Camera (very) Fundamentals

Version 2.1, page 3, © 2002, 2006 by Dale Cotton, all rights reserved

Chapter Three: Stops and Apertures

Fig 7. Look into my eye...

But modern cameras are more sophisticated than just lens and shutter. So let's go back to square one again and re-engineer our Mark II camera once more. To control how much light reaches the film we add a further refinement - one that is based on how the human eye works. The part of your eye that gives it colour is called the iris. In dim light the iris opens to make a bigger pupil opening to let more light into your eye; in brighter light the iris gets bigger to make a smaller pupil opening (see your iris in action using a mirror and flashlight).

Fig 8. The Mark III box camera with stop panel and shutter panel

We can do the same thing with our camera. We'll put a second slot in the box to receive a second panel, but the new panel will have a hole drilled in the centre. With the shutter pulled out of the way, light from the lens will still have to shine through this hole to reach the film. We'll call this a stop panel for now, since it will stop some portion of the light from reaching the film. If we make the hole in the stop panel to be the same size as the diameter of the lens (or even bigger) then all the light from the lens will reach the film. We'll call this a 1-stop panel. Now we'll take a second stop panel and drill a hole that only allows half the light from the lens to pass. We'll call this a ½-stop panel; it stops half the light. Clearly, we could make 1/3-stop and 1/4-stop panels, etc.

To operate the Mark III camera we

1. Put it in front of our subject with the shutter panel in place
2. Decide which stop panel we want to use
3. Put that panel into the stop panel slot
4. Pull out the shutter for a short period of time to expose the film.

Great ... but what has this added complication done for us? First, let's say that the light is very bright and we're having trouble removing then re-inserting the shutter quickly enough not to over-expose the film. If we have the ½-stop panel in place when we remove the shutter, we can leave the shutter out for twice the length of time before we have to re-insert it. However, as we experiment with our various stop panels we find out that something very interesting has happened to our pictures.

Fig 9. DOF blur at two different apertures (focus on centre bottle)

Let's say that the picture we want to take is a scene with a fence about 10 feet in front of the camera, then a cow just a few feet behind the fence, then a barn in the background about 50 feet away. With the 1-stop shutter in the camera perhaps the fence is sharply in focus, the cow slightly out of focus, and the barn totally blurry. With a 1/8 stop shutter in the camera fence, cow, and barn are all in reasonably sharp focus. So we can control how much of the depth of a picture will be in focus by using different stop panels. This is called controlling the depth of field (DOF). Similarly, in Fig 9 we see that the first and last bottles in the left-hand shot are both blurry, but in the right-hand shot both are much closer to being in focus.

Interesting fact: Many people are near-sighted. If you are one of us, check this out for yourself. Outside in sunlight so bright you have to squint, your pupil (lens aperture) is at its smallest. Consequently, your depth of field is so great you may almost be able to see sharply without your glasses on. At night indoors with only a 40 or 60 watt bulb to light the room your pupils will be open most of the way. Your DOF will be much smaller, so you will find a greater need for your glasses. The eye is a (motion picture) camera with a brain attached to it instead of film!

Fig 10. SLR lens diaphram with aperture set to f/4

Your camera's lens, being the marvel of modern technology that it is, has a variable stop diaphram (mechanical iris) inside it. You control how big the opening will be by means of the f/stop setting on the camera body. This opening is called the lens aperture, which corresponds to the pupil of your eye.

Boring fact: The strange name f/stop is actually simple to understand. In our Mark III camera to create our stop panels I said "drill a hole that only allows half the light from the lens to pass". Fine, but exactly how big a hole would that be? I turns out that we need to use half of the distance between the lens and the film, which is called the focal distance. In general, we divide the focal distance by our chosen stop fraction (1/2, or 1/4, or whatever), and call this an f/stop for short.

Going back to the Mark III, the first panel has a hole drilled in it that has a diameter equal to the distance between lens and film. This is our f/1 panel. Our second panel has a hole drilled in it that is half the diameter of the f/1 panel and is called our f/2 panel. Etc. This keeps our exposure calculations simple. Whatever time is correct to leave the shutter open at f/1, we need twice that time if we take out the f/1 panel and replace it with the f/2 panel. We need four times the shutter open time if we go from f/1 to f/4. Etc.

Once again, because of the way the human eye interprets brightness we want to use doubles and halves to control light. The standard sequence of f/stops is:

f/1 - f/1.4 - f/2 - f/2.8 - f/4 - f/5.6 - f/8 - f/11 - f/16 - f/22 - f/32 - f/64

Again: the "f" is whatever the focal length is for a given lens. The strange numbers, like f/1.4 and f/5.6 are due to the fact that we're dealing with the diameters of circles. (Not to frighten anyone but each number is a round-off multiple of the square root of 2.) Looking at Fig 1 again, we see the same 50mm lens shown in Fig 10, but from a different angle (upper left). In Fig 1 we can see that this lens offers aperture choices from f/1.4 to f/16.

Interesting fact: The lens in Fig. 10 has the markings 1:1.4 f=50mm on the front. Nearly all lens have similar numbers. 1:1.4 says that the ratio between the focal length and the widest aperture the diaphram allows is 1 to 1.4. This is simply another way of saying that the widest aperture of this lens is f/1.4. The f=50mm says that the focal length of this lens is 50mm.

Digital: Apertures and f/stops function identically in both digital and film cameras.