Wednesday, 17 October 2012

Mastering the Exposure Triangle

I am going to apologize for the length of this BLOG entry up front... this is a longer, fairly detailed explaination of what you need to think about while mastering the exposure triangle. But I would bookmark it and refer back to it as you do some real world implimentation of the techniques I discuss.


“Exposure” – is what photography is all about.

Photo courtesy of "Exposure Guide"... a great photography resource for you

Photography means writing with light in Greek, and exposure is the combination of three factors that determine what the light writes… this is called the Exposure Triangle. Those three elements are: ISO, Aperture and Shutter Speed.

The exposure triangle or photographic triangle refers to the inter-connected relationship of the 3 elements that make up an exposure or image. Aperture can be explained as the width of the lens opening which lets light into the camera, the wider this opening the more quickly light is allowed in to the camera and onto its sensor (or film). Shutter Speed is the amount of time which the shutter remains open to let light onto your sensor, it is measured in time ranging from 30 seconds or more to 1/8000th of a second or less. ISO is your sensors (or films) sensitivity to light. It determines how quickly your sensor can gather light.

The alchemical combination of these three elements then results in a given subject’s exposure value (EV). What is important to remember is that any change in any one of these elements will an effect on the other and consequently impact the final image (i.e. by changing the Aperture, you change depth of field; by changing ISO rating, you change the amount of light required to obtain an image, and by changing the Shutter Speed, you effect how motion is captured). Such that you will never be able to independently control a given element, because you have to take into account how the other two elements will interact for the final exposure.

Fortunately, the mathematics of photography happens to work in such a way that each element in the Exposure Triangle has a relative “stop of light” value. Such that if you increase the light by one stop by reducing the Shutter Speed, you can regain the original Exposure Value by either decreasing the Aperture by the same stop value and/or adjusting the ISO rating accordingly.

Here’s a real world example; I’m at the beach with the kids and the sun is going down, and I want to get a shot of their smiling faces. I take out my camera, do a quick meter reading with the shutter set 1/60th and get a EV on her face of f/4. I set the aperture to f/4 and take my photo. I look at the image on the display screen, and while I love the way the colored light dances on her face, I don’t like the depth of field – I can see too much of the background (the other people, etc.). I want as shallow a DOF as possible, which means I need to increase my aperture setting. I open the lens wide open, to f/1.8. This is a 3-stop difference, which lets in 8x as much light as I had with f/4. To compensate, to get back to the same EV that gave me such a pleasing image, I would need to increase the shutter speed by 3-stops – so I crank it up to 1/500th. I quickly take the picture again (that sun is going down)… and viola! I have my photo with the EV that gives me that amazing quality of light AND with the shallow DOF so you can’t make out what’s behind my girlfriend.

Shutter Speed is measured in fractions of a second and it determines how fast the shutter opens and closes, thereby controlling the key element in photography – light; specifically the time-frame in which light registers on the image sensor . The Shutter Speed captures the world in split seconds, but it can also be slowed down to a few seconds (or remain open longer at the photographer’s discretion). This enables all sorts of possibilities in determining what is actually recorded to the image sensor.

Aperture is the opening in the lens that determines the amount of focused light that reaches the image sensor. It’s measured in f-stops. The beauty of the f/stop arithmetic is that regardless of a lens’ focal length, the f/stop measures the same amount of light; such that f/4 on a 50mm lets in the same amount of light as f/4 on a 120mm. The opening’s diameter may differ, but the amount of light is the same because the length of the lens is different.

So what is correct exposure? That’s mainly subjective, but we can agree that it is when the camera effectively reproduces a subject on the image sensor where the most uniform amount of picture information is visible in the highlights, midtones and shadows. How do you determine the specific exposure you want? All dSLRs have an EV meter in the viewfinder that provides an EV on the subject that you are metering.

An effective way of ensuring a correct exposure is to employ Exposure Bracketing. This is a technique in which you’ll be taking from 3 exposures to 7 exposures– one at the designated exposure value (EV), and from one to three images 1/3 of an f/stop above, and from one to three images at 1/3 of an f/stop below.

On some robust cameras, you set the ISO, f-stop and shutter to acquire an exposure value (provided by the TTL meter), and press the shutter release. The camera will automatically shoot the upper and lower bracketed exposure. When you review the bracketed exposures, you’ll be able to see subtle, but key differences in the images – most specifically if there is any over- or underexposure. Professionals bracket all the time to make sure they get the best possible negative (film or digital neg) for later.

What exactly is under- and overexposure? It’s when there is excessive loss of image information within the highlights and shadows. There is typically no way of “finding” that lost image information with digital photography in particular (i.e. when the subject emits so much light that the image sensor is overwhelmed, it records that section of the image as zero; and the same thing is true when the subject emits so little light that image sensor believes there is nothing there). No matter how much tweaking you may try with a post processing software in the digital darkroom, there’s no recorded information to be discovered.

So how do you avoid under- or overexposing your pictures, before you master the art and craft of photography? You can use the Automatic Exposure Lock or AE Lock that’s available on most dSLRs. AE-Lock is a feature that, when you have the camera set to one of the automatic modes (i.e. Shutter-priority, Aperture-priority), it enables you to lock the EV and take continuous photos without have to resample the lighting in a given scene.

The Key to mastering the Triangle

The key to mastering the triangle is to understand what effect each element has on your pictures and to be able to prioritise what’s important for that particular image.

Most of my work for example is landscapes and for me picture quality is the most important thing because I sometimes make large prints to sell. Therefore I very rarely increase the ISO on my camera above 100. The only times I do so are when I wish to take a shot at a certain shutter speed (perhaps to capture the movement of water in a certain way or to ensure I freeze grass being blown about in strong winds.) and am unable to increase my aperture because it would result in a loss of depth of field.

Being a landscape photographer means most of my images are shot at apertures of f/8-f/16 to try and maximise depth of field. I do this by focusing one third up from the bottom of the image. When you look at my priorities when making images you can see that aperture is my first concern, followed by keeping as low an ISO as possible. It is only when moving elements are contained within a scene that I give much thought to shutter speed so this is the setting that is altered most freely when trying to create correct exposures.

A Wildlife photographer would no doubt replace my priority of aperture to one of shutter speed. This is because they need to be able to freeze moving subjects which requires the use of fast shutter speeds. Aperture will still have to be considered to ensure that enough of the subject is still in focus. ISO will therefore be the setting that is most free to alter in order to obtain a correct exposure. This is why pro sports and wildlife photographers really appreciate high ISO performance because it enables them to use faster shutter speeds in poor light.

If you are able to determine what your priorities are when capturing a shot then you will be able to make informed choices about which settings to use and which to alter to enable you to expose your photos correctly.

The wonderful thing about digital photography is that you can continue to experiment at no cost to you as you learn and master the three elements of the Exposure Triangle, going from semi-automatic to full manual. It takes a certain amount of practice and storing a great deal of information in your head… but you can master this.

I hope this helps you better master the exposure triangle,

Kev