Aperture

The ABC of Aperture, Bokeh and Composition.

By Bob Newman, first published September 2012

The aperture control, along with the shutter speed, is one of the main means the experienced photographer uses to gain a desired effect. The main effect controlled by the aperture control is that of depth of field, and when using shallow depth of field, the quality of the resultant out-of-focus areas, which has become known as ‘bokeh’. To many bokeh has become such an important property that lenses are prized for that quality. In this article we look at how the aperture mechanism works and how that affects the way a lens renders out of focus objects.

The effects of aperture

Often, the aperture control’s principle use is that of controlling ‘exposure’, by which is often (wrongly) meant the brightness of darkness of an image. However, the aperture also directly controls the depth of field in an image, and this it is this effect which will often be of more importance to the advanced photographer. To see how the aperture controls the depth of field, consider the situation shown in Figure 1.

Figure 1. Depth of field

A lens is focused on a particular point and all the rays from that point passing through the aperture of the lens will be focused to a single point on the sensor (assuming a perfect lens). The rays describe a cone of light based on the aperture with its apex on the focus point. Now lets consider what happens to rays coming from a point ‘A’ half way to the focus point. Rays from this point travel to the aperture, but instead of being focused on a point on the sensor converge somewhere behind. Because the rays travel in straight lines, the shape of cone that the sensor intercepts corresponds to the shape of the image, and its size corresponds to the image of an object the same size as the light cone in front of the lens at point A. Let’s put some numbers on this. Suppose that the lens is a 50mm f/1.4 focussed on 1m. The aperture diameter is 50mm divided by 1.4, which is 35.7mm. The distance of A from the lens is 0.5m, and by simple geometry we can determine that the size of the light cone at 0.5m is half of 35.7mm or 17.9mm. Thus, a point source at a will be rendered as though it were an aperture shaped disc 17.9mm in diameter. Similarly, if we consider a point source at B, 0.5m behind the point of focus, the ray cone is also 17.9mm in diameter, so there also the point source will be rendered 17.9 mm in diameter. At A or B, the smallest object that can be clearly imaged is the size of that disc, or 17.9 mm. From this geometry, we can quickly determine the depth of field for a given minimum feature size that is required to be rendered clearly in the final image. For instance, if we decide that we want to see things 1mm in diameter, then the depth of field extends between the points in front and behind the point of focus where the light cone is 1mm in diameter, which can be quickly calculated to be 1/35.7 of a metre (27mm) either side of the point of focus. At this point, for depth of field theorists, I should note that conventional calculations get a different result because they consider minimum size images (the ‘circle of confusion’) on the sensor, rather than minimum size objects in front of the lens.

The image that we see of objects at any given distance from the lens is the superposition of all the disc images from every point on the object, resulting in the kind of blurred rendering that you would get if you tried to paint a picture with a very broad brush.

Controlling the aperture

The size of the aperture is controlled by a mechanism in the lens called an ‘iris’ or ‘diaphragm’. As we have seen above, the shape of the aperture determines the way out of focus points are rendered. The best shape for most purposes is circular, so diaphragm mechanisms are designed to produce as nearly a circular shape opening as possible. This is done by constructing them of overlapping pivoted metal blades, as shown in Figure 2.

Figure 2: Iris

As the blades are swung round their pivot the size of the opening in the middle changes. The more blades there are, the more nearly circular the opening is. However, building an iris mechanism with many blades is expensive so cheaper lenses will tend to use fewer blades. The range is anywhere between five, which produces a pentagonal opening, to eleven, which produces a nearly circular opening. The circularity of the aperture can also improved by curving the blades, which does however make the diaphragm mechanism larger for the same maximum opening. Nowadays many lens manufacturers will say in their specifications how many blades the diaphragm uses (more being assumed to be better), but it can be checked simply by looking through the front of the lens.

Bokeh

Bokeh, originally derived from the Japanese word boke meaning ‘blur’ or ‘haze’ is a relatively newly minted photographic term referring not to the quantity, but to the quality of the rendering of the out of focus parts of an image. Thus some lenses are said to have ‘good’ bokeh, meaning that they render out of focus parts of the image pleasantly or ‘bad’ bokeh, meaning that they render them unpleasantly. From the discussion above we can see that what is actually rendered for a point on an out of focus object is a image of the aperture diaphragm, so the bokeh is fundamentally determined by what that looks like, both in terms of its shape and the distribution of light within it. The general rule with respect to shape is that the more circular the better. With regard to light distribution (which is determined by the optical design of the lens) the rule is that soft edges are better than hard, and that darker central regions are worst of all. One way to look at the shape of the blur circle is simply to take a photograph of an out of focus point source (or the nearest approximation that can be managed) – this will give an impression of both the shape and light distribution – remembering that both can change as the aperture changes.

Bokeh and composition

The use of shallow depth of field is a very distinctive style in photography. Like many things, it can be over used and become clichéd, but can also produce very beautiful images. If it is a style that a photographer wishes to adopt, it is likely that the lens kit will be shaped to cater for this, which usually will mean an array of large aperture lenses. From the discussion here, it can be seen that it is the size of the aperture and the subject distance that determines the amount of blur in relation to the size of the objects being photographed. Lenses are specified in terms or relative aperture or f-number, so the required f-number for a required amount of blur at a given distance will depend on focal length (and thus indirectly) sensor format. Apart from the aperture size it is worthwhile considering the bokeh, and if you do not have real world images with which to make an assessment, then the blur circle test above, or simply looking at the shape of the diaphragm, can allow you to make a good estimate of the likely quality of the out-of-focus rendering.