Aperture Size and Depth of Field
When you take a picture, light enters your camera through a 'hole'. This hole is referred to as the aperture. The size of the hole is referred to as the f-stop. The shutter that we talked about in the last section resides behind the hole or aperture. Like the shutter, the aperture size can (usually) be adjusted. It can be made larger, letting in more light. It can be made smaller, letting in less light. The shutter speed and aperture size combination is what determines the exposure (how much light hits the film) These f-stop numbers are usually written around the lens barrel, and can often be seen in the camera's viewfinder as well. What to do these numbers around the lens mean? They refer to the size of the aperture, how much light is getting to the film. They also determine depth of field. Depth of what? It's not as complicated as it sounds. I suggest getting your camera right now and checking the numbers around the lens barrel. The numbers always start out small and get larger i.e. 1.8, 2.8, 4, 5.6, 8, 11, 16, 22, 32. Your lens may not have all of these numbers but it will have some of them. Each number represents the size of the aperture or opening of the lens during the exposure. (Exposure once again, refers to the moment that the light passes through the aperture and the open shutter in order for the image to register on film). The smaller the number i.e. 1.8 or 2.8, the LARGER the opening of the aperture. The larger the number the smaller the aperture opening. Although this may seem contrary to logic, think of it this way. Pretend these numbers are all fractions of an inch or a centimeter. So f22 is SMALLER than f1.8 because it lets in a smaller fraction of light. A good thing to know is that each successive number lets twice as much light hit the film as the previous one. NOTE:THIS IS THE SAME THEME THAT WAS MENTIONED IN RELATION TO SHUTTER SPEEDS. So f 2.8 lets in twice as much light as f4. If we go in the other direction f11 lets HALF the light that f8 does because f11 is SMALLER. Ok now that the numbers are out of the way let's talk practical depth of field.
Depth of field-refers to the overall foreground to background sharpness of your image. This is most strongly determined by the aperture size you choose.
Try this exercise: go look out your window at a scene 50 feet away. Then squint with one eye or make a small circle the size of a pea with your fingers and look through the hole at the scene again. Keep making your finger smaller until the hole is teeny, let's say the size of a pea. You should notice that as the squint or finger circle gets smaller, the depth or how far you can see gets clearer or sharper. That's depth of field. And the lens on a camera works the same way. The smaller the aperture F22, F16, the sharper the details in BOTH the foreground and background. The larger the aperture 1.8, 2.8, only the element focused on will be sharp. The classic example is a man in a forest. If you focus on the man at 1.8, the man will be sharp but the trees behind him will be out of sharp focus. But at f22 both the man and the trees behind him will be in focus.
F-2.0 A shallow depth of field. Note how the foreground is sharper than the background.
F-16 A large depth of field. Note how both the foreground and background are sharp.
So why do we choose different apertures and not always the smallest or sharpest one? Basically for two reasons: Although the smaller apertures( F16,F22, F32) produce sharp focus from background to foreground, they require more light to achieve proper exposure. This is because the hole that the light passes through in order to hit the film is small. If the hole were larger, you would need less light. This is not a problem on a sunny day, but would be a problem on a cloudy day or at night. The second reason for choosing a larger aperture boils down to intent. Even if there is enough light to choose whatever aperture you desire, sometimes you want to ISOLATE the foreground from the background. This is especially useful when taking a portrait. Let's say we're talking about that man in the forest again. If we want the viewer to focus on the man, we will choose a larger f-stop. When we look at the resulting print, we will notice that the man is in sharp focus but the background is somewhat blurry. This will cause the viewer of the image to pay more attention to the man and less attention to the trees. If the man is a lumberjack however, and we want to show his environment, then we may wish to choose the smallest aperture (light permitting), and so the whole image from background to foreground will be sharp.
Film Speed
The last element in this trinity is film speed. What is the difference between a 100 speed film (prefixed by ISO or ASA) and a 200 speed film? You're gonna love it. AGAIN ITS BACK TO THE SAME THEME AS IN THE PREVIOUS SECTIONS. 100 speed film is twice as sensitive to light as 50 speed film. 200 speed film is TWICE as sensitive as 100 speed film. Going in the other direction, 200 speed film is HALF as sensitive as 400 speed film. The higher the film number, the less light you need to take the picture. This is why if you are taking indoor shots or shots at night (where there is less light) the people at the photo store will recommend a higher film speed. Film speeds can go as high as 1600 or even 3200. These films are extremely sensitive to light and are therefore often used when the light level is really low. So why not always choose a high film speed. What's the catch? The catch is in the grain. The lower the film speed the finer the grain. The higher the film speed the coarser the grain. When you look at a photo you are looking at thousands of teeny grains all clumped together to produce a continuous tone. The higher the film speed, the more you will see the grain. You might not see it so much in a small photo, but if the photo is enlarged to 8x10 inches, you can definitely see the difference. Grain isn't always bad. Many photographers use grain for artistic effects. Most people however, prefer finer grain. One more thing, 100 speed film always seems to have the lowest price.This is due to it's mass production.
Shutter speed - aperture - depth of field - film speed - summary
Based on film speed, the camera's light meter will evaluate the scene and produce a proper exposure. This exposure will be an f-stop along with a shutter speed. NOW THAT YOU KNOW SOMETHING ABOUT EXPOSURE, YOU CAN CHOOSE DIFFERENT SHUTTER SPEED/APERTURE COMBINATIONS FOR CREATIVE EFFECTS AND STILL MAINTAIN PROPER EXPOSURE.
So here we go: First put your camera in manual mode. Let's hope it's daytime. Go to the window and focus on something. Somewhere in the viewfinder you will see the exposure. Let's say it's f5.6 at 1/250 This means that the size of the aperture the 'scene' passes through is f5.6. It reaches the shutter which stays open for 1/250 of a second. Now as we have learned, shutter speeds and aperture sizes work mathematically . Let's say that we decide we want to shoot not at f5.6 but at f4. Since f4 is one f-stop(unit for the next section) larger, it will let in more light than we need. The final print will be too light or overexposed. Picture it this way: let's say the aperture is a window. If we make the window larger, more light will enter the room. Let's say that right behind the window are shutters. If we open the shutters for a longer time, again more light enters the room. Therefore the solution for letting in 1 unit extra light through the window (aperture) is to decrease the time the shutter stays open. So if the correct exposure was 5.6 at 1/250 and we change to the larger aperture f4, we must decrease the shutter speed by 1 unit to 1/500. Thus f5.6 at 1/250=f4 at 1/500. BOTH COMBINATIONS ALLOW EXACTLY THE SAME QUANTITY OF LIGHT TO REACH THE FILM. In this case the main difference between the two shots will be depth of field. The f5.6 shot will be sharper from foreground to background than the f4 shot.
Film speed works in a similar mathematical fashion. Let us say that it is an hour after sunset and the light is really low. We are using a 50mm lens with 100 speed film and taking a portrait of Bozo the clown on the beach. We look through the viewfinder and find that the exposure is f1.8(or the largest aperture on the lens) at 1/30th of a second. We know that since Bozo is standing he is moving slightly, and we therefore need to go to 1/60 to ensure that there is no movement which will blur the scene. What do we do? Basically we need 1 more unit of light to get the shutter speed to 1/60. If we are well prepared we would hopefully have another roll of film at a different speed on hand. Let's say we had a roll of 200 speed film. Because 200 speed film is twice as sensitive as 100 speed film, it would give us the extra 1 unit of light we need. So we could rewind the film wherever it is(what about the wasted pictures?....who cares.... this IS Bozo the clown) and then insert a roll of 200 speed film. We would then achieve proper exposure. What if all we had was a roll of 400 speed film. Since 400 is twice as fast as 200 we would be getting an extra unit of light (100 to 200 is 1 unit and 200 to 400 is another unit) In this case since we have to compensate for 1 extra unit of light, we could either make the f-stop smaller(F2.8) or increase the shutter speed(1/125) by 1 unit. This would balance the exposure. ASA 100 f1.8 at 1/30=ASA 200 f1.8 at 1/60=ASA 400 f1.8 at 1/125 And what if all we had was the roll of 100? Then we would hold the camera as steady as we could and hope for the best. We could also have used a flash if we had one, but that is a topic to be discussed at another time.