Tuesday, 3 April 2012

On Exposure Part I: The Photographic (or Exposure) Triangle

This first part of a four part series dealing with the fundamental principles of understanding exposure is both an introduction to the concept of exposure and to the foundation for understanding it - the photographic triangle. It also deals with the first element of that triangle - ISO.

The series is intended to set out, for beginners and in an understandable and accessible form, the essence of what I’ve learned as I have turned from the smaller cameras I normally take into the mountains to a digital SLR camera system. Of course, the principles are applicable to more than just the modern DSLR but that is what I am currently using so that will be my frame of reference. Note: I always use the Manual (M) setting on my DLSR which means each element of the Photographic (or Exposure) Triangle is set by me (rather than by the camera) for each and every exposure.

Exposure is the fulcrum of photography. As a term, it describes a number of linked concepts. At its most basic, it is used interchangeably with ‘image’ or ‘photograph’. Photographers refer to ‘making an exposure’. In reality, however, the most important use of the term exposure is to define the amount of light allowed to fall on film or a digital sensor.

Three main elements combine when determining the exposure of a particular photograph - ISO, Shutter Speed and Aperture. Taken together, they are elements of the Photographic (or Exposure) Triangle.

In crude summary, each setting controls exposure differently: Aperture controls the amount of light permitted to fall on the sensor; Shutter speed controls the amount of time the light is permitted to fall on the sensor and ISO is a measure of the sensitivity of your camera's sensor to a given amount of light.

Each of these three elements is affected by the choices made by the other two. In low-light conditions, for example, allowing as much as light as possible to fall on the sensor (aperture), for as long as possible (shutter speed), and making the sensor as sensitive to that light as possible (ISO), seems logical. Yet there are tradeoffs and it is not as simple as it might first seem from that statement.

Higher ISO settings can lead to grainy images - once used creatively in film photography, it is not quite so effective when it comes to digital sensors. A slower shutter speed, that is to say, having the shutter open for longer, could lead to blurring of the whole image if the camera moves at all during that time. Selecting a fully open aperture, to allow as much light in as possible, drastically reduces depth of field (the area in the image which is in focus and, hopefully, sharp) which means not everything in your exposure will be sharply focused. Any or all of these effects may be desired by you for creative reasons. Some, may not be.

By way of example (and don't worry if this is a bit technical right now, it will become clearer later and in subsequent posts) the image below, handheld and indoors, was at ISO 500, aperture f/4, shutter speed 1/15, focal length 50mm. It shows how aperture can isolate your subject (by blurring foreground and background) and how, even at ISO 500, images can look good. As it was handheld, and shutter speed was 1/15, it is not pin sharp but pretty close.

Understanding the Photographic Triangle means understanding why it is also called the ‘Exposure’ Triangle. Each element has a crucial role in determining whether you have captured a perfect exposure once the shutter is released. It is not all about light or dark either. A DLSR has a light meter which can be seen through the viewfinder. On the manual setting, that light meter will indicate whether the exposure is ‘under exposed’ (-) or ‘over exposed’ (+). It does this 
by virtue of a tracking spot which moves along the line towards the + and - sides as you alter whatever the camera is pointed at, and/or when you change aperture, ISO and shutter speed. Your aim, at the moment, is to ensure that your selection of the settings for the elements from the Exposure Triangle means that the tracking spot is bang on the middle (or 'perfect'), or within 1/3 or even 2/3 of a stop, of ‘perfect’. You might think that the image above is 1/3 underexposed. I probably agree with you.

All three elements, when used in concert, will create for you exposures which capture the emotion of the scene which moved you pull your camera out in the first place. However, in order to capture that scene, you need to understand what made you want to capture it in the first place and how each setting impacts on the final exposure. Changing one setting will require changes in at least one of the other settings to achieve a perfect exposure but each setting, on its own, has a creative influence.

For instance, a narrow aperture opening will mean much greater depth of field. A wider aperture opening will mean shallow depth of field. If you are intending on highlighting a particular subject by blurring the background beyond it, then an aperture setting of f/4 or lower would normally be required. This could potentially mean a high shutter speed. If you want everything to be sharp in a landscape image, then f/11 or higher might be required. This will necessitate a lower shutter speed which may preclude handholding. If you want the wispy, ghostly effect created by silky smooth waterfall images, a very low shutter speed is required. This will mean a consequential impact on aperture and depth of field. 

For the image below, again, I wanted to blur out the background and only have my son sharply in focus - so low aperture. I needed a high shutter speed to freeze the action as he was on a swing. I kept ISO at 500 even though there was plenty of light to get that high shutter speed. Again, this exposure was handheld. ISO 500, aperture f/4, focal length 96mm, shutter speed was 1/2500.

Aperture and Shutter Speed are discussed in more detail later, but it may be that you need to go outside the triangle to get the effect you want by deploying filters, for example.

ISO was first used in film photography and is an indication of how sensitive the particular film concerned was to light. It is measured through a numerical value and the lower the value the less sensitive the film and, importantly, the finer the grain in the exposures.

In digital photography, ISO reflects the sensitivity of the camera’s sensor and, again, the lower the value the less sensitive the sensor is (at that setting) and, importantly, the finer the grain in the exposures.

The baseline for exposures with very little grain, or ‘noise’, is 100 ISO. My Canon 60D does not have a setting below 100 ISO, but some high-end DSLR cameras do. A higher ISO, which makes the sensor more sensitive to light, will result in dramatically more grain or noise. As a result, you will usually only increase ISO from 100 if your desired aperture and shutter speed aren't otherwise obtainable. That said, a DLSR sensor, in contrast to that of a compact camera, will respond far more positively to that problem and even at ISO 500 or more, there will be little in the way of perceivable noise.

As I have said, your choice of ISO will impact on the aperture and shutter speed required for a perfect exposure. Increasing your ISO from 100 to 400 or 500 would mean a perfect exposure would require a faster shutter speed and narrower aperture. This might make handholding possible in low-light, for example. So, how do you choose ISO?

You must, ultimately, assess the light - do you need a high shutter speed for your exposure? If so, set the ISO as high as you are able so long as there is not too much noise. Indoor exposures will often require a higher ISO. Are you using a tripod? Shutter speed then becomes less of an issue so a lower ISO is possible. Is your subject stationary or moving? Are you attempting to achieve a particular effect as a consequence of high or low shutter speeds such as panning (where the subject is moving, but is kept sharp by moving the camera with it - this has the effect of blurring the background into movement lines and implying motion by the subject).

Next: Aperture.

The two books I recommend to learn more about Exposure are listed below. These are Amazon Associate affiliate links which means if you buy them here, I get some income from that purchase. I own both these books and endorse them completely. Peterson's book is a legendary starting point for beginner DSLR users. Freeman's book is hugely technical but tremendously detailed.


  1. Thanks for this post. As the new owner of a panasonic GF2 this has helped lessen the grade of the learning curve.   

  2. My pleasure - there are posts on Aperture, Shutter Speed and Achieving Sharpness Throughout Depth of Field to come over the nest few weeks. Feel free to contact me through my Contact Form on my professional website (see About Me above).

  3. good overview Maz, enjoyed reading this, it reminded me to think of the relationships (triangle) more formally again, which can be helpful.  Your 60d copes well at iso500, I try never to take my 4/3's sensor above iso200, especially in low light, it gets grainy fast.

    Btw, I think the first shot is a peach, and not 'under' at all ;)

  4. Thanks David. I was genuinely astonished, as my brother in law and I were comparing my Canon with his equivalent Nikon, when he started taking shots indoors, under poor light, and showed me exposures that were pin sharp and perfect. That's when ISO made sense to me and that's when I saw the divide between DSLR's and other types of camera. ISO 500 is the limit I set but I just know I could go higher with the 60D and still not get a great deal of noise.

  5. I have been using a cheap, old point & hope on the hill which continues to disappoint me. I makes mountains look like actual mole hills.

    I think a DSLR will be a bit much for me at the moment, not only learning how to use it but size/weight. I'm looking to get a Canon Powershot S100, I believe it comes under the heading "advanced compact". It has a good set of features with a good "stick it in the pocket" size.

    I expected your post to go over my head at this stage but it was well broken down and I didn't have to duck as it went past me. I learned something and look forward to the next instalments.

  6. I found that for the TMB and the CHR, the Lumix TZ10 was great for making straightforward images where the mountain environment created its own drama. I had little to do with it apart from getting composition right and being there (which is the hard part as it was sometimes nearly 4,000m up). For some instances, having a DSLR would have allowed me to capture some of the drama of the smaller, less obvious but equally significant parts of the treks and told a slightly different story through them. The Lumix could not do that for me as I had not learned how to use its (albeit limited) manual settings. An advanced compact, once you can experiment with aperture, shutter speed and, hopefully, ISO, will give you more freedom, certainly.

    I hope you enjoy the next part, posted today - now it gets really interesting!