I originally provided the information below to the Rhee Tae Kwon-Do Perth Region Photography Group in 2008, and perhaps it might be of some help to publish it here (slightly edited).
Terminology
Photography involves a certain amount of technical terminology. This can be daunting at first, but hopefully this guide will help clear things up for you.
Focal Length
In general terms, the focal length is the physical length of your lens set-up. If you have a zoom lens, you will notice that when zoomed out (wide angle), the lens section actually becomes physically shorter. When zoomed in (telephoto), the lens section actually becomes physically longer. There are some zoom lenses that reverse this situation—typically, these are newer lenses—but the optical effect is the same. Focal length is measured in millimetres (mm).
- Shorter focal length (e.g., 24 mm) = wider angle of view = you see more of the scene (e.g., for landscapes and group portraits).
- Longer focal length (e.g., 105 mm) = narrower angle of view = you see less of the scene, but the subject is magnified more (e.g., for sports and solo portraits).
In between wide angle and telephoto focal lengths are the ‘normal’ focal lengths (e.g., 50 mm), which approximate the views that most people see using their own eyes.
Differences at shorter focal lengths make a more noticeable difference to the scene than differences at longer focal lengths. For example, a picture shot at 14 mm will have noticeably more in it than one at 18 mm, while a picture shot at 90 mm will look fairly similar to one at 105 mm.
Focal length also affects perspective, or how objects in your picture look in relation to each other. With a short focal length (wide angle) the distances between objects are exaggerated, while with a longer focal length (telephoto) the distances are compressed or minimised. A couple of examples of the telephoto compression of perspective come to mind.
- The incident where the late Steve Irwin was captured on video feeding a crocodile while carrying his baby son. Many of the pictures that were published made it look as if the pair were very close to the crocodile, but a side-on shot showed that they were actually quite far away.
- In televised cricket, any picture where you are looking over the bowler’s shoulder during a delivery. The batsman at the other end of the pitch (and indeed, the wicket keeper) looks close by, until you see a side-on view and realise how far apart they actually are.
Going back to wide angle (short focal length), you can probably remember pictures of people taken ‘too close for comfort’—their facial features appear exaggerated (e.g., big nose, stretched jaw) due to the particular perspective of these focal lengths.
Prime Lenses and Zoom Lenses
A prime lens (or fixed-focal length lens) is a lens that is made to function at only one focal length, while a zoom lens is a lens made to function across a range of focal lengths.
Prime lenses are typically of higher optical quality than zoom lenses at the equivalent focal length; it is much easier to design a lens to operate well at only one focal length than to operate well at several focal lengths. Prime lenses feature larger apertures (and are thus more suitable for use in low light photography) than zoom lenses, but zoom lenses are more convenient if you need to zoom in or out quickly (a turn of the zoom ring is usually faster than running forward or back).
Focus Distance
The focus distance is simply the distance from the sensor (or in older cameras, the film) to the subject (or target) on which the camera is actually focused. Years ago, all focusing was done manually, but autofocus became mainstream in the 1980s.
Although it might not sound too important (as long as you’re focused on your subject correctly), focus distance actually does have an effect on other aspects of a photograph, which we will see in a moment.
Depth of Field
You have all probably seen professional portraits—the subject’s face is nicely in focus, but the background is blurred (this effect is often known as boke or bokeh, from a Japanese word). This is an example of a picture with shallow (or narrow) depth of field. Only a specific part of the picture is in focus, and the rest is blurred out of focus.
In contrast, just about every landscape or group portrait you see will have great (or broad or wide) depth of field. A lot of the picture is in focus, ranging from things close to the camera all the way to things in the distance.
It turns out that a shorter focus distance (focusing in something close to the camera) results in shallower depth of field (blurred background), while a longer focus distance results in greater depth of field. The aperture also affects depth of field (and is usually the main consideration for most people in this regard, i.e., they tend to think more about aperture rather than focus distance when trying to achieve shallow depth of field).
Aperture
For light to hit the sensor, and thus be recorded as a picture by the camera’s electronics, it has to be focused properly and, of course, there must be some opening in the camera for the light to actually get to the sensor. Most lenses are actually clusters of glass elements that take light from the scene and shape it so that it hits the sensor in the right way to make a picture. One thing that you can control is how big the hole is that is letting light hit the sensor; this is the aperture.
Apertures are specified in terms of fractions of the focal length (f). Thus, if someone writes f2.8, this should actually be written as f/2.8 or f = 1/2.8. Thus, a ‘bigger’ f number (e.g., 16) actually means a smaller aperture than a ’smaller’ f number (e.g., 2.8). If you think of them as fractions (i.e., 1/16 and 1/2.8) you should be able to see how this makes sense.
- Larger aperture = shallower depth of field = faster shutter speed (a bigger hole means less time needed to get the right amount of light in; important for sports photography).
- Smaller aperture = greater depth of field = slower shutter speed (a smaller hole means more time needed to get the right amount of light in; important for landscapes).
One issue to note: large aperture zoom lenses are harder to make and so cost a lot more than other types of lenses. Most amateur/consumer zoom lenses have maximum apertures of f/3.5 to f/5.6. Professional zoom lenses are typically f/2.8. Prime (non-zoom) lenses tend to be the largest aperture lenses, but are also easier to make because they don’t zoom; these typically have maximum apertures around f/1.4 to f/2.
Just as with focus distance and focal length, you can change the aperture. What is critical when looking at lenses is what the maximum aperture is—this is a hard limit on how much light you can let into the camera using that lens.
Exposure Duration (commonly referred to as Shutter Speed)
You can also set how long the camera keeps the aperture open for (while also activating the sensor to record the picture). Your exposure duration will affect what kind of action you can shoot; for landscapes or other scenes where you aren’t interested in capturing action, a long exposure is perfectly fine. Typically, you would also mount your camera on a tripod for such photography.
For hand-held photography, you will probably want to use exposures well under 1 second. If you use longer exposures, you will just get blurred images (if trying to capture action) or else your picture will be affected by camera shake (i.e., you can’t hold the camera perfectly still—and remember that you’re trying to hold your camera still while light is passing through a lens and hitting a sensor about the size of your thumb … even a tiny fraction of a mm of shake can blur your picture).
Most people can hold a camera fairly still for 1/60 of a second, which I believe is why that is the default exposure duration for most flash photographs. Any longer (e.g., 1/2 s) and camera shake will probably blur the picture. Shorter shutter speeds (e.g., 1/100 s) are usually better.
Note that other factors described above will affect the longest exposure you can use.
- Longer focal length = shorter exposure needed (the lens will magnify both the scene and the shake of your hands).
- Shorter focal length = longer exposure may be used safely.
- Larger aperture = more light let in (bigger hole) = shorter exposure will result.
- Smaller aperture = less light let in (smaller hole) = longer exposure will result.
For action photography, you would probably want 1/250 s or faster, though this depends on the type of action you’re trying to shoot.
Sensitivity (of the Sensor/Film)
Following the International Standards Organisation (ISO) standard, film/sensor sensitivity often starts at ISO100 (less sensitive) and can go up to ISO1600 or ISO3200 (more sensitive). I believe that ASA and ISO are equivalent as far as film/sensor sensitivity is concerned. If you are familiar with ASA ratings from film photography, effectively, ASA100 = ISO100.
- Lower ISO = sensor is set to be less sensitive to light = longer shutter speed needed = larger aperture becomes very handy if you need to retain a fast shutter speed (better picture quality—less electronic noise in signal).
- Higher ISO = sensor is set to be more sensitive to light = faster shutter speed is possible = smaller aperture may be used if desired (poorer picture quality—more electronic noise in signal).
You will normally want to use the lowest ISO sensitivity you can without compromising your shutter speed. For low-light action shooting with a prime lens, you can probably use ISO800; with a zoom lens, you will probably need to go to ISO1600 (or higher if your camera supports it).