A quick non-camera aside: Exposure!
I am going to take a lot about exposure in the next few sections. Let’s take a half-second (or longer) to make sure we know what it means.
The Exposure is the amount of light that reaches the camera sensor. Basically, it’s how much light makes up the image.
As photographers, the exposure settings are the primary settings that we are worried about when taking a photo. Aside from, of course, where we point the camera. Getting the exposure correct is considered more technical than artistic, but the way we get the exposure correct can often influence the photograph to a great artistic degree. This will be clear when we look at shutter speed, and at depth of field.
If there isn’t enough light reaching the camera, the photo will be really dark - and lots of the sections will be totally black. If you leave the lens-cap on, all of the image will be totally black. Post-processing can sorta fix mildly underexposed images, like if everything is shadowy, but once part of the image is as dark as it can get - that’s it. Data lost! We need tonal difference between pixels, even mild differences, in order to perceive an image as, well, an image. If there was no difference (“contrast”) between any of the images, it would just be a solid color - and no amount of post processing could turn that right again.
This photo was shot with an automatic setting that got the exposure on the sign correct. Sadly, the rest of the photo is unusably underexpoed. This image was not taken at night, but dusk, and this exposure loses wonderful sunset light and color.
If too much light reaches the camera, the image is going to be really bright, and parts of it will be totally white - we call that “blown out”.
This Image is super over-exposed
No need to include a photo of a correct exposure - any good photo, that isn't noticable missing details, blown out, or dark, is "correct". There are no rules as to what constitutes a perfectly correct exposure1. If the image looks good, than it is good! In fact, some images might report as under or over-exposed, but still be what we want! Silhouettes are a good example of this - automatic settings would not have gotten the following two shots, in their aim for an average ’correct' image.
*This image is underexposed, but, like, on purpose?
This image is both underexposed, of my friend and the hallway, and overexposed, with the windows in the background being totally blown out. It's an example of "High Contrast" photography, which is all about shapes (and usually black/white).
The above image was shot underexposed - top left - but not that underexpoed. I was able to "fix" the exposure in post production. This image was probably a bit too dark to be really usable. Note the "noisiness" of the bottom-right part - an artifact of the underexposed image. Later, in the sections on Dynamic Range and the Historgram, we'll talk all about what images are literally able to be fixed or not.
Sometimes only a section of the photo is "wrong", like an over-exposed window or distractingly bright light bulb. Sometimes all the post-processing we need to do is selectively fix the trouble spots2.
As camera sensors get better, the range of images that will be 'fixable' will increase!
1 Actually, there are such rules. Well, guidelines. There are such guidelines. We'll talk about good guidelines to follow in the section on dynamic range and the histogram.
2 This isn't a modern invention - back in the darkroom days, photographers would "dodge" and "burn" their photographs. See the section on the film photography workflow for more.3
3 If you use photoshop and you think "Dodge and Burn, those remind me of some tools in photoshop" you are absolutely correct. Those tools were named after the darkroom technique. Same with the sponge tool that desaturates an image, and, uh, the magnifying glass. Also 'loupe' view in Lightroom. also the name 'lightroom' as a pun on darkroom. There are lots more - which is no surprise, really.