Photographic film is basically a coating (known as an emulsion ) on a transparent film backing. The emulsion contains a number of very tiny grains of light-sensitive chemicals suspended in an inert medium. These
very tiny grains of chemicals respond to the light falling on them. When the film is exposed to the light, these chemical grains change in a more or less permanent way. Before the exposed film is developed, we say the image is latent in the altered chemical grains. Development alters the grains in such a way that it creates a real image (either a negative or a positive image, depending on the type of emulsion) on the film.
a. Emulsions and film speed
Not all emulsions behave the same way
ASA/ ISO (different rating systems) FILM SPEED p.58 of text
RATING- how sensitive it is to light
25 50 64 100 125 160 200 400 800 1600 2000
told you to buy 100 or 400 speed film
the higher the # is the faster the film – the more
sensitive it is to light
benefits in low light situations
a faster film also means that it is more grainy (large and small silver halide crystals), less detail, less contrast SHOW picture from text
ideally, you’ll want to use the slowest film speed possible in a given situation
100 speed film better to use outside on a really sunny day and the 400 speed film on a cloudy day
like the textured look of the grain
o a given amount of light. Some respond to light more readily than others, and so are called "fast" films. That is, it takes less light to produce a properly exposed film than a "slow" film. Film speeds are
expressed in ISO numbers . ISO-50 film is slower than ISO-100, and ISO-100 film is slower than ISO-400. The rating of a film in terms of ISO numbers increases with the speed of the film. We will be talking about f-stops shortly, but there is a relationship between f-stops and ISO numbers ... a change of ISO by a factor of 2 corresponds to a change in the f-stop of one full stop. That is, if we double the film speed (in ISO units), we can decrease the aperture size by one f-stop and get an equivalent exposure.
Sounds like you always would want to buy the fastest film available, right? Well, unfortunately, it's not that simple. The faster the film, generally speaking, the larger the grain size. Large grains can give an image a "grainy" look ... "fast" film often shows the grainy character of the emulsion that can detract from image clarity. Some typical ISO film speeds are shown in Table 1.
25 50 64 100 125 160 200 400 800 1600 2000
Table 1. A selection of typically-available film speeds, rated in ISO numbers. Note that not all film speeds are at one full f-stop intervals.
The notion of an emulsion's latitude is an important one. The human eye is a remarkable instrument for seeing ... one of its characteristics is a large dynamic range . That is, when we look at a scene, some parts of it are dark and some parts of it are light. What we mean by dynamic range is the range of light and dark between seeing something as totally black and seeing something as totally white. If we can see structure and detail in shadow areas as well as in brightly lit areas ... at the same time ... then we have a large dynamic range. Of course, if it gets too bright or too dark, our eyes can adjust to the amount of light, but that's another issue. Emulsions vary as to their dynamic range, but virtually all films have a much narrower dynamic range than what our eyes can see.
If the contrast between light and shadow in the scene is large, then the film needs to span a large dynamic range, or latitude, if all parts of the recorded image are to retain detail in both shadow and brightly lit areas at the same time.
Slide films (positive images) typically have a latitude that is less than typical print films (negative images). Some slide films have only about a 2 f-stop latitude. Some print films have perhaps 6 f-stops of latitude. Our eyes can see with a range of much greater than this, but I don't know the actual value in f-stops.
When an emulsion has relatively little latitude, then scenes with a lot of contrast between light and dark pose a problem: if the shadow areas are properly exposed, the highlights "wash out" (lose detail) and just look white; if the highlights are properly exposed, the shadow areas "block up" (another form of losing detail) and just look black. There are ways to compensate for this at times, but if you are using film to record what you see, remember to account for this whenever possible.
When using print film, remember that even if it has more latitude, the final image has to account for the latitude of the emulsion being used to make the final print (usually on paper). Print emulsions typically have less latitude than the negative emulsions used to record the image!
3. The camera as a light-measuring device
The lens/camera is the primary light measuring device, but it also acts to focus the light onto the film. I am not going to discuss focusing very much ... details of focusing depend on the lens elements (usually glass of one sort or another). The glass in a typical lens involves several elements (2 or more), in an attempt to account for various kinds of distortion (or aberration ) that a single element produces. The complex structure of a good quality lens is the result of trying to design a lens to minimize simultaneously several different kinds of abberation in an attempt to get the
maximum clarity (focus) possible and still be affordable.
a. Shutter speed
However, there is another critical element of the camera that is usually contained within the body of the camera: the shutter . Basically, the shutter is a mechanism that stays closed most of time. It is only open for a precisely measured amount of time, usually measured in fractions of a second, called the shutter speed (or exposure time). Most cameras have a range of shutter speeds more or less as shown in Table 2.
1 1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500 1/1000
sec sec sec sec sec sec sec sec sec sec sec
Table 2. Typical lens shutter speeds. Some cameras have much faster and/or much slower set shutter speeds.
Observe that, roughly speaking, the pre-set shutter speeds change by a factor of 2 (more or less), so that slowing the shutter speed down to the next value provides twice the exposure. In real cameras, the shutter often is of the "focal plane" type and not part of the lens ... it is a slit that travels across the image at a fixed speed. The effective shutter speed is determined by the size of the traveling slit ... if it is wide, the film is exposed for a longer time than if it is narrow.
b. Apertures and f-stops
Not only is the time of the shutter's opening is controlled, however. Also, the size of the lens's opening is variable; the size of the opening is called the lens's aperture . Basically, the aperture is controlled by a sort of diaphragm with a circular hole that increases or decreases in size according to how the lens is set. A simplified version of the lens/aperture is shown in Fig. 1.
Fig. 1. The lens/aperture combination, showing how the aperture determines how the light is restricted by the aperture. For convenience, the focal length and focal plane are illustrated on this schematic. In (a) the aperture is large, in (b) the aperture is small.
In what follows, I have included some equations. If you are capable of understanding the equations, so much the better, but it is not absolutely necessary to do so in order to read the conclusions ... go ahead and skip down to the paragraph beginning "The largest possible aperture ..."
Mathematical discussion of the f-number.
The aperture is not described in terms of its actual size, however. Rather, the aperture is described in terms of its f-number (or f-stop), which accounts for the focal length of the lens ... the f-number (f ) is the ratio of the focal length (f) of the lens to the diameter of the aperture (D , where the aperture is assumed to be circular). Thus, if a lens has a focal length of 50 mm and an aperture with a diameter of 25 mm, then its f-number is 2.0, usually written as "f /2" so that
The area of the aperture A that determines the amount of light let through the shutter is given by
Suppose the area of the aperture at one setting of the lens's aperture happens to be two times the area at another setting; that is A2 = 2A1 . Hopefully, it is easy to see from the preceding formula that in this case the diameter must have increased by a factor of = 1.41421...., such that D2 = D1. But that means
That is, every time we double the area ("open up" the aperture), the f-number changes by a factor of ( ). Recall that for a 50 mm lens with a 25 mm aperture, the f-stop was f /2. With twice the area (i.e., opening up by one full stop), the f-stop changes from 2.0 to 2 /=1.41421... . This would be denoted as f /1.4.
What if we go the other way, and cut the area in half ("stop down" the aperture)? It should be clear by now that each time we cut the area in half (i.e., stop down by one full stop), the f-number changes by a factor of = 1.41421... , so for half the area of the aperture initially set at f /2, the new f-stop would be 2 *= 2.8285, written as f /2.8.
The largest possible aperture is for the diameter is when D = f, or f /1.0. We say that each factor of 2 in aperture size is considered a full stop . That is, when the aperture area changes by a factor of 2, that corresponds to one full f-stop. It is also common terminology to refer to the speed of a lens in terms of its maximum aperture. Thus, a 50 mm lens with a maximum aperture of f /1.4 would be a full stop faster than a 50 mm lens with a maximum aperture of f /2. That lens would be described as a 50 mm f /1.4 lens. A lens two full stops faster than an f /2 lens would be rated at f /1, the fastest possible lens speed for that focal length. Many lenses have aperture rings that have click stops at 1/2 stop intervals, such that it takes two clicks to equal a full stop.
It is important to note that as the f-numbers increase, the size of the aperture decreases. When we say we are "stopping down the lens" that means we are increasing the f-number or decreasing the aperture area. If we are "opening up" the lens, this corresponds to decreasing the f-numbers or increasing the aperture area. [Note: if this seems silly to you, just remember that I am only reporting on what terms mean ... I didn't develop this terminology!] The following table shows the f-numbers associated with full stop differences. Table 3 shows f-stops as they are typically marked on lenses
1 1.4 2 2.8 4 5.6 8 11 16 22 32
Table 3. f-numbers at full stop intervals, beginning at f /1.
I will leave it as an exercise for the interested readers to figure out what the f-numbers would be at 1/2-stop intervals; also, to calculate (for example)
how much faster an f/3.5 lens is than an f/4.0 lens, as measured in f-stops.
c. Focal length
I have been talking about focal length, f; what does focal length mean? If you can imagine a lens focused on an object at an infinite distance, so that light rays from the object are coming in as straight parallel lines, the focal length is simply the distance from the center of the lens to the point where those rays converge after passing through the lens (see Fig. 2)
Since items must always be focused on the plane of the film (which, therefore, is known as the focal plane ) Longer focal length lenses typically are longer in length. The longer focal length means that a narrower region of the subject is projected onto the film. This means that longer focal lengths result in an increase in the "magnification" of the resulting image ... see Figure 2. Therefore, "telephoto" lenses have long focal lengths and "wide angle" lenses have short focal lengths. A "normal" lens for a 35 mm camera is about 50 mm; such a lens is neither a telephoto nor a wide-angle. Its images correspond roughly to the perspective we would see with our eyes.
Fig. 2. A schematic illustration showing how focal length corresponds to magnification. Some exaggeration has been used to make the point.
d. Exposure reciprocity
We are now ready to explain exposure reciprocity . The total amount of light that the camera allows to reach the film is determined by the combination of the shutter speed and the aperture. Let's suppose that some combination of exposure time and f-stop produces an ideal exposure ... for example 1/125th of a second at f /8.0 ... but wait ... if we doubled the aperture size (which would let in twice as much light ... a full stop, corresponding to f /5.6) and cut the exposure time in half (to 1/250th of a second), that would correspond to exactly the same total exposure! This is the reciprocity rule ... if you double the aperture size
and cut the exposure time by half, you get the same exposure. Or, you could stop down by a full stop (cut the aperture size by half, in our example to f /11) and double the exposure time (in our example, to 1/60th of a second) ... bingo! ... the same total amount of light, once again.
O.K. ... that's all very interesting. Why would we want to do such a thing? To see that, we have to consider two other topics: depth of field and the motion of the subject.
e. Subject motion
If all subjects were absolutely still all the time, then having the capability to vary the shutter speed would not be very important. However, most real subjects move, to a greater or lesser extent. If we want the capacity to "freeze" that motion, we need a fast shutter speed. For example, suppose you wanted to take a close-up shot of some wildflowers that were waving in a slight breeze. If your shutter speed is fast enough, say 1/250th of a second, the image will appear to have "frozen" the wildflower's motions and it will be sharp and clear.
On the other hand, you might want to photograph a moving stream and have its motion turn the water into a sort of blur. This is a very common thing to want to do ... in order to do this, you would want a slow shutter speed, say 1/2 a second.
Therefore, the ability to vary the shutter speed is an important factor under your creative control.
f. Depth of field
If you have your lens focused on some subject, it turns out that there is a range of distances, both larger than and smaller than the distance to the subject, where other objects in view are also shown clearly. They are very nearly in focus, as well. This is called the depth of field, and it turns out to depend mostly on the focal length of the lens and on the aperture
setting. As illustrated schematically in Fig. 3, if you increase the aperture size, you reduce the depth of field, and vice-versa. If you want objects nearby and far away to be in focus at the same time, this is going to require you to stop down the aperture (increase the f-number). There also are times when you want the depth of field to be small; for example, if objects in the background of a photograph's subject would be distracting, it is common to have them so badly out of focus that they simply become a blur, allowing the image to emphasize the subject rather than the distracting background. In such a case you would want to open up the aperture (decrease the f-number)
Fig. 3. Schematic showing the effect of changing the aperture on depth of field, where the horizontal lines indicate the range within which objects will be in focus for each aperture setting. The X- marks indicate the hyperfocal distances (only schematically!). At f /2, only the near foreground objects would be clear, while the midground and background would be blurry. At f /5.6, at this particular hyperfocal distance, only the midground and background would be in focus, while foreground objects would be blurry. At f /16, objects from the foreground to the background would all be in focus. Shifting the hyperfocal distance alters which objects are in focus, and can change the range in which objects are in focus. See Fig. 4 caption below
Note that since there is a range associated with the depth of field at a particular f-stop, if you want the foreground and background to be in focus simultaneously, you would want to focus the lens at some distance in between the nearest and farthest object. This distance, which optimizes the focus to achieve the desired result is referred to as the hyperfocal distance. Many lenses have depth of field marks on the focusing ring, showing the depth of field range for several different apertures. Decide the distances to the nearest and farthest subjects you want to be in focus and the hyperfocal distance will be shown clearly on the lens. Simply focus at that distance and you're in. A schematic example is shown in Fig. 4.
Fig. 4. Schematic example of the depth of field marks on the focusing mechanism of a lens, as seen looking down on the lens as it is mounted on a 35 mm camera. In this example, the lens's pre-set aperture is f /16; the depth of field marks indicate that at f /16, everything from infinity (?) to about 2.5 meters (m) is in focus, with a hyperfocal distance of 5 m already set on the focusing ring. Observe that the distance scale is not linear ... that is, the depth of field range (as measured in feet [ft] or meters [m] changes as the focus of the lens changes ... to the right of the hyperfocal distance (5 m), the focal range is from 5 m to infinity, whereas to the left of the hyperfocal distance, it's from 2.5 m to 5 m (only 2.5 m). If the focus were to shift from 5 m to, say, 10 m, the range to the right would be 10 m to infinity, but the range to the left would be from about 4 m to 10 m (about 6 m).
Another method is to use your camera's depth of field preview feature, if it has one. Most modern cameras use the maximum aperture for focusing and through-the-lens (TLL) metering, and the lens is stopped down to the pre-set aperture only when the shutter is pressed. Otherwise, what you see in the (TTL) viewfinder would, in general, be darker than what you see at a wide-open aperture ... this facilitates focusing. However, it can prevent you from seeing what the aperture's depth of field does to the final image. Some cameras permit you to stop down the lens to "preview" what the depth of field at the pre-set aperture looks like. If something looks fuzzy when you preview the depth of field, it will be out of focus in the image. Thus, you can experiment with different apertures and see for yourself what the shot will look like ... and you can experiment with trying to find the hyperfocal distance this way, as well.
The drawback to using the depth of field preview feature is that the image can be too dark in the viewfinder when the lens is stopped down using the depth of field preivew. The darkness of the view through the viewfinder can make it difficult to see what ther resulting image might look like. I have used