Glossary of photographic terms
Canon use APS-C sensors for all their EOS cameras but the professional 1D X, 1D, 1Ds and 5D series. The size of an APS-C sensor is smaller when compared to a full frame sensor (which uses the 35 mm film format as used in analog / chemical photography). Therefore the effective focal length of any lens is longer by a factor of 1.6 (called the crop factor). If you use a 100 mm lens with an APS-C sensor the angle of view will be identical to the one a 160 mm would create on a full frame sensor. Some lenses can only be used with APS-C sensors (such as the Canon EF-S series of lenses). Note that using an APS-C sensor has an influence on the in-focus range as well. Not for the sensor itself but for the lenses that you use. Confused? Bear with me just a little more. If you use an APS-C camera and wish to make a regular wide-angle photo you might choose a focal length of 17 mm which will give you a certain (huge) in-focus range. If you wanted to take the same picture with a full frame camera you would choose a different lens with a focal length of 27 mm which - because of the longer focal length - would give you a much narrower in-focus range. You would only have the same in-focus range if you used the same 17 mm lens with the full frame camera but that would give you a completely different angle of view for the resulting image.
Canon use APS-H sensors only for EOS cameras of the 1D series (not 1D X, not 1Ds). The size of an APS-H sensor is smaller when compared to a full frame sensor (which uses the 35 mm film format as used in analog / chemical photography) but larger than an APS-C sensor. The effective focal length of any lens is longer by a factor of 1.3 (called the crop factor). If you use a 100 mm lens with an APS-H sensor the angle of view will be identical to the one a 130 mm lens would create on a full frame sensor. Canon APS-H cameras cannot use lenses that are made for APS-C sensors (no EF-S lenses).
Beauty of blurred image parts
[or Bokeh] As the name already suggests the beauty of blurred image parts is a matter of opinion but in general blurry parts should at be as evenly blurred as possible. This mainly depends on the physical characteristics of the aperture blades. The more blades are used the more evenly round can the aperture be and in addition some manufacturers curve the blades themselves to achieve are more even curvature of the aperture. Color fringes in blurred parts of the image also impair their perceived beauty.
The camera itself without lens is called "body".
Circular polarizing filter
Circular polarizing filters are used to reduce (or amplify) reflections on non-metallic surfaces that are produced by lateral rays of light. Reflections on glass windows or on the water of a lake can be distracting or desireable - depending on what story your image tells. The color of skin, plants, animals, stones and other surfaces can be intensivied by removing the gleam of lateral light rays. A boring sky can get much more dramatic and show more contract with the clouds when photographed with a polarizing filter. Circular polarizers must be adjusted (before each shot) to the angle your camera has to the sun. That's usually done by rotating the upper part of the filter while looking through the viewfinder (that's why your lens's front element should not rotate while focusing or zooming).
[or Light Fall-Off, Vignetting] Most lenses create an image that is slightly darker towards the corners than in the center. The reason for that is partly just unavoidable physics but the way the lens is build also influences how dark the corners get. The lower the chosen f-stop the more intense the shadow gets. Many people use this creatively for portraits by choosing very low f-stops. Attached filters or hoods can further increase the corner shadow while software today is pretty effective at removing it (in post or even in-camera). A good lens should at least get rid of the corner shadow when higher f-stops are selected. Some wide angle lenses create an image that looks almost as if you were peeking through a keyhole.
Using a lens on a camera sensor that is smaller than a full frame sensor results in a more magnified image (compared with the image a full frame sensor would record). The one taken with the smaller sensor seems to have been taken with a longer focal length than the full frame image - for APS-C cameras the difference between these two focal lengths is 60% or a factor of 1.6. It's called the crop factor because you could also create the magnified image from a full frame image by cropping its width and height (dividing both figures by 1.6).
Curvature of the focal plane
Because camera sensors are flat (not curved) they ideally need lenses to project a flat image onto them. In reality however the projected image is usually slightly curved which results in either the center of the image being in focus or the corner (but not both at the same time). This has nothing to do with the lesser resolution you always expect from the corners of an image when compared with the center. A lens with pronounced curvature can still give you good resolution at the corners but only if you focus on them. If you are shooting a straight wall or a large group of people with a lens like this you'll have to decide whether to focus on the center or on the corner and will be struggling to get both in focus (higher f-stops help). Simly put: the focal plane should be just that - plane and not curved.
see In-focus Range
[or Teleconverter] An extender is mounted between the camera and the lens to extend the lens's focal length by a certain factor. The extender does this by using magnifying glasses that reduce the image quality and absorb some of the light. Typical extenders are available with zoom levels of 1.4 (40% gain in focal length, 1 f-stop light loss) and 2.0 (100% gain in focal length, 2 f-stops light loss). Because of mechanical / optical contraints not every lens can be used with extenders. In case of Canon only some of the better telephoto lenses can be used. The image stabilizers efficiency is usually not affected by the use of an extender however the autofocus might stop working. That's because cameras need a certain maximum aperture for autofocus to work (and that varies for the different camera models).
The f-stop gain of an image stabilizer is a measure for its efficiency at removing motion blur (mainly) caused by the natural tremor of your hands. An f-stop gain of 2 exposure values will allow you to use shutter speeds two times lower (e.g. 1/100s instead of 1/400s) than you normally could without inducing motion blur.
Full Frame Sensor
Full Frame Sensors have the same size that 35 mm film as used in analog / chemical photography has. Because film strips have perforations on both sides the actual space that is exposed is not 35 mm but just 24 mm in width (width × length: 0.9 " × 1.4 "). Full frame sensors are used in all Canon EOS 1D x, 1Ds and 5D models (but not 1D models).
An image stabilizer mainly reduces motion blur caused by the natural tremor of your hands when handholding the camera. The longer the focal length the more important is an image stabilizer because if your hands move just a tiny bit with a wide angle lens the image framing changes much less dramatic than with an ultra telephoto lens. To help that you can choose faster shutter speeds. If no image stabilizer is available you should use shutter speeds faster than or equal to the reciprocal value of the lens's focal length (focal length: 250mm means shutter speeds faster than or equal to 1/250s). This of course is an approximation and depends on how still you can hold your camera. Also if you use an APS-C camera you will have to consider the crop factor as well (250 × 1.6 = 400, so you are looking at 1/400s without image stabilizer).
An image stabilizer will allow you to use slower shutter speeds without inducing motion blur. The efficiency of the stabilizer is measured in f-stops (or exposure values) that you gain by using it. For our example above an f-stop gain of 2 exposure values will allow you to use 1/100s instead of 1/400s (or 1/60s instead of 1/250s respectively).
Using a tripod also prevents motion blur caused by the tremor of your hands and most image stabilizers should therefore be disabled when using a tripod. Some however detect tripod use automatically and still reduce the motion blur caused by the mirror and shutter curtain movement. But remember: if your subject moves (even slightly because of the wind) an image stabilizer will not help you one bit with this type of motion blur.
[or Depth-of-field] Whether the image should be sharp from the foreground to the background or whether just a small part of the image should be in focus depends a lot on what story you are telling with your image. Wide landscape are often completely in focus but if you'd like a single flower to stand out of a meadow one way of doing this is to have a narrow in-focus range that covers just the flower and blurs everything else. The in-focus range depends on several factors that you can influence. The lower the f-stop and the longer the focal length and the closer your subject is the narrower is the in-focus range. Please also note that the smaller your camera's sensor is the larger is the in-focus range (but that's a bit of a simplified statement).
The ISO setting is the sensitivity of the camera sensor which can be configured in-camera. In analog / chemical photography every film has a fixed ISO rating. When photographing in bright sunlight you would choose a low-sensitivity film like an ISO 100 film and would swap that with an ISO 400 film for indoor shots. Digital photography allows you to configure the ISO rating individually for every picture you take but (like with analog film) the higher the ISO rating the lower the overall image quality and the more noise is visible in the resulting picture. In many camera specifications you can find which ISO range can be configured for the sensor but please be aware that this does not say anything about how good the sensor's image quality is in a high ISO shot.
The numbers of the ISO rating are similar to shutter speeds in that a number double as big means double as sensitive to light and thus only half the amount of light is needed. So if you'd like to take a picture at f/8 and find out that for an ISO 100 setting a shutter speed of 1/25 s is needed for the amount of light present you can up the ISO to 200 to allow for a faster shutter speed of 1/50 s or even to ISO 400 for 1/100 s (and so forth).
L Series Lenses
Canon call their high-end luxury lenses "L"-lenses.
A lens hood is attached to the front of the lens in order to keep lateral light rays from falling onto the lens's front element. It's just a mechanical shield - if constructed properly does not influence image quality at all. However, some poor implementations are constructed so that they obstruct the angle of view of the lens which will induce darker corner shadow.
see Corner Shadow
The magnification ratio is the ratio between a thing's real size and the space it takes up on the camera's sensor. If a man is 6 ' tall (= 71 ") and takes up 0.71 " on the sensor the magnification ratio is 1:100 or 0.01 x. Since there is a limit to how close a subject can be to the lens and still be in focus lenses have fixed maximum magnification ratios which are usually noted in the lens's specification. Real macro lenses have maximum magnification ratios of 1.00 x (1:1) which means that something that's 0.71 " in real life will take up the same 0.71 " on the sensor.
The maximum aperture is the lowest f-stop that can be used on a lens. It lets the maximum amount of light through and is usually mentioned in the lens's name - e.g. f/2.8. On a zoom lens the maximum aperture often depends on the chosen focal length - hence the range given in some lens names e.g. f/4.5-5.6.
Prime Lenses are lenses that only offer one focal length (no zoom).
Stopping down means to use higher f-stops (e.g. f/8 instead of f/5.6).
When using a tripod usually the camera is mounted onto the tripod's head. But if your lens is long and heavy you are putting a lot of stress on your camera's lens mount and furthermore the tripod might fall over because it is not well balanced with a large and heavy lens protruding it. Instead attach a tripod mount to your lens and then attach the lens (rather than the camera) to the tripod head. That way the weight can be balanced for the tripod head and there is a lot less stress on the camera's lens mount (it only has to carry the weight of the camera now rather than the weight of the lens). Some lenses come with a tripod mount for others it can be purchased as an accessory. Tripod mounts usually also allow for a quick change between portrait and landscape orientation.
see Corner Shadow