There are several types of cameras you may encounter. While the lines defining the categories may blur as time goes on, several basic conclusions can be surmised about a camera given the category it's part of. It seems everybody has a camera phone nowadays. Image sensors on camera phones are generally quite small. This fact means we can draw some generalizations about their performance. The benefits of a camera phone are that you will nearly always have it with you. It's easy to use. Most will upload images to a website automatically. Most will do video. Most will take excellent macro shots providing light is good. The drawbacks are that they're slow. Most camera phones do not offer any kind of manual control. Low light images are generally poor quality. Most will not have a shallow DoF. Most will be too slow for action shots. But, the camera you have with you is better than the one you don't. You can get some excellent shots with one despite any shortcomings.Next up on the pecking order of quality are point and shoot cameras. These are pocket sized digital cameras for the most part. Some point and shoot cameras can be quite large though. They will generally have larger sensor sizes than camera phones, but small in comparison to DSLR or Medium format cameras. They'll have better low light capabilities than a camera phone, though there may be some exceptions. DoF will be fairly deep. Macro production will be fairly good due to the small sensor size. Many will have some sort of vibration reduction. Most will record video. Some will do well with action shots, but most will not. Some allow you to manually adjust settings. The more compact ones are very easy to stash in a pocket and will be easier to use than a camera phone.Moving on up the quality ladder we have DSLR cameras. Sensors in DSLR cameras are come in two basic sizes. APS-C size sensors, which are smaller than 35mm film. 25.1 × 16.7 mm is generally the normal size for APS-C. This size sensor allows 1.5 to 1.6x higher zoom levels than a full frame 35mm DSLR. Full frame DSLRs will have a shallower DoF than APS-C size DSLRs. Full frame DSLRs will generally collect light better than an APS-C DSLR with the same number of pixels. DSLRs in general will be much better at action shots due to their quick handling and faster processors. Better DSLRs will have buttons and external controls which can be used to access functions rapidly instead of digging through menus to change settings like on point and shoot cameras.
The eye is functionally quite similar to a camera. A Camera sensor is composed of multiple photo sites which are similar to the rods and cones in our eyes. Without going into too much technical detail, the sensors are generally arranged in what is known as a bayer pattern. This pattern is comprised of rows of photosites which alternate green, red and blue filters over each photosite. Photosites are color blind. So they need colored filters to filter out the unwanted light over each photosite. This way, the photosite will only register the color that is allowed through the filter.
50% of the sites use green filters. 25% use red and 25% use blue. The reason being, our eyes our built the same way, and this has proven to be the most effective way. Also, the dyes needed to filter other colors were less available at the time the technology was developed. Each group of 4 photosites then becomes a pixel. The photons measured within each photosite are averaged together to produce a color.
Sensor size is important for two distinct reasons. Sensor size dictates the crop factor of a particular camera. Smaller sensors crop more out of an image produced by a lens than a larger sensor. Smaller sensors also produce a larger DoF.
Aperture, shutter speed and ISO are what we use to control the exposure of photo. Aperture controls the amount of light coming in through the lens. The aperture an analogous to the human iris. The smaller the aperture opening, the less light comes in. The larger the aperture opening, the more light comes in. Aperture is expressed as a fraction of the focal length of a lens.
The focal length is generally, but not always, the distance between the front lens, where the image enters the camera, and the sensor where the image will be in focus. Usually Aperture is just shown as f/1.8 or f/5.6 for example, with f being the focal length. The higher the number, the smaller the aperture size and the less light enters the lens. Another way aperture affects captured images is called depth of field. A larger aperture produces a narrower depth of field. The depth of field is the area of an image that appears to be in focus. Things closer and further than the subject in focus, tend to be out of focus. The depth of field determines how deep this focused area is.
See how the background and foreground are out of focus?
Shutter speed is the length of time the shutter remains open. The longer the shutter is open, the more light enters. Also, the longer a shutter is open, the more motion blur an image will show. Usually shutter speeds range from 1/8000 of a second to 30 seconds. At 1/8000 of a second, very little light gets in, however, motion is frozen. Conversely, at 30 seconds, every motion in the image will show up as a blur, but a lot of light will be captured. Longer shutter speeds are useful for shooting in low light situations or adding intentional motion blur for artistic effect.
ISO refers to the sensitivity of the film or sensor. A lower number denotes less sensitivity to light. Higher numbers indicate more sensitivity to light. The higher the ISO the more noise there will be in the image. Increasing ISO behaves like an amplifier. Amplifiers will make unwanted as well as wanted signals larger or more visible in this case. So noise gets amplified as well. Higher ISO will result in less crisp images.
Getting the image you want may sometimes require a juggling act between these three major settings. For example, if you wanted to freeze action in low light, you'd probably start by using the highest acceptable ISO setting. You'd naturally need to have a high enough shutter speed to freeze the action. This may dictate that you're required to use a large aperture to let in as much light as possible. So, you'd be stuck with a narrow depth of field which would be a product of the large aperture. The opossum was taken at night with just a bare 60 watt bulb for illumination. Notice the grainy image and the shallow depth of field.
Now if you wanted to capture something like a bird in motion during day light and freeze most of the action, you could get away with using a lower ISO since you'd have plenty of light at your disposal. You could use a smaller aperture to get deeper field to ensure more of your subject was in focus. You could use a fast enough shutter speed to freeze all the motion, or as with the following example, you could slow down the shutter speed a bit to get a sense of motion by providing a bit of background and wing motion blur.
Most modern cameras feature automatic modes. There are scene modes that will select from pre-determined programmed settings. There are also modes which will allow you to prioritize shutter speed or aperture. The camera will use built in light sensors and scene analysis to choose what it thinks are the best settings. What if you don't like the automatic settings the camera chose? You can either try manual, or you can tweak the settings a bit using the exposure compensation adjustment. Most cameras will have a way to access this functionality. Basically, exposure compensation tells the camera you want the image to be lighter or darker than what the camera chose. How it accomplishes this, depends on the priority mode selected and any automatic options which have been previously selected.
Metering modes will affect how the camera interprets a scene. Spot metering vs Matrix.
Focus modes vary with cameras as well. Most cameras will offer options to allow you to choose the focus point. This is particularly critical with larger aperture settings since you want to be able to choose what is in focus. You'll want to be able to control where the shallow DoF is focused. Some cameras will do face recognition and automatically focus on the eyes. Some will do focus tracking. With focus tracking, you lock focus on your subject and the camera will automatically follow them as they move around in the view finder.
White balance consists of finding the color temperature of the light in the camera frame and adjusting it so that white in your subject looks, well, white. The temperature is usually expressed in Kelvin and is a characteristic of the frequency range of the light source and any media it is filtered through. Air, haze, clouds and windows are just some forms of the the media light can be filtered through. In the days of film, white balance was pre-determined by the film used and adjusted with filters on the lens. Today's digital cameras provide for this setting in the software. You can usually choose from several automatic white balance modes.
Many cameras also offer manual white balance setting. You can either photograph a white or grey card within the area of your subject and the camera will determine the white balance, or you can manual choose the kelvin level yourself.