While ISO certainly has an impact on a perfect exposure, for what Bryan Peterson called, is his book Understanding Exposure, a ‘creatively correct exposure’ aperture and shutter speed are the more important corners of the Exposure Triangle. A 'creatively correct exposure' is, in short, a 'perfect' exposure which follows a particular creative avenue chosen by you to inspire, in your viewer, a desired emotional reaction. You are attempting to recreate the drama which inspired you to make the exposure in the first place.
You will recall from my previous post - On Exposure Part I - that a 'perfect' exposure is one where the shutter speed and aperture, along with ISO, combine to make an exposure where the light meter sits on '0' or is 1/3 to 2/3 of a stop under (-) or over (+) exposed. An exposure made as a consequence of one 'perfect' aperture/shutter speed combination will be very different from another, also 'perfect', aperture/shutter speed combination. Which combination you select will depend on what feeling you are trying to inspire in the viewer and what type of image you're trying to create. In order to understand how to select an aperture setting, you need to understand what aperture is and what effects it creates.
What is Aperture?
Aperture values, expressed as ‘f-stops’, relate to the diameter of the opening through which light hits the camera’s sensor. The lower the numerical value, the wider the opening and vise versa. And, as we saw in Part I, the wider the opening, the more light can fall on the sensor.
Traditionally, aperture values or f-stops, are as follows, which reflect the diameter of the hole halving each time:
f/1.0, f/1.4, f/2.0, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 and f/32.
That said, most DSLR cameras have aperture settings in between these - my Canon 60D for example has among others F/9, f/10 and f/13, all of which can be quite useful. The lower and higher ends of the aperture scale are a function of the lens you have selected subject to your DSLR itself. High quality ‘fast’ lenses (those with f/2.8 or lower) are usually very expensive.
Depth of Field
The principal creative reason for changing aperture is to influence depth of field. A lower f-stop value (a wider opening) generates a shallow depth of field. The subject in focus will be sharp, but the background will be blurred. In fact, and more specifically, on my Canon 24-105 f/4 L-series lens, f/4 produces a very shallow depth of field indeed. In an image of my partner holding my son, at 100mm focal length, my son is in perfect focus but even my partner, inches away from him, is slightly blurred.
Which leads me onto the other point about utilising a low f-stop in order to create what Peterson terms so vividly an ‘isolation’ shot - the longer the focal length selected (the higher the level of zoom used), the more blurred the background will be. Use a 300mm lens and zoom in to the fullest extent and the background beyond your subject may in fact be so indistinct as to be almost a single texture with just some limited separation in colour.
For a variety of different 'textures', take a look at this exposure I recently took for a magazine feature on wild camping (f/5.6, 65mm) - the subject was the stove, but I wanted the far background very blurred but the near foreground and background also blurred but not to the same extent as I wanted to show the presence of the grass but not focus on it:
A higher f-stop value (also known as ‘stopping down’) produces significant depth of field. Not only will the subject be in focus but the foreground and background as well. This sort of aperture setting is commonly used in landscape shots. There are issues about retaining perfect sharpness throughout an exposure which revolve around calculating hyperfocal distance and where in the scene you should focus but I’ll deal with those in the next post on Depth of Field and Hyperfocal Distance Focusing. For the exposure below I used f/22, 17mm (and a 3 stop ND Grad filter - but more on that in another post) and focused on the second row of trees to the left of the exposure.
As we know, a high f-stop value, or narrow opening, requires a lower shutter speed to achieve a perfect exposure. Given many landscape shots are best taken, for the ‘creatively correct exposure’, in low-light conditions such as dawn or dusk, almost all landscape photographers need to use a tripod and remote shutter release.
A note of caution on high f-stop values - whilst f/16 and f/22 may produce prodigious depth of field, the exposures they generate also suffer, to varying degrees, from diffraction. This effect, a consequence of light being forced through a narrow opening, means the sharpness of the image begins to become compromised. The optimum sharpness on most lenses is two or three stops down (i.e. higher in numerical value) from the lowest aperture value on the lens. Usually, this is f/8. Because of this, many landscape photographers will select f/8 or f/11, or possibly f/13, for their landscape exposures. Whether diffraction is an issue for you will depend on how sharp you want your exposures to be. However, as long as you don't want subject in the immediate foreground to be sharply in focus, say within 3-4m of you, then f/8 or f/9 will have sufficient depth of field and produce sharp exposures. I'll tell you why this is in the next post.
For example for this exposure at RHS Wisley I wanted everything in focus but the foreground interest was 6m away. f/8 was fine at 29mm.
And for example, for this exposure, I wanted the tree branches to be sharp as well as the sky. The branches were around 8m away from me, so f/9 was perfect.
So, making exposures of the same scene, with different aperture settings, produces wildly different images.
Next: Depth of Field and Hyperfocal Distance Focusing
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.