Yes, with a terrible piece of punnage I'd like to introduce a mini topic worthy of being thrown into the mix at this point, which is Depth of Field. Also known as Depth of Focus, this concerns what I can only really summarize as 'front-to-back' sharpness. Look in any magazine where there are both portraits and scenery shots, and the main difference you will usually see is that in the landscape shots, the whole of the scene will be sharp, from the closest sheep to the distant hills. Look at a portrait and you tend to find that the only sharp part is actually the subject, and the background is thrown out of focus, thus partly or entirely rendering it a blur.
The distance between the closest and furthest elements which are rendered 'acceptably sharp' is described as the depth of field. If you're using a digital compact or small bridge camera then your control over this effect will be limited, and can best be demonstrated when shooting as close as possible to a detailed subject, using the longest setting on the zoom lens. Some may show the effect on the LCD screen before you press the shutter, while on others you will need to compare captured images. If you're using a reasonably well specified SLR camera, whether film or digital, it's much simpler to demonstrate the effect, due mostly to the additional control available over the settings, and the 'depth of field preview' button which I personally regard as essential on any film SLR camera and highly desirable on digital SLRs. The circumstance where the resulting differences will be most noticeable will be the same scenarios as for the digi-compact i.e. close subject, long lens.
So to the demo.
Set your camera or lens to a wide aperture (small number) such as f/4 or as wide as your kit will allow, put the camera on a tripod or beanbag, focus on a nearby subject such as an ornament - something with some front-to-back depth to it. Look through the viewfinder or at the screen. You should see that only a small part of the subject is in focus. At the widest aperture, what you see is what you get. The image will record like this if you fire the shutter. Take the photo.
Now 'stop down' the lens or camera to the smallest (large number) aperture available. This may be anything from f/8 to f/32 depending on your kit. The image in the viewfinder will not change, so if you're shooting film, use of the depth of field preview at this point is your only route to check the result before your prints or slides come back, and it's well worth a look if using a DSLR too. As you press the button or lever the viewfinder will darken significantly. As your eye becomes accustomed to the reduced brightness you should be able to see that much more or all of the scene is now sharp. What the button or lever is doing is closing the lens down to the aperture you selected, allowing you to see what the film or sensor will 'see' when the shutter fires. On a well specified digital compact I would expect the LCD screen to show the result of the change, but many may not and you will have to take the photo to see the difference. The results should be obvious in these circumstances.
Which brings me back to my earlier comment, about depth of field control being limited on digital compacts. DoF is affected by three main variables. The focal length of the lens :- shorter lens = more DoF, the aperture of the lens :- smaller aperture = more DoF, and the size of the recording medium :- smaller size = shorter lenses = more DoF. On a digital compact, the sensor tends to be very small, increasing the depth of field. Due to the small sensor size, lenses tend to be very short, increasing the depth of field again, and the available aperture range is limited, thus having a further limiting effect on your level of control. These factors all conspire to make it difficult to achieve shallow DoF on many digital compacts unless you are very close, zoomed in, and using a wide aperture - using portrait mode will achieve this if your camera does not have aperture controls.
