A properly exposed photograph is achieved by balancing shutter speed, aperture and ISO. An underexposed photo is too dark, an overexposed photo is too bright, while a properly exposed photo is close to how your eye would perceive the image in real life. Most modern cameras are excellent at determining exposure settings for you automatically, so don't be afraid to leave your camera in full auto mode while getting to grips with things. The more you learn about camera settings, you more you will understand why your cameras does what it does in auto mode. The more you shoot, you more you learn. We will now look at Shutter Priority (TV Mode), Aperture Priority (AV Mode), and ISO in a little more depth.
Shutter Priority (TV Mode)
Shutter priority controls the shutter speed of your camera. Shutter speed is the time it takes for the camera's shutter to open and close when you click the button. The longer your shutter is open, the more light gets in. The shorter time your shutter is open, the less light gets in. This, of course, affects the exposure. Too little light and the photo will be dark and blocked up with shadows. Too much light, and the photo will be blown out.
Another important consideration with shutter priority is that the longer the shutter is open, the more blur you will have in your photos because of camera movement. This is known as motion blur. If you are hand holding your camera and not using a tripod, it is therefore important to use a shutter speed that is fast enough to prevent your photos being blurry.
If you use full auto mode your camera will decide what the shutter speed should be. Cameras are so good these days at determining exposure that leaving your camera in Auto while you learn should produce sufficient pleasing photos to keep you happy.
In shutter priority, high numbers mean faster shutter speeds. 1/2000 for example = one/ two thousandth of a second. That’s very fast, and is good for freezing sports action. 1/4 = 1 quarter of a second, which is far too slow for hand held shots.
What should the slowest shutter speed be then when hand holding a camera? Memorise this formula - 1.5 x focal length. This is the slowest speed you should consider for hand held photography, and it is the only formula I'm going to ask you to memorise. Focal length is the reach of your lens. If you are using a 50mm lens, your focal length is 50mm, therefore the slowest shutter speed to prevent motion blur should be 50 x 1.5 = 75, or 1/75 of a second. As there is no 1/75 of a second setting go up to the next setting which is 1/125. If you are using a 55-200mm zoom lens and you are zoomed all the way to 200mm, the slowest shutter speed should be 200 x 1.5 = 300, or 1/300. As 1/250 is slower, go up to the next setting which is 1/500th of a second. If you are zoomed right out at 55mm then 55 x 1.5 = 82.5, so again 1/125 would be the slowest recommended shutter speed.
If you learn this formula, you will always have a handy way of determining the slowest shutter speed you can safely get away with. This will become especially important later when you start shooting in low light conditions.
As motion blur can spoil your photos, learning how to hold your camera will help you to take better photos. If you take a photo while swinging your camera around, everything will be blurred because of the motion of the camera. With one hand, hold the camera steady with your finger on the shutter button. With your other hand, cradle the camera as shown in the first picture below and lock your fingers together. Finally, using your elbows as a makeshift tripod, lock them firmly into your side. Another excellent method is to find somewhere to rest your elbows to help you steady the camera.
Of course, there will be times you will want to have slow shutter speeds, for example with night photography, or producing dreamy effects with running water, and we will get to that later. For now, just let this information sink in, memorise that formula, and keep an eye on your camera to see what shutter speeds it is selecting for each photo.
Aperture Priority (AV Mode)
Aperture priority also affects exposure, but in a totally different way to shutter speed. Where the shutter controls light by how long or how short a time the aperture is open, you can also adjust the size of the aperture which again affects the amount of light that hits the sensor. The larger the aperture, the more light is allowed into the camera, the smaller the aperture, the less light is allowed into the camera. As you control the size of the aperture, you control how much light hits the sensor. Balancing shutter speed and aperture together is how you control exposure.
Depth of Field
Just as changing the shutter speed will affect a photo by either freezing fast action or by introducing blur, so too changing the aperture will affect a photo but in a totally different way by changing the depth of field. This can all seem a little bewildering to begin with but don't worry, there is nothing difficult about it so track with me. In the photo of the seal pup below I wanted a shallow depth of field. In other words I wanted the foreground and the background to be out of focus and all the focus to be on the pup. To achieve it, I opened the aperture to the widest aperture of my telephoto lens when fully zoomed in at the long end. As you open up the aperture, the depth of field becomes shallower, meaning more of the foreground and background goes out of focus.
In the next photo I wanted the background more in focus to give an idea of the pup's environment. To do that I used a wide angle lens and stopped down the aperture to make it smaller. As you open your aperture towards your lenses fastest settings, which usually varies between f4 and f2 or even faster depending on the quality of your glass, the depth of field becomes shallower. The shallower your depth of field the more of your background and foreground goes out of focus. As you stop down the aperture, the depth of field increases, meaning more of the foreground and background comes into focus.
Bokeh comes from the Japanese boke (pronounced boh-kay or boh-ka) which means blur or haze, and refers to the aesthetic properties of the soft out of focus areas in a photo. Good bokeh is very pleasing to the eye. It is used when you want to separate your subject from distracting backgrounds. Bokeh is desirable in portraiture when you would use a shallow depth of field so the subject stands out. If you take a photo at f2.8, the depth of field will be much shallower than if you take the photo at f/8, when more of the foreground and background would be in focus. The beautiful bokeh behind this spider's web is sunlight sparkling on the surface of a river, and I made it so creamy by opening the aperture as wide as possible.
As well as opening your aperture, there are other ways to achieve good bokeh. One thing you can do is find a position where the background is as far away as possible from your subject. If will be difficult to achieve good bokeh if the background is close to your subject. Another thing that affects bokeh is how close you are to your subject. The closer you move to your subject while keeping them in focus, the more the background will go out of focus. Telephoto lenses can be excellent tools in this regards for producing soft dreamy bokeh.The best lenses for achieving pleasing bokeh tend to be expensive glass with apertures of f/2.8 or faster, but don’t let that put you off learning how to get the best bokeh you can from your existing glass. Get the depth of field right along with good separation between subject and background, and you can achieve pleasing bokeh with any lenses.
ISO and Noise
ISO stands for International Standards Organization, the body that sets the sensitivity ratings for camera sensors. Before digital cameras, ISO was an indication of how sensitive a film was to light. In digital cameras it is an indication of how sensitive your camera's sensor is to light. As you increase the ISO, you increase the sensor's sensitivity to light. As the sensor becomes more sensitive to light it allows you to use faster shutter speeds and still get good exposures in low light. However, there is a trade off. It's a bit like turning up the volume on a hifi system, go too far and you get noise distortion. The higher you bump up the ISO, the more noise will be introduced to your image. Noise?
Noise is nasty horrible ugly stuff and you should always avoid introducing noise into your images by shooting at low ISO settings. Only increase ISO if there is no other way to get enough light for a good exposure. When there isn't enough light to set a shutter speed that is sufficiently high to keep your photos sharp and crisp, and your camera's aperture is wide open, then you must turn up the ISO. The good news is that the very latest cameras have excellent sensors that can control noise well even at high ISO settings. Compare the two images below which are 100% crops taken from much larger images. The one on the right was taken at ISO 6400 and has lots of noise while the image on the left was taken at ISO 200 and has no noise. The image on the right however has better focus because the high ISO allowed me to use a faster shutter speed. The image on the left looks better, but as I used a low ISO in low light, the shutter speed was too slow to take a crisply focused image.
To conclude this section of the workshop then, it is by balancing shutter speed, aperture, and ISO that you produce properly exposed focused images.
Credits - Lens apertures, Leonrw, Wikipedia. All other photos copyright George Maciver - all rights reserved.