Objectives
After working through this chapter, you should be able to:
Explain what an ‘optical instrument’ is
Explain how cameras work
Explain how Galilean, Keplerian and reflecting telescopes work
Explain how optical instruments can help people with low vision
Introduction
An optical instrument can include any device or equipment which can alter an image for enhancement or viewing purposes. This includes cameras, binoculars, telescopes and monoculars – some of which are clinically relevant as they’re utilised as low vision aids, which are devices that help people with low acuity to be able to see. This chapter will explain the principles of these devices and then discuss their use in practical settings.
Cameras
I think it’s useful to start this chapter by going over cameras, because we all use cameras on our mobile devices fairly regularly, so we can apply the knowledge that we already have. A camera , then, is a device that focuses light onto a screen, digital sensor or a film for the purposes of viewing the image. Historically the term originates from the use of camera obscuras , which were dark rooms with a pinhole to let the light in; this pinhole projected an image onto a flat wall which could be viewed by the observer (for details on how pinhole cameras work, see Box 13.1 and chapter 18 ). Since then, cameras have developed significantly to include the use of fancy lenses and digital enhancing software.
A pinhole camera (or camera obscura ) is a name for an imaging device that utilises the theory behind pinhole photography. When light emanating from an object shines through a pinhole, it produces an upside-down image. This is because light rays have to travel in straight lines (to a limit, see chapter 10 to review how light can bend around corners when diffraction occurs). This means that light coming off the tip of the object (e.g. the top of the mug in Fig. B13.1 ) will travel in a straight line through the pinhole and end up near the floor, whilst light rays from the base of the object travel in a straight line through the pinhole and end up near the ceiling. When these rays form a focus, it necessarily produces an inverted image.
If we think about it logically, then, in the most basic of forms, the camera needs to be constructed to allow light into the camera itself, without letting too much in that it will affect the quality of the image, so it needs to be a box with a hole in the front of it. In optics, the hole that lets light into an optical instrument is called the aperture stop (or iris ), and importantly, this hole should not affect the field of view (what the instrument can see). If it helps as an example, the human eye can be thought of as an optical instrument and our iris is the aperture stop. When we’re in brightly lit places, our pupils (the hole in our iris) get smaller (to reduce the amount of light entering the eye), but this only helps with the brightness, it doesn’t restrict our field of view at all – and the same is true for aperture stops in cameras. Our basic camera will also need a screen (or equivalent) at the back of the box to project the image onto, so in theory it could just be an enclosed box with a pinhole on the front ( Fig. 13.1 A), or it could be an enclosed box with a slightly larger aperture and a simple lens, to focus the image ( Fig. 13.1 B).
However, as we’ve hinted at already with our example of the human eye, sometimes we need to alter the amount of light that can enter the instrument (e.g. if it’s very dark, maybe we want our camera to let in more light), or we might want to change the focus (e.g. if we want to take an image of something that’s very far away). In this case, we need our camera to be able to alter the size of the aperture stop (depending on our lighting requirements), and we need to be able to zoom, or replace the lenses to those of different focal lengths (f) , which as you probably know is how modern-day cameras work.
The focal length of a camera (usually reported in millimetres (mm)) is determined by where the lens will focus light that enters with zero vergence (from infinity). So, for example, a camera lens with an 18 mm focal length would focus zero vergence light 18 mm behind the lens. This means that as the focal length of the lens changes, the lens will be closer to, or further away from, the sensor at the back of the camera in order to work effectively. This is why some telephoto lenses are so long (e.g. 300 mm focal length). The focal length also determines the magnification (m) and the field of view . Long focal lengths will have narrow fields of view and high magnification, whereas short focal lengths will have wide fields of view and lower levels of magnification ( Fig. 13.2 ).
In terms of varying the diameter of the aperture stop to let in more or less light, it’s relatively straightforward as we just need a size-changing aperture within the camera. However, photographers describe the diameter (ø) relative to the focal length (f) of the lens – called the f -number – as this helps them understand what effect the aperture stop will have on the final image. The equation for calculating the f -number is shown in Equation 13.1 . Here you can see that if a camera had a lens with a focal length of 100 mm, with an aperture stop 25 mm wide, the f -number would be 4 (100/25), and so it would be written as f /4, which means the aperture stop in this case has a diameter that equates to a quarter of the focal length. This also highlights that changing either the focal length or the diameter of the aperture stop would alter the f -number.
N=fø
f−number=focal lengthaperture diameter
Importantly, smaller aperture stops (which let in less light) might need longer exposure times, which is another thing to take into consideration when planning your shots. In general terms, the smaller the f -number, the more light is let into the instrument, and the better the lens will perform in low levels of light. Altering the aperture size (and f -number) can also affect something called depth of field , which describes the range in which something can be in focus. For example, let’s say I’m taking a photo of my two cats sitting near each other, but one is approximately one metre closer to me than the other. A large depth of field would mean I could take a photo that has both of the cats in focus at the same time, whereas a small depth of field would mean I’d have to focus my camera on one or the other, and the one that wasn’t in focus would be blurry ( Fig. 13.3 ). In general, smaller apertures lead to larger depths of field, and larger apertures produce smaller depths of field.
In summary, cameras are very clever, and there’s an awful lot to think about when trying to take a photograph!
Telescopes
Telescopes are optical instruments designed with the purpose of helping us see objects that are far away, and they do this by magnifying the far-away object so we can see it more clearly. For example, if we use a telescope to look at the moon, the image of the moon is magnified to help us see it in more detail.
A ‘basic’ telescope will need to be made of two lenses – an eyepiece lens (the one near your eye) and an objective lens (the one nearer to the object). These lenses work together just like the multiple lens systems we discussed in chapter 3 to produce an image that is in focus for us as the observer. An interesting thing to learn here is that the image we see through the telescope will be visible through what’s called the exit pupil of the instrument. An exit pupil is the view of the aperture stop from the back of the instrument, so it’s only possible to see the exit pupil through the eyepiece, which makes sense (the view the other way round would be called the entrance pupil , but we’re not going to worry about that in this book). The position and size of the exit pupil in a telescope will control the field of view , which means it also serves as the field stop . A field stop is an aperture in an optical instrument that controls the field of view (how much of the scene is visible at any one time).
There are a few different types of telescopes, so it’s helpful to discuss them separately.
Galilean telescope
A Galilean telescope is made using a negatively powered eyepiece lens and a positively powered objective lens ( Fig. 13.4 ). This produces a virtual (upright), magnified image for the observer. The issue with a Galilean telescope is that the exit pupil doesn’t coincide with the pupil of the observer; as you can see in Fig. 13.5 , the exit pupil (the clear window that the image forms in) is produced in between the two lenses, meaning it can be likened to looking through a keyhole. The observer will need to line up their own pupil with the exit pupil in order to get the best view. It also means that if the observer is holding the eyepiece lens too far away from their eye, it will restrict what they can see.
