Lesson 6 -Exposure

Camera Metering Systems

Basically, there are three different types of metering systems that you may find yourself using. You may be using a fully Automatic Metering System. This system is used in basic point-and-shoot cameras that are automatic everything. The camera does it all-right or wrong. You may be using a camera with a built-in metering system that allows you to have some hands-on control of exposure, that is manual override-that is a way to turn off the automatic metering and take hands-on control of exposure. Most SLR's and DSLR's offer a choice of exposure modes-that is, you can set the camera in a variety of automatic " programmed" exposure modes, or you can set the camera in the Manual Mode, which gives you complete personal control of exposure.

What is Good Exposure?

Good exposure first and foremost avoids losing detail in large areas of the image. For example, there should be detail in the highlights rather than washed-out white areas that make it impossible to get a sense of form or detail in those areas. In addition, there should be detail in the darker portions, or the shadow areas.

Program Mode and Camera Shake

Program Mode and Automatic Mode usually select a safe shutter speed for the lens you are using, one that should be fast enough to minimize camera shake. For example, if you are using a 50mm lens, it would select a shutter speed of , say, 1/60 sec. Or, if you are using a longer lens, such as a 200mm, it will select a faster speed-say, 1/250 sec.

Shutter Speed and Aperture

The shutter is the device that lets light reach the sensor or film for a precise length of time. Shutter speeds are marked on the camera as 1, 2, 4, 8, 15, 30, 60, 125, 250, 500, and 1000. These are actually fractions-1, 1/2, 1/4, 1/8 and so on. Many cameras offer even shorter shutter speeds, such as 1/4000 sec and 1/8000 sec, and have settings for longer exposure times of up to 30 sec. As you move from one speed to the next shorter speed, you cut the time of exposure in half. Thus, an exposure of 1/60 sec allows light to reach the sensor for half the time allowed by 1/30 sec-which means that half as much light will enter the camera. This is the basic function of shutter speed: To control the time during which the light hits the sensor. The shorter the time, the less light, in direct proportion. If you cut the time in half, you cut the light entering the camera in half. Another way to control the amount of light entering the camera is to change the size of the aperture. A larger aperture opening admits more light. A smaller aperture opening admits less light. You have two variables that control the amount of light that enters the camera, aperture and shutter speed. There are three exposure controls, namely shutter speed, aperture size, and ISO. We are going to concentrate on the selection of shutter speed and aperture size first; not only do they affect exposure, but your selection of a given shutter speed or aperture size will determine some visual aspects of the photograph you take. Aperture size will vary the depth of field in your photo. Shutter speed can freeze or blur the subjects in your photo. Most of the time you will select an ISO that allows you to use the aperture size and/or shutter speed that you feel will make the best photo of your subject.

Exposure Modes

There are 4 basic exposure modes: Automatic/Program Mode For some camera brands, these are two separate modes, with Automatic designed to be goof-proof. Basically, in Program Mode, you let the camera's hardware and software make the exposure decision based on the ISO you have set for the camera and the light reading taken by the camera's reflected light meter. Under normal lighting conditions, Program Mode works very well. Shutter Priority You pick the shutter speed and the camera's meter will select a corresponding aperture that will give correct exposure. If you know you need a fast shutter speed or a slow one, this allows you to pick the speed and let the camera pick the matching aperture. This may be faster than using Program Shift in Program Mode. Aperture Priority You pick the aperture and the camera will match it up with what it thinks is the appropriate shutter speed based on the meter reading. This can be helpful for people working with a fast lens who want to make sure every photo is taken at f/2.8 to use shallow depth of field in order to isolate the subject from the background. Manual Mode You will get a meter reading from the camera's meter displayed in the viewfinder, but it is up to you to pick the aperture opening and shutter speed that you want. Scenic Modes Most cameras have a "Sports" mode that simply pushes for a fast shutter speed, a "Landscape" mode that pushes for a small aperture and greater depth of field, and a "Portrait" mode that will favor a large aperture to separate the portrait subject from the background.

Exposure Meters

There are two basic types of meters. * Reflected-light meters * Incident-light meters Reflected-light Meters The meter that is built into your camera is a reflected-light meter. That means it's designed to read light that is reflected off the subject that you plan to photograph. Since the meter is built into the camera, it will read the light reflected off the objects that are in front of the camera. If you aim the meter at a bright subject, it will give a high reading. If you aim it at a dark subject, it will give a low reading. If you aim it a mixed scene of light and dark subjects, it will give a reading of the average overall brightness in the scene. The reading will be the same whether you are shooting in color or black and white. Many cameras also provide a reflected-metering mode in which the meter reads a very narrow area of light-perhaps just one or two degrees wide. Since the meter can be pointed at and read just a small area in front of the camera, it's called a spot meter. Spot metering can be extremely useful when you want to meter just a small area of the scene. However, if you do not know what you are doing, a spot meter can steer you in the wrong direction. Every DSLR camera offers multi-segmented metering. This type of metering system goes by different names, depending on the manufacturer. Nikon, for example, calls it Matrix Metering, and Canon calls it Evaluative Metering. In effect, such meters divide the frame into many different zones. The meter reads the light in each zone and feeds the information into computer chips, which give weight to each of the readings and determine the right exposure. They do an astounding job, but, despite their sophistication, such meters can still only guess what you want. What do you want properly exposed-your subject's face or the sunset sky? The meter can't decide for you...even a multi-segmented meter.

Automatic Exposure (AE)

To determine exposure, you use an exposure meter. Almost all cameras have a built-in exposure meter, which also offers you the benefit of Automatic Exposure. Today, virtually all point-and-shoot cameras and DSLRs have Automatic Exposure. In theory, all you do is point the lens toward your subject and press the shutter button. The built-in meter does the rest, calculating the "right" exposure and causing the camera to set the appropriate aperture and shutter speed. So far, so good. Unfortunately, what the built-in meter considers as the "right" exposure is often wrong. Why? Because the meter can't think. Only you can think. Do you want the subject's face to be properly exposed, or do you want to silhouette the face and expose the sky to capture the drama of the sunset? Even when some camera software offers "face recognition" and other suggestions, the AE meter system can only guess what you want to achieve. Only you know what you want, and in this Lesson you are going to learn how to use your camera's built-in metering system to ensure that you get the exposure you want every time.

What Is Exposure?

We don't expose properly, we either overexpose the scene, causing it to be light and "washed out" or underexposed, causing it to be dark. And we made this error even though our camera provided us with the latest foolproof, built-in exposure meter and automatic exposure. What can go wrong with these "goof-proof" metering systems? The answer is usually simple. The meter didn't go wrong. We did! The meter will read anything we point at it. But the trick is to know where to point the meter and how to interpret its reading. That's where you come in. The smartest meter in the world can't make a perfect exposure unless you-the photographer-point it at the right part of the scene and then use its reading intelligently. The trick is to know how to expose correctly. In a digital camera when you make the exposure, each pixel on the image sensor records the brightness of the light that falls on it by accumulating an electrical charge. The more light that hits a pixel, the higher the charge it records. Pixels capturing light from highlights in the scene will have high charges; those capturing light from shadow areas will have low charges.

Lesson 6 - Exposure

Your Equipment Will Determine How You Expose Your Photographs. If you have a DSLR or digital point-and-shoot camera, getting the proper exposure for a photograph can be accomplished in several different ways. Some photographers still place their faith in the exposure meter in the camera or a hand-held meter, while others adjust the exposure based on viewing results on the camera's electronic viewfinder or on a monitor. How you choose to work will depend on the type of equipment you are using, the type of subject matter you are photographing, and your preference. Today, there is no single correct way of achieving correct exposure. If you use a digital camera, you will have more options, but to some extent they will be dictated by the nature of your subject matter. The landscape photographer can take several images of the same scene and vary the exposure slightly in each (bracketing), confident that back in the studio there will be time to pick the image that has the best overall exposure. Both digital and film photographers have the option to improve the appearance of the image. The best way to achieve good exposure is to get it right in the camera. The first step is to understand what constitutes correct exposure. After that, we will review how traditional metering can help you correct exposure. After that, we'll look at the "shoot, review, and adjust" options open to photographers using a digital camera and the flexibility offered by image-editing software. The path to good exposure using a digital camera is as simple as a three-legged stool: 1) Shutter Speed 2) Aperture Size 3) ISO

Program Shift

Your camera may have a very handy feature called Program Shift. When you are in that mode, you can change the shutter speed/aperture combination. Suppose your camera's meter gives you a reading that causes the camera to select an aperture/shutter speed combination of f/8 at 1/250 sec, but you want a different shutter speed or aperture and still want to give the sensor the same amount of exposure as the f/8 at 1/250 sec recommended by the camera. Turning the control and you can temporarily switch the exposure to f/5.6 at 1/500 sec, to f/4 at 1/1000 sec, or f/11 at 1/125 sec, f/16 at 1/60 sec, or any other combination that gives the same exposure as f/8 at 1/250 sec. While f/5.6 at 1/500 sec will give the same exposure as f/8 at 1/250 sec, a shutter speed that is twice as fast may help freeze a fast-moving subject in a sports photo. Similarly, f/16 at 1/60 sec might give you the maximum depth of field for a landscape. If f/8 at 1/60 is the correct exposure for a given scene, other combinations will include f/5.6 at 1/125, f/4 at 1/250, and f/2.8 at 1/500, as well as f/11 at 1/30 and f/16 at 1/15. All these combinations transmit the same amount of light to the sensor inside the camera. Equivalent Exposures: Aperture (f-stop Shutter Speed f/2.8 1/500 f/4 1/250 f/5.6 1/125 f/8 1/60 f/11 1/30 f/16 1/15 What are the two other factors that affect exposure? The first factor is ISO. If you change the ISO setting then the correct exposure combination will change, as will the equivalent exposures. So, if f/8 at 1/60 is the correct exposure for a given scene at ISO 100, and you change the ISO to 200, then f/8 at 1/125 becomes the correct exposure. That is why we suggest you vary ISO as the last step in setting your exposure. The second factor is the illumination of the scene.