Ahhh histograms!! If like me you were never good at maths, don't worry! Histograms really aren't that difficult to get your head around! In fact, alongside the built in light meters in most modern cameras that offer manual controls, i would argue that they are probably the most useful tool for beginner photographers to begin to understand exposure! Used side by side, they allow you not only to tweak your setting in an instant, but in the long term, following regular use they can help teach you what to expect in different situations and allow you to guess/estimate (or guesstimate) the settings that are most likely to give you the best possible exposure.
The camera's built in light meter, "reads" the tones in the scene and averages them out as mid-tone grey AKA 18% grey. In many cases this works fine, however if you've ever tried to take a picture in snowy conditions (or any unusually bright scene) using automatic or semi-automatic modes (aperture or shutter speed priority), you may have noticed that the snow is most often rendered as grey and rather dirty-looking. In fact, if you take a picture of a white object and then take a picture of a black object using your camera's automatic or semi-automatic modes, they will both be rendered as grey! Using manual modes one can easily calculate by how many stops a scene is brighter or darker than mid-tone grey and adjust setting accordingly. With automatic and semi-automatic modes, most cameras (for example android smartphones do but iphones don't as far as I know) should offer the choice to apply what is known as exposure compensation (usually indicated by a +/- symbol) which can be set to overexpose or underexpose depending on how much brighter or darker the subject is than mid-tone grey.
Alternatively, exposure issues can be dealt with by snapping off a shot and examining the histogram. Snowy conditions tend to require at least half a stop of exposure compensation (i.e. more than the light meter suggests) but by using the histogram and using the ETTR (exposing to the right) technique, you can ensure that snow (or any other particularly bright area) is rendered as white, but also that as many details as possible are recorded and that the highlights aren't clipped. On the other hand however, when photographing dark subjects, the opposite applies and you may need to expose by slightly less than the light meter suggests. Inexperienced wedding photographers, for this reason often talk of the difficulty with getting pictures of the bride and groom where the bride's dress is white and the groom's suit is black!
A histogram (in the photographic world) is a graph which maps 256 tones ranging from absolute black (on the left side) to absolute white (on the right side). The ETTR method works best when using an RGB histogram but the more common luminance histogram is just as good in most situations. With this method, exposure is set so that the histogram reaches as far to the right as possible without any clipping (without it spilling over the edge, referred to as highlight clipping). The dynamic (tonal) range differs from camera to camera, but using ETTR will ensure that the maximum possible amount of shadow detail is recorded and that highlights are not clipped, thus giving more freedom in post-processing.
In certain tricky situations, when lighting is uneven (e.g. concerts), switching the metering mode to "spot metering" will allow you to get a reading from a smaller area (i.e. a model's face) within the frame so you can expose more accurately for the main subject rather than the surrounding elements. When using spot metering, in some situations accurate meter readings can be taken off roads and in some cases grass (provided these are within the mid-tone range).
To be sure though, you can always get an 18% grey card from pretty much any photographic retailer, hold it up in front of your camera, adjust your shutter speed and aperture so that the meter falls on the 0 and get a reading which is most likely to yield accurate results (these are also great for setting white balance). In order for this to work however, the card needs to be in the same light as the subject that you are photographing.
Back in the day, before these fancy gadgets were widely available, photographers relied on what is known as the sunny 16 rule, which says that on a sunny day setting aperture to f/16 and shutter speed to the reciprocal of the ISO film speed for a subject in direct sunlight. So for example when shooting at ISO 100 and using the sunny 16 rule, the shutter speed would be set at 1/100s (or 1/125s). Alternatively, when shooting at ISO 200, shutter speed would be set at 1/200 (or 1/250s), at ISO 400 shutter speed would be set at 1/400s (or 1/500s) and so on.
Once you become proficient at reading your camera's histogram, and as your camera starts to feel more and more as an extension of your own body, these methods will start to feel irrelevant as you'll come to rely to a larger extent on your experience in order to get the correct exposure.
Peace Advocate Blogography
Welcome to the Peace Advocate Photography blog, where you will find everything from gear reviews to my opinions on photography and recaps of my recent shoots!