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Exposure Basics

Light Quantity

The magic of photography lies in the light. Light can do more than make an image on film or on digital media. Light can emphasize, subdue or alter moods. It can help you say many things about your subject beyound the physical shapes of the objects in the frame.

The first property of light to deal with is intensity or brightness (quantity). The meter in the camera measures this and converts it into
f-stops (aperture) on the lens and shutter speeds that control how long light enters the camera.  The key to understanding intensity is making sure that there is enough light to record an image on the light sensor. Too little light and the image is dark and too much light and it is white and washed out.

The light that enters our camera lens is either direct light from a light source such as the sun or light bulb OR it is reflected light that has come from light that has bounced off of people or things. This reflected light is called ambient light or available light.

The light meter measures the light that it sees and must convert it into a proper exposure. Most cameras use an average of all of the light that falls on the frame of the camera while some select just a spot in the photo to measure the light. This quantity of light is converted into aperture and shutter speeds based on the sensitivity of the film (ISO) which determiens the amount of light needed to convert that light into a neutral gray on the film.

grayscaleNeutral gray represents a shade of gray that is half way between white and black. It is a shade of gray that is the same on a negative as it is in real life and is sometimes referred to as 18% gray since it reflects back 18% of the light that hits it.

This is the shade that all light meters aim to convert the exposure into. Sometimes the average type light meter makes a mistake if it is aimed at a scene that has a bright spot in the frame that throws the average higher or a dark spot that throws the average lower. A snow scene or a night shot or concert shot will fool a light meter. To cure this some photographers use a spot meter which measures only a small portion of a scene or frame or they take a meter reading of a gray card and adjust the camera in manual mode. We will spend more time on this subject in future lessons.


Exposure Factors

What are the elements of exposure that a photographer must be aware of? The following are considered key:
  • Film Type or Camera Sensitivity measured in ISO
  • Quantity of Light
  • Shutter
  • Aperture
Film Type before the digital age this was the start of the challenge of making an exposure. Film, which is based on silver halide chemicals, can be made more sensitive to light by making the grains larger and adding additives to the emulsion. In digital the sensor is made sensitive electronically but still deals with issues of picture sharpness. Larger gain in film means grain in your final photo which was not good for display but not a problem if the image was put in a magazine where grain was not visible because of the way they print on press.

Film sensitivity was measured at first by a number called ASA which stood for American Standards Association which did the measuring. They gave film a number with most normal film at ASA 100. A number of 200 let in twice as much light as 100. In general a higher number means the film is more sensitive to light and thus needing less light to get an exposure. An ASA of 1600 was possible with just grain size changes in the manufacture of the film but to get to ASA 3200 or more (which was about as high as you could go) required special chemicals to process the film. Color film never went to high ASA settings much above 400 which is why Black and White was the film of choice for most photo journalists.

In the digital age ASA has been changed to ISO and represents International Standards Organization as the group that sets the standard. It is the same number and same meaning as ASA so you may hear older photographers use ASA as that is what they grew up with. Film Type then was the first decision for the photographer. Where would the images be viewed? If in a gallery then use a low speed film but if in a book then move to the higher ASA like 400 for Tri-X which will get good shots indoors or out. Work that was going into a magazine was always shot in Kodachrome slides because of the quality of color. We will talk about color later in the class. In digital our first decision is what ISO will we set the camera to. The low numbers like 80 or 100 force the camera to adjust letting in a lot of light to make a photo. High numbers like 800, 1600 or even 3200 take very little light to make an image but can result in other problems.


The Digital Age and Exposure

Now we are in the Digital Age and film sensitivity changes to camera sensitivity which can be adjusted for every photo and no longer fixed for a whole roll of film. Once an old camera had film placed in it there was no turning back. You had to shoot the whole roll. That is why photographers like Ansel Adams used a View Camera that shot one sheet at a time. He could record the exposure data and then adjust the processing in the darkroom one sheet at a time to match the best photographic printing conditions. Now it is the electronic circuit and software and hardware that set the sensitivity. This can be adjusted one shot at a time.

The grain issue is gone because all cameras use a
PIXEL element as their minimum point to records data and these have no effect on grain in the photo. BUT now we add something new. NOISE. This is when the photo picks up errors in recording and puts in pixels that are the wrong color in an image causing spots. The noise is an electronic and hardware problem and actually can be reduced if the camera is COLD or by special software in the camera. The noise is most visible in the shadows so photography outside in normal lighting is not a problem. Noise is not a problem at an ISO at or below 200. The longer the shutter speed the more noise is likely. A time exposure at night is likely to create noise. But, the good news is many quality cameras today use electronic circuitry to remove or minimize the noise. Today the photographer still needs to decide what ISO to set the camera to but that setting can change from one photo to the next. For film to change ISO meant the photographer had another camera in the bag to carry. How does the digital photographer decide on ISO -- mostly based on the shutter and aperture.

Quantity of Light is beyond control of the photographer unless we move to adding a flash or spot light which we will not discuss at this point. SO, the important thing is that we have a good
light meter that can measure that light and advise us on exposure based on that quantity. Since the 1990's every good camera now comes with a built in metering system. Because this is part of the camera many photographers forget about lighting and exposure. Don't!

shutterSHUTTER is the first camera control of importance to the photographer. This controls how long the camera has light entering into it. In film cameras the shutter also had an effect on camera handling. If the camera was a range finder or viewfinder type the shutter was in the lens and was very quiet. Most compact digital cameras still have the shutter located in the lens. For the SLR the shutter is a curtain with a slit in it that moves across the film or sensor. This is called a
focal plane shutter. The size of the slit in the curtain determines shutter speed. Early SLR's had this curtain move horizontally across the film which meant that as the curtain moved on a subject that was moving quickly it could cause some distortion. In my early days of photography of race cars it was common to get oval shaped tires from this. Today all Focal Plain or Curtain shutters move vertically which seldom causes distortion.

The important point now is to understand that the shutter captures the movement of our subject. A slow shutter will allow a moving object to blur and a fast shutter will stop the motion. In some digital cameras there is not a mechanical shutter and the process of triggering the camera to accept light is done electronically. Regardless of how the process is accomplished the shutter is the part of the camera that starts and stops the exposure process. Because it is time related the shutter controls how much motion our subject will show in the final image. A short shutter time (fast) will not let the subject move very far and will thus seem to stop the motion.

APERTURE is the opening in the lens that lets light in. For many of the lower priced digital cameras this is a weak area. The aperture is measured in a number called f-stop which can be between f1.8 up to as high as f64 or more. Each number is a fraction so f2 is 1/2 and f16 is 1/16. The larger the number the smaller the opening and thus the less light let in. That is the exposure part of aperture but what does it do to our photo? An aperture of f2, which is a large hole or opening, causes the background of our photo to be out of focus. An aperture set to f16 of the same subject will cause the background to become sharp and clear. We call this depth of field. Lower cost digital cameras have a limited range of aperture and may only be adjustable between f5.6 and f8 causing very little control in depth. The higher priced digital SLR cameras like the Canon EOS 5D give greater control and resembles the controls film photographers grew to depend on for quality pictures.

The photo of the new moon at sunset was an exposure challenge. The sky was bright but the moon was just a little bit brighter. Setting the exposure required getting the sky just dark enough to let the moon stand out while still having some color. Now if it were possible to get the dark foreground to show up we would have a photo that captured what our eye actually saw. Often in photography what we see and what the final print displays is a lot different because our eye can manage a huge range of brightness and still see something. Film or digital sensors are limited in the range of light they can absorb so measurement of the light becomes critical to a good photograph.

For this shot the camera was first pointed to just the sky with about 25% of the viewfinder including the trees. The exposure was locked in by gently holding the shutter release down until the auto focus / auto exposure sets in. Holding the release there the camera was moved to make the composition shown here. Click - then we take the photo. Check it in the LCD screen.

Digital cameras respond to light much like color slide film did in the analog photography days. Slide film had less
exposure latitude or less ability to still record an image even though the exposure was off. In slide film or digital the exposure must be more accurate. But, in digital we can shoot and view and change the setting and try again. We can even change how it looks in the computer later on.

The following is from
Wikipedia and is an excellent article about light quantity or measurement.


Exposure metering

Almost any scene of photographic interest contains elements of different luminance; consequently, the “exposure” actually is many different exposures. The exposure time is the same for all elements, but the image brightness varies with the gray value of each subject element.

meterExposure is often determined using a reflected-light exposure meter. The earliest meters measured overall average luminance; meter calibration was established to give satisfactory exposures for typical outdoor scenes. However, if the part of a scene that is metered includes large areas of unusually high or low reflectance, or unusually large areas of highlight or shadow, the “effective” average reflectance may differ substantially from that of a “typical” scene, and the rendering may not be as desired. The meter illustrated on the right was my first meter. The meter actually measured foot candles. The number was then set using the slide rule calculator above it to determine the exposure. It took some serious thinking to operate it. It was designed to records the amount of light that was reflected back to it by the subject so it was held by the photographer and aimed at the subject.

A meter like this one does not know what it is looking at. They are designed to take the brightness value recorded by the meter and convert that into a neutral gray shade on a negative or print. If the meter is aimed at something that is neutral gray it will do a good job. Aim it at something bright or white and it will turn it gray. Snow will always be exposed wrong with this meter.

It is possible to make a meter reading of an individual scene element, but the exposure indicated by the meter will render that element as a medium gray; in the case of a dark object, that result is usually not what is desired. Even when metering individual scene elements, some adjustment of the indicated exposure is often needed if the metered scene element is to be rendered as visualized.

Early photographers carried a gray card that they would put up near their subject and then aim the meter at that card. Since the card and the subject had the same light falling on them the exposure could be set. In a pinch a photographer would use the human hand as it is close to neutral gray in value and usually the most important shade to capture in a photo.


Examples of metering modes

Cameras generally allow the user to select between spot, center-weighted average, or multi-zone metering modes.
Various metering modes are provided to allow the user to select the most appropriate one for use in a variety of lighting conditions.

Spot Meter: With spot metering, the camera will only measure a very small area of the scene (between 1-5% of the viewfinder area). This will typically be the very centre of the scene, but some cameras allow the user to select a different off-center spot, or to recompose by moving the camera after metering. A few models (including the Olympus OM-4 and Canon T90) support a Multi-Spot mode which allows multiple spot meter readings to be taken of a scene that are averaged. Both of those cameras and others also support metering of highlight and shadow areas.

Spot metering is very accurate and is not influenced by other areas in the frame. It is commonly used to shoot very high contrast scenes. For example, if the subject's back is being hit by the rising sun and the face is a lot darker than the bright halo around the subject's back and hairline (the subject is "backlit"), spot metering allows the photographer to measure the light bouncing off the subject's face and expose properly for that, instead of the much brighter light around the hairline. The area around the back and hairline will then become over-exposed. Spot metering is a method upon which the zone system depends.

Center-weighted average metering: In this system, the meter concentrates between 60 to 80 percent of the sensitivity towards the central part of the viewfinder. The balance is then "feathered" out towards the edges. Some cameras will allow the user to adjust the weight/balance of the central portion to the peripheral one. One advantage of this method is that it is less influenced by small areas that vary greatly in brightness at the edges of the viewfinder; as many subjects are in the central part of the frame, consistent results can be obtained.

Average Metering: In this metering mode the camera will use the light information coming from the entire scene and averages for the final exposure setting, giving no weighting to any particular portion of the metered area.

Partial Metering: This mode meters a larger area than spot metering (around 10-15% of the entire frame), and is generally used when very bright or very dark areas on the edges of the frame would otherwise influence the metering unduly. Like spot metering, some cameras can use variable points to take readings from, (in general autofocus points), or have a fixed point in the center of the viewfinder. Partial metering is found mostly on Canon cameras.

meterMulti-zone metering: This mode is also called matrix, evaluative, honeycomb, segment metering, or esp — (electro selective pattern) metering on some cameras. This metering mode was first introduced by the Nikon FA, where it was called Automatic Multi-Pattern metering. On a number of cameras this is the default/standard metering setting. Here the camera measures the light intensity in several points in the scene, and then combines the results to find the settings for the best exposure. How they are combined/calculated deviates from camera to camera. The actual number of zones used varies wildly, from several to over a thousand. However performance should not be concluded on the number of zones alone, or the layout. In general, the most advanced metering is found on single-lens reflex cameras.

The photo above shows the zones in a typical Canon camera. These are used to adjust focus as well as exposure. In this type of system the camera electronics will be aware of a bright background or a white object surrounded by dark gray objects. These meters will produce great results and for many situations that is all we need.

Many manufacturers are less than open about the exact calculations used to determine the exposure. A number of factors are taken into consideration, including the following: Autofocus (AF) point, distance to subject, areas in focus or out of focus, colors/hues of the scene, and backlighting. Multi-zone tends to bias its exposure towards the autofocus point being used (whilst taking into account other areas of the frame too), thus ensuring that the point of interest has been exposed for properly. A database of many thousands of exposures is pre-stored in the camera, and the processor can use a selective pattern to determine what is being photographed.

Some cameras allow the user to link or unlink the autofocus and metering, and allow the option of locking exposure once AF confirmation is achieved, AEL, (auto-exposure lock). Using manual focus, and on many compacts/bridge cameras, the AF point is not used as part of the exposure calculation, in such instances it is common for the metering to default to a central point in the viewfinder, using a pattern based off of that area. There is considerable variation from different manufacturers as to how multi-zone metering is implemented, and even from the same maker in their model range, and how much "priority" is given to the AF point itself. Some "Scene" modes, such as sunset, sports, night exposures etc, also often affect the calculations of this metering pattern.

However, some photographers may be uncomfortable with multi-zone metering. This tends to stem from a lack of clarity about "how" the camera reacts in certain situations. The design concept behind multi-zone is to reduce the need to use exposure compensation. Some users have problems in wide-angle shots with high contrast, due to the large area which can vary greatly in brightness. It is important to understand that even in this situation, the focus point can be critical to the overall exposure.


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