As you preview the back of the screen, it is hard to tell how bright or dark the image actually is. You can just rely on your camera's meter, shoot in one of the automatic modes (nothing wrong with that), and hope for the best. To further hedge your bets, you could take a look at that funky looking wave looking graph that pops up in your camera's display options. What does the wave mean? Why is it so spiky? Why does my camera not have a cup holder? Check out the video and the detailed explanation below.
I can help you out with those first two questions. You are on your own with the cup holder. That wavey graphy thingy is called a histogram. Once you know how to better read it, it can provide you with some great information about the exposure levels in your image that just looking at the image on the screen cannot.
The histogram represents the darkest parts of a given image on the left hand side of the graph. The brightest part of the image is represented on the right. Where are midtones (where the most saturated colors live) represented...you guessed it, the middle! That takes care of where exposure values lie, so why are some parts of the graph taller than others? Each point of height at a given left to right point on the graph represents the amount of pixels in the image that have that were exposed to that exact brightness.
This answer will vary. The reason for that is that the light levels in different scenes can vary greatly. For example, a scene like the one below will have what you might call a more balanced histogram. There are a lot of values represented from dark to light across the whole image. This creates a wave that, while it does have peaks, has no real low points where exposure values are not represented. These types of images tend to have saturated color values, somewhat dark areas, somewhat light areas, and a few extremely dark and bright areas thrown in for good measure.
When should I adjust my exposure based off my histogram reading?
I am so glad you asked. If you are photographing a landscape scene on a sunny or overcast day and the histogram wave seems to be smooshed against the right or left side of the graph, you may be losing detail in your image. When using a digital camera, you usually do not want your histogram to be smooshed on the righthand side because you will start losing detail in your highlights. Digital imaging sensors tend to hold onto detail better in the darker or shadow values than they do the brighter or highlight values. In short, a little smooshing on the lefthand side is alright. Smoosh the right, well you will see what kind of terrible things will happen if you decide to go down that road.
Alright, so you were reading everything and deserve an answer. In lighting situations with large portions of a scene falling into the brightest or darkest part of an exposure, you will end up with a wave that is heavily weighted toward one end or the other of your graph.
Did you not just warn me that the smooshing thing was bad?
Hey, rules were made to be broken. I am trying to explain the exceptions here. For example, any event that takes place on a stage at night or at an indoor venue will tend to have extremely dark and bright image values. A spotlight will give great exposure on a subject, while letting the rest of a stage go black. As you might expect, this will leave you with a histogram that favors the lefthand side of the graph. This is perfectly fine and means you are on the right track to exposing your image naturally. It gives you a nice, bright, and isolated subject surrounded by black as they perform onstage.
Good for you. You can also start to find out how far to push your edits in your post production software of choice. This will render images that not only look good on your screen, but also hold up well on the bajillion other types of screens that your online viewers are using to look at your work. This will also give you a leg up when judging your work for printing purposes (although this is a whole 'nother post in itself).
Go forth, make friends with your histogram, and shoot with confidence! Your photos, I mean.