By Joseph Grosjean
Photographer and Photograph Editor
Last issue we discussed the pinhole camera and introduced the importance of a lens. This issue we will explore how a full-sized lens works, what an aperture is, and what effects can be created once the aperture has been mastered.
A modern lens is composed of around a dozen individual curved pieces of glass called “lens elements,” which focus light. The elements are also able to move forwards and backwards to adjust the focal length (amount of zoom) of the lens. Towards the rear of the lens is the aperture, a variable diameter hole that light passes through. By varying the size of the hole, photographers can control the amount of light that reaches the film or image sensor.
There are some additional components to a lens, including the servo motor that controls the auto-focus feature; switches to select autofocus or manual focus (AF and MF, respectively); and manually setting, on and off, the vibration reduction (VR for Nikon, IS for Canon), if the lens has this feature.
The aperture is typically controlled using wheels on the camera body. The way that aperture is quantified is a bit strange. Smaller f-stop numbers mean a larger opening, and larger f-stop numbers mean a smaller opening. Typical values for consumer grade lenses are between f3.5 (full open) and f32 (full closed), with high quality prime lenses coming in at f1.8.
There are two styles of lenses, variable aperture and fixed aperture zoom lenses. Most consumer grade lenses are variable aperture, so when you increase the focal length the image gets darker. This is due to the longer distance that light must travel to your film or image sensor. The camera manufacturers make it easy to quantify this change by displaying the effective aperture value, even though the aperture itself is not actually closing. This is done to make the lenses lighter, less complex, and cheaper to manufacture.
A fixed aperture zoom lens does not have this same issue. As such, the image does not darken when the focal length is increased due to the lens itself not changing in length. This does add significantly to the mechanical complexity, the weight, and the cost of the lens. So, do you need a fixed aperture lens? Probably not unless you are a professional photographer. Most people, including myself, can live without that extra light as there are other ways to compensate for this light loss.
As discussed in last issue, a smaller hole (pinhole or a small aperture, i.e. f32) will restrict the amount of light that reaches the film or image sensor but filter the light so that only the most focused light may pass. This creates an infinite plane of focus (everything focuses, foreground to background). The inverse of this would be a larger hole that allows a lot of light through and has no focusing effect beyond the lens elements, so the plane of focus can be very limited, especially if the lens is of a high focal length and close to the subject. This creates a depth of field effect where only a plane of the scene is in focus.