Feel free to suggest, ask questions, or add. This is a repost from one I did about a year ago, with some minor tweaks:
Bokeh- The portion of an image (often the background in a portrait) that is rendered out of focus. Japanese origin. Became trendy term in the late 1990s-early 2000s. It has a more positive connotation than “blurry,” “soft,” or “out-of-focus.” Meaning that the photographer most likely dropped portions of the image into bokeh intentionally, for a specific reason. Basic bokeh formula = Longish focal length + Wide aperture (low f/#) + Camera as close to the subject as possible + Subject as far away from the background as possible
Blurry- Most often used to describe motion rendered in a still image, like a car in motion rendered as a blurred streak by use of slow shutter speed, or the blur you see from camera shake when you hand-hold while using too slow of a shutter speed. Sometimes used to mean out of focus, not sharp, though the better term for these qualities is “soft.”
Soft- An image or portions of an image that are not sharply focused. Usually used to describe something we feel should be in focus, but isn’t. “The eyes in this portrait look a little soft to me.”
Sharp- Usually used to describe parts of the image that are rendered clearly. “I like how sharp all the leaves on the tree in this shot are, I can see detail in all of them.” Also related to sharpness, which is how well, overall, a lens can resolve the edges and color differences between different areas in the image.
Focal Point- Often misconstrued as the part of the image that is focused on, which is not entirely true. The focal point is the part of the image, usually the subject, toward which the photographer is steering your view. It is the point of interest or activity. Focus may or may not be held on this point, depending on what the photographer is trying to do visually. Can sometimes also mean the point where lines tend to converge at infinity. We often use leading lines, the rule of thirds, or a color or tone that stands out to draw the viewers eye to the focal point.
Point of Focus- Self-describing. The point on which sharpest focus is held. Not really a single point, necessarily, because there is actually a total area of front-to-back distance in an image that is sharp, not just a single point or plane. It is a factor in Depth of Field, which I’ll get to in a second.
Depth of Field- See “Point of Focus” for the lead-up to this. Besides the actual area where the lens is focused, there is a greater area of front-to-back distance in the image frame that can also be rendered sharply by a lens. How large this area of sharpness is depends mainly on the aperture selected. A smaller aperture (higher f/number) will render more of the image frame, both ahead of and behind the point of focus, in sharp focus than a larger aperture (smaller f/number).
Stop- We have to have something to measure levels of exposure. Just like a ruler is divided into inches or centimeters, exposure is divided into stops. If you shoot a random image, increasing the exposure by one stop doubles the light hitting the sensor or film and makes the image “brighter.” Decreasing the exposure by one stop halves the light hitting the sensor or film and makes the image “darker.”
You can change basically 3 things to move stops up or down: 1) ISO (note that with digital, ISO sensitivity is adjustable and can be used as an exposure control, but film ISO is fixed and cannot be adjusted), 2) camera shutter speed, or 3) lens f/number (lens aperture opening).
Doubling or halving the ISO equals one stop up or down. Generally, manually-set ISO is adjusted in one-stop increments (100, 200, 400, etc.). This is easily changed on a digital camera, but film ISO is fixed. The only way to change ISO with film is to physically switch to film with a higher or lower ISO- changing the ISO setting on a film camera does nothing to the film sensitivity.
Doubling or halving the shutter speed equals one stop up or down. 1/125sec is one stop less exposure than 1/60sec. 1/30sec is one stop more exposure than 1/60sec. Most cameras increase or decrease shutter speed in 1/3 stop increments, so 3 clicks up or down from where you are is one stop down or up from the current exposure.
Multiplying the currently-set f/number of the lens aperture by 1.41 equals one stop down, dividing the currently-set f/number of the lens by 1.41 equals one stop up (the exact numbers are ever-so-slightly different, but the principle holds true), so f/5.6 to f/8 is one stop down (less exposure), f/5.6 to f/4 is one stop up (more exposure). Sounds kind of complicated, but it’s not. Most lenses increase or decrease aperture in 1/3-stop increments. So 3 clicks up or down from wherever you happen to be is one stop down or up from the current exposure. Just remember, with f/numbers, bigger number equals “darker” image, smaller number equals “lighter” image. See also “depth of field,” because changing f/number (aperture) does more than just change exposure.
Shutter Speed- Simply the amount of time the shutter is left open (or that the sensor is activated in the case of an e-shutter). You have to open the shutter to expose the film or sensor. If it’s open for a short time, less total light hits the film or sensor. If it’s open longer, more total light hits the film or sensor. Shutter speeds faster than 1/125 are better for stopping motion and preventing blur. Longer shutter speeds allow moving objects to drop into blur. The faster the object is moving, the faster the shutter speed has to be to stop it from blurring.
f/number or f stop- Used as a measure of how much light the lens diaphragm/aperture is letting through the lens. Small f/numbers, like f/2.8, f/2, etc. allow more light through the lens. Larger numbers let less light through. See also: "Stop," and "Depth of Field."
Aperture- The opening in the lens controlled by the lens’ diaphragm. It can be adjusted to be larger or smaller. See “f/number or f stop.”
ISO Number- (Sometimes referred to as ASA or DIN, but in digital cameras “ISO” is the norm). A carry-over from film. The number indicates how sensitive the film is to light. Lower ISO equals less-sensitive film. Higher ISO is more sensitive. The same applies to digital sensors. Low numbers are less sensitive and higher numbers more sensitive. With film, lower numbers usually have finer grain and can render sharper images. Similarly, with digital, lower numbers have less “noise” in the image and can render sharper images, larger numbers introduce noise that can affect the perception of quality and sharpness. It’s kind of like turning up the volume when you’re listening to music. There is a sweet spot where everything sounds good. Turn it up, and it gets louder, but also possibly more distorted. If you're shooting film, it's important to note that, unlike digital, changing the ISO setting on your film camera does nothing to the sensitivity of the film, and nothing happens in camera to make the film more or less sensitive to light; your ISO 200 film will always be ISO 200 film, for example. What changing the ISO on a film camera does do is re-calibrate the light meter-- if you set the camera ISO to 200 and you actually have 100 ISO film loaded, you've told the meter to show you correct exposure for film that is 1 stop more sensitive than what you've loaded; if you shoot "correct" exposures according to the meter, your film will actually be underexposed by 1 stop and your images will be darker/flatter than expected. Setting ISO 400 on the camera with 100 film loaded will give you shots that are underexposed by two stops. Set the camera to match the film ISO, unless you have done test shots at different ISO settings and know for a fact that the film type you're using is actually more (or less) sensitive than how it's labeled.
Flat- Describes light quality in an image that is dull or has low contrast, a lack of differentiation between light and dark areas. Usually this is from dull, featureless light or underexposure. “This image looks a little flat to me.”
Hot- Part of an image that is far too bright/overexposed. Usually to the point that it either has no detail and/or is distracting. “This corner is a little hot, it keeps pulling me away from the subject.” Or “The highlights in the face are hot, I can’t see any detail in the skin tones.”
Focal Length- This is not how physically long or short your lens is (you can't whip out a ruler, measure your lens from front to back and determine focal length, that isn't what it's about). Strictly- it's the distance from the lens element/s to the single point in space where the light is focused behind it. Again, has nothing to do with the physical length of your lens. A lot of technical things apply, but let's keep it as simple as possible: Measured in millimeters, longer focal length lenses give greater magnification and a narrower field of view. Shorter focal length lenses give less magnification and a wider field view. A "long" lens is one of larger focal length. A "short" lens is one of shorter focal length. On the common APS-C crop-sensor DSLR, wide-angle lenses start at a focal length of about 18mm and shorter, normal lenses at about 35mm to 45mm, and telephoto lenses at about 50mm or longer. These numbers will be different for full-frame-sensor digital, 35mm film, or Micro 4/3 cameras.
Crop Factor- Back in the day, before digital, the most common film camera used by pro or serious photographers was the 35mm SLR, which had a film frame 36mm wide and 24mm tall. "Full frame" in digital means a sensor of the same dimensions as that 35mm piece of film. When the digital SLR (DSLR) became a thing, a sensor smaller than full-frame was most-often used to save space and cost; the APS-C sensor has a sensor about 1.5 times smaller than a full-frame sensor or 35mm film frame. Everything worked the same, but to get everyone on board and to help film photographers make the transition and understand the differences the smaller sensor made, we were introduced to crop factor.
Here's the easiest way I know to explain crop factor: Imagine you set up a movie projector to project its image on to a 36’ wide, 24’ tall screen. Nice big projected image. Ooh. Ahh.
Now leave the projector and its lens exactly where they are. Change nothing. But take down the 36’x24’ screen and replace it with a smaller 23.6’ x 15.7’ screen in exactly the same location. Now the smaller screen catches a smaller portion of the total projected image, the rest of the projection is lost to space around the edges. You are seeing a cropped version of what is available,
In the above example::
Did the focal length of the projector’s lens change? No.
Did the amount of light passing through the projector’s lens change (get brighter or darker)? No.
All that happened was you put up a smaller screen, so you don’t see all the available projection. The screen is the camera sensor, and the lens and whatever it’s focused on are the projector. Get it?
What it means in practical terms is that if you're used to shooting on 35mm film, or if you're transitioning from a full-frame to a crop-sensor DSLR, and you want to use lenses that have the same field of view as your full-frame lenses, then you'll need to divide the focal length of the full frame lens by 1.5 (1.6 for Canon) and use that focal length on your APS-C camera. Example: I take portraits on my full-frame camera with a 75mm lens and like the field of view it gives me. My standby camera is an APS-C DSLR. If I switch to my APS-C camera, I need to use a 50mm lens to get the most-simlar field of view to what I saw at 75mm on the full-frame.
New Stuff Added Jan 2nd, 2018:
Dynamic Range- Okay, let's start by talking about something most of us have, your eye and it's retina. Currently, I'm sitting in a fairly darkened room under window lighting (no interior lights on) and it's sunny early afternoon outside. My eye is easily capable of looking into the interior of the room and seeing detail, clarity, and color in extremely dark, dull, shadowy conditions where light barely reaches; I can also look outside through the window and see full clarity and detail in the brightly-lit outdoor space, even into the shadow areas under the trees and the eaves of the buildings. My eye has no trouble discerning the details and features of both scenes, even though they are in light levels that vary in intensity by hundreds or even thousand of times. Moreover, I can back up and include both the bright window and dim room in my view and see detail in both sides of the scene at once with my eyes with little to no difficulty. My eyes have extremely high dynamic range.
My camera's sensor or film, by comparison cannot come even remotely close to doing what my eyes can do, and herein lies one of the things that all photographers need to wrap their heads around as early as possible: It is impossible for a camera/lens to reproduce the world exactly as the eye sees it. I can shoot an image of the interior of this building using my camera's light meter to select settings that will give me detail and contrast in this dim inside light (a low shutter speed, wide aperture, and possibly a high ISO) and it will look fine, but if I then turn the camera to the window and try to take a pic of the scene outside without changing my settings, then the image will be blown out; overexposed to the point of looking pure white with no detail. So I have to again use my camera's light meter reading to reduce my shutter speed, make my aperture smaller, and probably lower my ISO to get the window shot exposed correctly. But now, if I turn the camera back to the inside scene with those settings, the shot will be underexposed and black with no detail. It's even worse if I try to take a shot including both the window and the interior scene-- while my eyes can easily adjust and see the bright outside and dark inside essentially in the same view, allowing me to see details in the whole scene from bright to dark, the camera simply can't-- it doesn't have the dynamic range to capture both the bright parts and dark parts of the scene simultaneously and allow the image to show detail in both areas. The best I can hope for is to pick an exposure setting somewhere in between the bright and dark areas that will let most of the things in the scene be reasonably exposed, but I'll lose detail in the highlights, which will still get blown out and overexposed, and the shadows, which will still be nearly black and underexposed. The camera has low dynamic range compared to my eyes. There are ways to work around this problem, but it's not something your camera can do in one shot.
So, when we talk about dynamic range and say that one camera sensor or film type has a higher dynamic range than the other, what we are saying that it is better at capturing a usable image from a scene with large differences between dark and bright areas before it loses shadow details or blows out highlights than the other sensor or film, or that it's very good at holding image detail/color/sharpness in extremely bright or extremely dim light. Sensor and film design affect this ability, as does sensor size. A smaller sensor typically has less dynamic range than a larger sensor. Some types of film emulsion are better at capturing both highlight and shadow details than others.