Not the Length of the Grass...

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.

Decent Exposure

Exposure is a term that's scattered liberally around any text on photography. It basically stands as a shorthand for all the settings and decisions that enable you to get an image on screen that has the brightness, colour rendition and clarity that matches the natural 'eye' view of the scene, or whatever version of that you have set out to create. Variables in question are shutter speed, aperture and film or sensor speed (ISO). I'll cover each of these in turn as they'll all be useful to you the very moment you start to see your camera as a creative tool.

Shutter speed
This is the amount of time the film or sensor (i'm going to use the term 'image plane' to refer to these two generically) is exposed to the light. Pretty obviously, longer time open equals more light hitting the image plane. Shutter speeds traditionally run on a doubling pattern, but generally the number displayed needs to be considered as a fraction. For example a speed of '1000' displayed means one-thousandth of a second, a relatively fast speed. '1' means one whole second, and inbetween we generally find 2 (half a second), 4 (quarter), 8 etc up to 64, which is then 'almost' doubled to 125, 250 etc up to whatever your camera maker deems necessary. Mine goes to 4000. Some go much faster than this, and many go to much slower speeds than 1 second. A shutter speed of 'B' signifies 'Bulb' exposure - a throwback term which basically means the shutter is held open for as long as you hold the shutter release. sometimes accompanied by a 'T' setting ('Time') which opens the shutter on one press of the release and closes it on a second, these settings can be used for very long exposures lasting several minutes or even hours when shooting star trails etc. Fast shutter speeds freeze action or movement, slow ones blur it. Different subjects will look good at different speeds. For example you may assume that a racing car should be shot at the fastest possible speed, but if that causes the wheels to be frozen too, the car will appear not to be moving. If you want to experiment with the effects of different speeds, find a fountain or fast-moving stream and shoot it at all different speeds. From swirling milky streaks to sparkling droplets, the effects are very different. Shutter speed also has an impact on flash photography. Your camera will have a maximum sync speed, above which the flash cannot be used. On most digital SLRs this is a respectably fast speed above 1/125 sec. On many (specifically Canon) budget film SLRs it was/is much lower, around 1/90 sec, though some flash units can get around this limitation.

Aperture
The Aperture setting controls the amount of light entering the lens in a different way to shutter speed, though the two are related. The aperture is the name for the hole or gap through which light passes on its way to the image plane. The diameter of this hole can be varied using either a ring around the lens barrel, or a setting on the camera body. For all Canon EOS SLRs and lenses and many of the modern budget nixon lenses, this is a camera body setting, if you're on an older camera, especially anything manual focus, the aperture tends to be set from the lens. Settings range on normal lenses from about 1.4 to 32, in numbers which increase by multiplying by approximately root2, thus the range goes 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, 22, 32. These numbers are often referred to as 'f-numbers' or 'f-stops' and written as 'f/5.6'. This is because they are expressed as a fraction of the focal length of the lens, and thus the same number can mean the same exposure setting regardless of the lens length. F/32 is thus much smaller than F/2.0, one sixteenth the diameter in fact. The gap between each number in the range is known as a 'stop' and setting your lens to a smaller aperture is known as 'stopping down'. One stop of aperture corresponds to one doubling or halving of shutter speed, an arrangement known as the reciprocity law. If your camera suggests, for example 1/250sec at an aperture of f/11, you could equally shoot with settings of 1/60 @ f/22 or 1/1000 @ f/5.6, and obtain the same exposure, albeit with different effects. We'll come on to aperture and shutter speed related effects in a future entry. For now I'm just concentrating on exposure and as such we can leave aperture there.

ISO (or 'film') speed
This is controlled on film cameras by physically changing the film, or sometimes changing how we choose to treat film. As such it is a 'whole roll' decision that must be lived with for up to 40 frames if we don't wish to waste film. On digital cameras it tends to be much easier, and can be set differently for each and every shot if we so wish. It is a measure of the sensitivity of the recording medium, be that film or sensor. Usually available in doubling hundreds from 100 to typically 800 or 1600, it can be treated as simply another exposure variable, allowing you greater flexibility with your shots and the conditions in which you shoot them, it does have tradeoffs however, which are perhaps not so obvious as those related to shutter speed. Suppose you are set at 1/100sec @ f/4, your lens aperture as wide open as it goes, and your ISO on 400. The light levels drop and you need just one extra stop to expose correctly. Aperture is at maximum so that leaves just shutter speed or ISO. If you reduce the shutter speed (increase the time for which the image plane is exposed) then you risk not being able to hold the camera still. If you take the other option and push the ISO up to 800, your image quality may well be compromised by increased image 'grain' or 'noise' (extra patches or blotches of colour or light where you don't want them) which is a common side effect of higher ISOs. Going one stop higher to 1600 will seriously affect your image usability on all but the highest quality SLRs. It's there to be used, but bear in mind that speeds typically above about 400 ISO will adversely affect your images in some way. One quick addendum for any pedants in the building - the technically correct definition of ISO includes both the European DIN specification and the more familiar numeric American standards. The 200 or 400 or whatever is technically called the ASA rating, but in most publications etc, ISO and ASA are used interchangeably,

This has become something of a monster post, so I'll leave it there for now. Once you've digested the settings, I'll try my best to explain how and why and most importantly when to use them.