikon_average, Sony sony_center, Olympus oly_center)

Uses the centre, but also considers the rest of the image (fading out towards the edges).

Spot (Canon spot_mode, Sony sony_spot, Olympus oly_spot)

Uses a small spot in the centre of the frame (usually approx 1.5% of the image). Like Partial, just with a much smaller area.
Because the image under the spot can quickly change with small camera movements (consider the difference if it was over a white or a black area of penguin feathers) I do not recommend this mode unless you’re working on a tripod in a very methodical process. For action shots I find it too unstable.

Spot (Nikon nikon_spot)

Sometimes better than Canon’s Spot, because instead of being locked in the centre of the image the spot is located at whichever AF point you’re using. If you’re always using the centre AF point obviously it’s very similar to Canon’s Spot.
But also be aware that if you’re using one of the “Auto-area” modes where the camera is choosing the AF point, the camera can meter on the centre of the image no matter where the AF point has roamed. Again it becomes the same as Canon’s Spot.

Evaluative (Canon eval_mode), Matrix (Nikon nikon_matrix), Multi (Sony sony_multi), ESP (Olympus oly_ESP), Pattern (as reported by Lightroom)

The camera splits most of the image up into a coarse grid (depending on the camera this can be 35, 63, or even thousands of grid squares) and measures the brightess of each.
It then compares this to a huge database of grid brightnesses in its memory to find one that matches, and then uses the appropriate settings. Some cameras have a database
of brightnesses, but some have colour memories. Some also use distance information provided by the AF system.

Often this works well as a default, without needing as many changes to Exposure Compensation as the other modes. But if the memory the camera matches your scene against doesn’t actually match your scene, you will need to apply compensation. For example with the dark outline of a person against a sky, how does the camera know if you want them as a silhouette or if you want to see their face?

Each of these metering modes is there for a reason, and various teachers will recommend the use of different modes in different scenarios. I have found Evaluative/Matrix to be a useful default, but I’ve still had to build up experience in recognising which sorts of scenes my cameras need how much compensation for.

Exposure modes

Using whatever metering mode you’ve selected, in the auto-exposure modes the camera will use the metered brightness to set:

Program: P

Shutter speed, aperture, and ISO (if on Auto-ISO) are all set by the camera.

Shutter (time)-priority: Tv (Canon), S (most others)

You choose the shutter speed, and the camera chooses the aperture (and possibly ISO).
If the scene is too bright or too dark often the aperture number in the viewfinder will flash as a warning, and you’ll have to change the shutter speed or the ISO to bring the aperture back into a reasonable range.

Aperture-priority: Av (Canon), A (most others)

You choose the aperture (controlling things like depth-of-field) and the camera chooses the rest.
If the scene is too dark, you can easily run into problems where the shutter speed gets very slow, at which point you have to change the aperture or ISO. Make sure you keep an eye on the shutter speed without blindly trusting the camera.

In those modes you set the exposure compensation using a dial (or sometimes a button+dial combination). In the viewfinder the compensation is visible below or beside the image. Usually as a sliding scale. I find it very important to be able to adjust the compensation without taking your eye away from the viewfinder.

Screen Shot 2015-12-21 at 3.34.09 pm
Here the exposure scale can be seen (at the 0 mark) between the aperture (“16”) and the ISO.

In Manual exposure mode (M) you get to set both the shutter speed and the aperture (and the ISO: auto-ISO in Manual isn’t really “manual” and is a topic for a different discussion).
The metering is still done by the camera using whatever metering mode you’ve set, but instead of setting exposure values, the camera just indicates to you in the viewfinder whether your exposure matches its idea or not.

Usually this is shown on a sliding scale (sometimes the same one used for exposure compensation in the auto modes). However some Pentax models instead show it beside that scale as a numeric value. If it’s positive (usually to the “right” of the scale, although some cameras do it differently) the camera thinks your settings are too bright. If it’s negative, the camera thinks it’s too dark.

So if you set your aperture and ISO and then twirled the shutter speed to a setting where the meter indicated “0” (in the middle of the scale) you’d have the same settings the camera would have chosen in Aperture-priority mode. If you would have used positive exposure compensation (sometimes in snowy scenes I use +2 stops to make the snow white) in an auto mode, you just set the shutter speed so the meter indicates that same amount brighter than “0”.
Of course you can instead choose a shutter speed and then vary the aperture, and have similar behaviour to the camera in shutter-priority mode.

An advantage I find working in Manual mode is that if I then move the lens so the arrangement of dark and light tones in the scene changes (maybe my penguin subject turns to the side) the exposure settings don’t change “by themselves” and I don’t need to keep adjusting the exposure compensation up and down. Of course, you do need to keep an eye on the meter in case the overall lighting changes (e.g. a cloud crosses the sun).

Using your viewfinder

When shooting wildlife action, it’s important to see what’s going on!

  • Make sure you can see the shutter speed, aperture, meter (or exposure compensation) and usually ISO within the viewfinder.
    And without looking away from your subject! Not only might you miss some animal activity: as soon as your camera points at a different subject the exposure numbers can change.
  • If you can’t see all the numbers, it might be that you just need to get your eye closer to the viewfinder.
  • Make sure your viewfinder’s focus is adjusted (using the diopter control beside it) to match your eyes and/or glasses.
    The easiest way of getting this right is to de-focus the lens so the scene is very blurred, but then get the numbers crisp by adjusting the diopter. I often tweak mine during the day depending on whether I’m using my reading glasses or not.
  • As well as seeing the numbers at a glance, make sure you can change the shutter speed, aperture, ISO and exposure compensation WITHOUT TAKING YOUR EYE AWAY FROM THE VIEWFINDER.
    Learn which buttons and dials to manipulate without having to look for them.

Reviewing images

If your camera has an optical viewfinder you have to occasionally review images on the camera’s rear LCD to check your exposures/etc. But never trust the brightness of the LCD: always look at the histogram as well as the image itself. On my cameras this means the image is relatively small, but I can enlarge it if necessary with a single button press.

7D2_histo

I usually set my cameras to automatically show (review) images on the LCD for several seconds after they’re taken. Most of the time because I half-press the shutter button the preview goes away, but if I want to can glance down at the screen and “chimp” my exposure (actually, it’s probably only chimping when accompanied by “Oooh!”). Of course I can press PLAY at any point to review recent images.

If my camera has an electronic viewfinder it can often provide a preview histogram overlaid on the image before you take the photo, and this can be very useful. But with electronic viewfinders I try to set the camera to NOT automatically review the image in the viewfinder: I want to be looking at the subject so I don’t miss any photo opportunities. I can always press PLAY when it’s time to review an image.

 

This will probably do for today: an overview of exposure and metering modes, with a bit of viewfinder management thrown in. Maybe it will help you get your head around what your camera’s doing in the background! By the way, the image at the top of this post is a “brain coral” turned into a small boulder on a beach (at Australia’s Lord Howe Island). Hopefully your own brain doesn’t feel as beaten up after reading this!

Auto Focus

This discussion has hardly mentioned focus as it’s a huge topic of its own. But suffice it to say that being able to see what’s going on in the viewfinder (which AF points are being used, whether the camera has managed to lock on, etc) is very important. So all of the above is still relevant.

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