In spite of point-and-shoot cameras in mobile phones, there are still people who take pleasure in photography as an activity in itself. If you count yourself in this group, then this website is for you. This Blog will provide an introduction to SLR photography, whether your interest is in Film or Digital. Each individual page will deal with a separate topic. If you have questions, please use the comments boxes, and I can incorporate the comments and my replies into the Blog.
Camera Controls, and their part in Image Sharpness
The diagram below represents a typical manual focus film S.L.R. camera with a short zoom lens fitted. Different models of camera will possess features in addition to those detailed above; digital cameras will not possess the features associated with film transport; and compact cameras with a lot of automation may not physically permit user-access to these controls; these, then, are the fundamental controls as far as photographers are concerned, and they are common to all cameras.
Single Lens Reflex or SLR Camera.
Single Lens - the same lens is used both to view the scene and to take the photograph;
Reflex - the use of this single lens is possible because the construction utilises a mirror which directs light up through the pentaprism for viewing, but which moves out of the way to allow light to pass through the shutter when the photograph is taken;
WYSIWYG - Because the use of the mirror allows the same lens to both view the scene and take the photograph, what you see in the viewfinder is what you get in the photograph.
These are the three characteristics of the SLR Camera.
The two principal camera controls of shutter speed and lens aperture are used in the control of both image appearance and camera exposure. Their use in control of image appearance (specifically with regard to image SHARPNESS) is what is under consideration here. Their use in the control of camera exposure will be addressed separately.
Holding the Camera.
The largest single factor contributing to unsharp photographs is camera shake due to inappropriate handling of the camera. The most stable hand-holding technique is illustrated below left: the left hand cradles the lens and supports the camera body, while the right hand operates control switches (or dials) and the shutter release button.
A second factor contributing to camera shake is the (unintentional) use of too-slow a shutter speed. The rule of thumb suggests that the slowest shutter speed capable of being hand-held is the reciprocal of the focal length of the lens being used. For example: 28mm lens, 1/30th second; 200mm lens, 1/250th second. The use of a sturdy tripod and remote release minimises the risk of camera shake in all but the windiest conditions.
The focal length of a lens is defined as “the distance (nowadays in mm.) between the rear nodal point and the image plane when the lens is focussed on an object at infinity”. However, without some understanding of the construction of lenses and the optical functioning of compound lenses, such a technical definition is less than helpful. As practical photographers, you need some useful practical way of understanding what focal length means in terms of translating the picture in your head into the image you make with your camera. The simplest way to do this is to think in terms of angle of view, or conversely, in terms of image magnification.
A short focal length lens is generally less than 50mm. So, for 35mm. format, 35mm., 28mm., 24mm., 20mm. are all short focal length. These are Wide Angle focal lengths, that is, the angle of view which is seen by the camera covers a wide horizontal angle. Because the image area is fixed by the format of the camera, in order to fit all of a wide angle of view into the image area, image magnification of a short focal length, or wide angle lens, must be small.
The focal length of the so-called Standard Lens for any given camera format is approximately equivalent to the diagonal of the format. Thus, for 35mm. format cameras, with a format of 36mm. by 24mm. and a diagonal of 45mm., the standard focal length is 50mm. This roughly approximates to the angle of view as seen by the unaided human eye. In the case of modern half-frame digital SLR cameras, with a format of 24mm. by 18mm. and a diagonal of 30mm. the standard focal length is nearer to 30mm. The dividing line between short and long focal length lenses is, arbitrarily, the standard focal length.
A long focal length, then, is generally taken to be longer than the standard. The generic term for long focal length lenses is Telephoto. This implies that these lenses are like telescopes, so make distant objects appear closer, and make small objects appear to be larger. We can thus consider a long focal length, or telephoto, lens to have a narrow angle of view, and a large degree of image magnification.
A Zoom Lens, often erroneously confused with the term telephoto, is by definition a variable focal length lens. Without information about the specific focal range, a zoom lens can only be said to be variable, and its range can encompass short (wide angle) or long (telephoto) focal lengths, or straddle the standard focal lengths, for example in the so-called ‘standard zoom’ of 18 - 55mm.
So, in terms of the picture in your head, thinking about how much or how little of the scene should appear in your photograph helps you to think about which focal length of lens (how much image magnification, or how wide or narrow a field of view) is required to achieve the desired composition. Reference to the series of pictures to the right will give some idea of how the focal length of a lens will affect a photograph of the same scene from a fixed viewpoint.
There are three aspects of image sharpness that must be differentiated: focus; depth of field; and movement blur.
Focus is the control which affects sharpness associated with objects at a specific distance from the camera viewpoint. Focussing of the lens is achieved manually by means of the lens focussing ring, or automatically with the first pressure on the shutter release button, and can be seen in the viewfinder.
Focus allows you to choose which object or plane within the scene you wish to render as sharply as possible. Thus, you can choose to make the foreground sharp, by focussing on the foreground (picture left).
Or you can choose to make an alternative, more distant object or plane sharp (picture right), by focussing on the distant object or horizon. It is important to note that a camera lens can only ever be focussed to a single plane at any one time.
Depth of Field.
There are times, however, when you wish to produce an image in which everything appears to be “in focus”. As seen above, a camera lens can only be focussed to a single plane at any one time. But, as the picture (below) shows, it is possible to produce an image in which the foreground and the distant horizon both appear to be sharp at the same time.
It is often said that there are similarities between the eye and the camera. Both use a lens to focus light rays onto a light sensitive medium. In the case of the eye this is the retina, in the case of the camera it is photographic film or the digital sensor. However, the ability of the eye to rapidly change focus, and thus for nearby objects to be perceived in sharp focus virtually simultaneously with more distant objects, is not shared by the camera. The camera lens may be focussed on one plane only; therefore in a photograph only objects lying on this plane will be in objectively measurable sharp focus. What is found in practice, though, is that there are usually objects in front of and behind this plane of sharp focus which appear acceptably sharp (or alternatively, may be seen as acceptably unsharp) in the photograph. This phenomenon is known as Depth of Field. Depth of field is a very useful tool that is used by the photographer to imitate the ability of the eye to apparently see everything simultaneously in sharp focus. Conversely, it may be used to deliberately isolate a single object or plane. This is known as differential focus, and is best seen in the first of the three pictures above, in which the viewer's attention is directed to the foreground rocks.
In practical terms depth of field varies with the following factors:
i). the focal length of the lens;
ii). the lens aperture;
iii). the focussed distance.
i). Depth of field is greatest with lenses of short focal length (wide angle), and least with lenses of long focal length (telephoto).
ii). Depth of field is greatest with small lens apertures, and least with large lens apertures. It follows, therefore, that the greatest degree of depth of field will be achieved by using a wide angle lens stopped down to its smallest aperture, and the shallowest depth of field will result from a telephoto lens opened up to full aperture.
iii). For each focal length and each lens aperture there is a particular focussed distance, known as the hyperfocaldistance, such that when the lens is focussed to that distance and the diaphragm set to that aperture, maximum depth of field is achieved - it extends from half the hyperfocal distance to infinity. For photography at normal focussing distances, the extent of depth of field can be assumed to be in the ratio one third in front of the plane of sharp focus, to two thirds behind. In situations of very close focussing (often referred to as macro) then depth of field becomes extremely shallow, and lies symmetrically about the plane of sharp focus.
From the point of view of general photography, which may require that the picture be sharp from foreground to background, it would seem sensible to advocate the use of small lens apertures. However, conditions prevailing at the time of taking the picture will govern whether small apertures are possible or desirable. The need to hand-hold the camera may necessitate a fast shutter speed and possibly preclude the employment of a small aperture, whereas a tripod will permit the slower shutter speeds that the use of a small aperture will entail. However, a slow shutter speed may adversely affect the appearance of the image if some portion of the subject is moving: see below for “Effect of shutter speed...”.
Access to shutter speed and lens aperture control, in the case of a digital SLR, will be via some form of push-button/command dial combination (refer to camera manual for specific instructions). Actual control of shutter and aperture will be by the use of manual or semi-automatic exposure control. For example, if using a particular aperture setting to achieve a certain depth of field, then the ‘AV’ mode (aperture value) may be used; the photographer selects the chosen aperture value, and the camera automatically selects the corresponding shutter speed. Once the appropriate settings have been chosen and applied, then the image is recorded by pressing the shutter release button. The majority of S.L.R. cameras permit the use of some form of remote release, which allows hands-off, vibration-free shutter release. This has an important application when the camera is mounted on a tripod.
Every lens possesses its so-called Optimum Aperture. In the absence of any pictorial requirement for a particular aperture setting, there is a good argument for the use of a mid-range aperture when this proves feasible. Camera lenses, even expensive, highly-corrected ones, produce what are known as image aberrations in their margins. By stopping the lens down, ie. making the size of the aperture smaller, (rule of thumb, two to three stops down from maximum aperture) only the central portion of each of the various lens elements is used, thus minimising any marginal faults and optimising performance.
Effect of Shutter Speed on the Appearance of the Image.
Shutter speed does not refer to a linear ‘miles per hour’ type speed, since the shutter blinds used in an SLR camera always move at the same speed. What is being referred to is the duration of the shutter opening, usually expressed as a fraction of a second, but in certain circumstances can be periods of seconds, minutes or even hours. The shorter the duration of the shutter opening, (faster) there is less opportunity for moving objects within the camera’s visible field to be ‘seen’ to move while the shutter is open, and thus to appear blurred in the final image.
The effect of shutter speed on the appearance of the image essentially splits into two: fast shutter speed (short duration) to “freeze” any movement; or slow shutter speed (long duration) to allow the motion to become more-or-less blurred.
1). Freeze Movement for Sharp Subject (picture right).
Moving objects are viewed by the naked eye as “moving”, and not as a sequence of static, still images. There may be times, however, when it is desirable to record the detail, within a moving subject, that is not generally visible to the naked eye; for example, the individual water drops in a waterfall. To capture this detail necessitates a fast shutter speed (short duration) in order to “freeze” the movement. The practical shutter speed appropriate to a particular subject will depend upon a number of variables, such as subject speed, direction of motion, distance of subject from the camera, desired degree of “freezing” or blurring, film speed (or ISO setting) and so on.
2). Allow Movement for Blurred Subject. (Bottom picture)
The opposite approach can also be used to produce interesting image effects. By holding the camera perfectly still, ideally on a tripod, and using a slow shutter speed (long duration), it is possible to allow the moving object to pass the camera and record as a blur against a sharp background. The success of this technique depends on the shutter speed used and the speed at which the moving object passes the camera, so, as with so many photographic exercises, a number of practical trials may be necessary before success is attained. This technique is used regularly and successfully to photograph waterfalls and the like, when a slow shutter speed allows the water to record as a smooth, soft flow, rather than appear frozen into its individual gouts and splashes.