The Exposure Triangle

Did you know that very few professional photographers actually use manual mode all the time on their cameras? When the light is good most of us work in aperture priority and let the camera choose shutter speed and ISO.

In aperture priority we control the depth of field. So if the light is good and the shutter speed the camera selects is fast enough for sharp photos, and I'm not shooting fast moving subjects like sports or birds in flight, shutter speed and ISO are things I can forget about unless the light goes. How I determine the light is simple. Let's say I have my 16-55mm lens on my camera. What is the slowest shutter speed I can select without risking blurry photos? Remember the formula 1.5 x focal length? The longest focal length of my 16-55mm lens is 55mm and 1.5 x 55 is just over 80, or 1/80th of a second. So any pictures I take at 1/125th of a second are going to be sharp without even considering the extra stops of image stabilisation. Once I know that, I take a test exposure in aperture priority. On a decent light day the camera will usually select shutter speeds of 1/500 or higher so what does this tell me? It tells me I can leave the camera in aperture priority and simply forget about the shutter speed and ISO settings.

Usually when I'm out and about I'm more interested in controlling the depth of field than anything else. If I'm photographing seals lying around on the shore, I need to control the depth of field so I can control the bokeh and that's why I select aperture priority. When I open my lens I get the shallowest depth of field my lens will produce, and if the light is good I simply leave the camera to select ISO and shutter speed knowing they're going to be just fine. Once I've selected the right aperture, all I have to do then is make sure the eyes are in focus. If I change the aperture for any reason, the camera will automatically change either the shutter speed or the ISO or both without me having to think about it.

Let's say however, that you want to photograph moving subjects, like children or animals running around. In this situation, depth of field may not be your first priority and all you want are sharp photos. You're outside and the light is good. How can your camera help you and how does the exposure triangle relate to this situation?

In this case, aperture priority is not our best option as a shutter speed of 1/500 of a second is not fast enough to capture a sharp photo of a child running as fast as they can, dogs scampering around, or birds in flight. As depth of field isn't the main consideration, we must go into shutter prority and select a shutter speed fast enough to ensure nicely focused images that are not blurry. When I first go into shutter priority, I take a test shot at 1/2000, which is perfect for freezing fast action. If the camera takes a well exposed image, terrific, but if it struggles I try a shutter speed of 1/1000 and test again. You will also have to put your camera in continuous focus (tracking focus) and concentrate on getting the children or the animals in sharp focus. Half press the shutter button to lock focus and follow them around with the camera keeping them in the centre of your frame. As you do so the camera will automatically track them and keep them in focus.

If you have a burst mode on your camera, keeping the button pressed will take a series of shots for you. Until now DSLRs have been the cameras of choice for rapid action shots, but the latest mirrorless systems are extremely impressive at tracking and keeping focus. As you take your photos, your camera will automatically adjust the aperature and ISO settings for you to ensure you shoot well exposed focused images. The more you practice, the better you will become.

As you shoot in aperture priority you only have to think about depth of field and focus, and your camera will automatically adjust shutter speed and ISO. As you shoot in shutter priority you only have to think about motion blur and focus, and your camera will automatically adjust the aperture and ISO. The more you shoot in these modes the more you will begin to understand the exposure triangle.

What if the light isn't great? Or you are indoors and your camera struggles to select a shutter speed fast enough to keep your photos in focus? What if you open your aperture all the way, but your photos are still blurry? There will be times your camera just will not be able to do a good job in either shutter priority or aperture priority modes, and when that happens you will have to go into full manual mode. If the light is awful and you are in aperture priority, your camera may select shutter speeds that are too slow for the situation and your pictures will be soft and blurry due to camera movement. If you need to manually control shutter speed in such situations it is time to take the plunge and switch to manual. Don't be scared of it though, it's actually really quite easy.

The secret to using manual successfully is simple. Here's how I do it. First I select the depth of field I want, which in low light is usually, but not always as open as possible at the lenses fastest settings. This allows the most possible light into the camera through the aperture and will give me the best background blur, or bokeh. If my subject is stationary, I do my little formula calculation (1.5 x focal length), and then select the shutter speed I know will give me a focused image. I then take a test shot. If it's too dark, I turn up the ISO. If it's still too dark, I turn up the ISO again. I keep doing this until I get a properly exposed photo. This method ensures that I never turn up the ISO more than is absolutely necessary to get a sharp nicely exposed photo that's in focus while keeping the noise to a minimum.

You will usually need to be in manual mode in low light situations such as indoor sports or on very dull and cloudy days when shooting moving subjects. In the photo below I needed a shutter speed of 1/250 to be sure there was no blur (I was using a 24-105mm lens at the long end), and I also needed to control depth of field because if it was too shallow I would have risked only one of the subject's eyes being in focus. To achieve 1/250 shutter speed with f/5.6 I put the camera in manual, took a test shot and then dialled up the ISO until I had a good exposure. This photo was challenging because not only was it heavily overcast, the sun had actually set and the light was absolutely awful. I'd been out in the Cairngorms all day and didn't have any artificial lights and I will not use camera flash on people. To be honest, I did have to use noise reduction software to clean up the image, but more on that later. And that's all there is to the exposure triangle. There, told you it wasn't difficult.

Credits -  All photos copyright George Maciver - all rights reserved.