Focus, Exposure, and Shutter Speeds Printing Knowledge
Focus, Exposure, and Shutter Speeds

Many digital cameras have auto focus and auto exposure modes that simplify the process of capturing the image. However, not all cameras offer the auto focus and auto exposure features, so it is best to consider the value of each for the intended purpose of the digital camera.

Focus Options

  • Fixed Focus:  Fixed focus is usually found in consumer level, lower priced cameras. A specific range of area is selected by the camera manufacturer to serve for the camera's focus. The range may be 5 feet and beyond for which the focus is fixed and cannot be altered.
  • Manual Focus:  a manual focus allows the user to adjust the lens to create the desired sharpness for both the foreground and background of the subject matter.
  • Auto Focus:  A selected area in the center of the lens is used as the target area around which the image is focused. When the subject is centered in the viewfinder, the auto focus adjusts the image clarity for the shot. If the auto focus cannot be manually altered, a user will be unable to sharply focus all aspects of the subject, especially if the subject is not centered in the focus.
  • Dynamic Focus:  Dynamic focus allows for the selection of focus positions that are not necessarily centered but may be to the right or left of the center of the viewfinder. With dynamic focus feature, the subject can be placed out of the center of the picture and yet remain in focus.
  • Tracked Focus:  Tracked focus is a feature that holds a subject in focus while the subject is in motion. The camera automatically tracks the speed and anticipates the direction of the subject's movement. The camera then adjusts the focus automatically for the action shot.

Exposure Options

If no manual intervention occurs, exposures can also be controlled automatically as the camera's internal light meter measures the amount of light reflected from the subject matter and surrounding area. The camera will then either digitally display the proper aperture and shutter speed setting so the user can make the adjustments, or the camera makes one of the adjustments but not the other, or all of the adjustments are made with fully automatic processes.

  • Aperture Priority:  Apeture Priority refers to the feature that will enable the user to control the depth of field or the clarity of the background by selecting the aperture setting. The shutter speed is then automatically set in accordance with the aperture setting to provide the best exposure.
  • Shutter Priority:  The shutter priority refers to the feature that will enable the user to control the shutter speed, keeping it open for longer exposures or closing it quickly to capture moving action. The aperture is then automatically set by the camera for the best exposure.
  • Fully Automatic:  Fully automatic refers to the feature that will set both the aperture and the shutter speed for the best exposure.

Focus and Exposure Locking

As the picture is taken, it is usually best to partially depress the shutter button to give the camera time to adjust (length of time varies on camera models) the exposure and focus settings. Shutter speeds can range from 30 seconds to 1/10000 of a second. Once the settings are locked in place, the user can then finish depressing the shutter to take the photo.

During the delay between pressing the shutter release and taking the picture, the camera performs a number of electronic tasks. It clears the CCD, corrects the white balance, meters and sets the exposure, focuses the image, and finally fires the flash (if needed) and then takes the picture. The camera then compresses the image and saves it in memory. Then, another picture cannot be taken until the camera completes the compression process, which may create a delay depending on the camera's processing speeds.

Light Sensing and Metering

For cameras using film, there are different types of film material available to provide the best color reproduction in specific lighting conditions. Since digital cameras do not use film, varying lighting conditions are controlled by the camera's exposure system instead of through the use of different film. It is important to understand the camera's exposure system in order to make sure the camera's lighting sensor is capable of handling a variety of lighting conditions that will be encountered.

  • By checking the camera's "equivalent ISO number", which is a measurement of the sensitivity of the lighting sensor to varying lighting conditions, images can be improved when taken in different levels of light.
  • The higher the ISO number, the better the image in low light and conversely, the lower the ISO number, the better the image in high or intense lighting.
  • ISO sensitivity settings can range from 200 to 800 on consumer level cameras and above 1000 on professional level models.
  • Additional options for improving image quality can be provided with cameras having a wide range of shutter speeds and lens apertures.
  • Check the number of modes available on the camera's internal or external flash and its distance range. This feature can be used to increase the lighting of objects within close range.

Lighting conditions are affected by two distinct factors: the quality of the light and the quantity of light. Insufficient light results in underexposure while excessive amounts of light will overexpose the image. To measure the light and make the proper adjustment for the proper exposure, digital cameras are equipped with metering systems to properly set the exposure. Metering systems include the following:

  • Spot or Partial Metering:  Light is measured in a small circular area around the center of the image. Spot metering will typically measure less than 4% of the image and partial metering will measure almost 10% of the image. Spot and partial metering is used most often for close-up photography or when there is a significant difference between the brightness of the foreground and the background.
  • Bottom-Weighted Metering:  The light is measured by averaging the tonal values around the lower half of the picture setting and adjusting to a middle gray value to create the proper exposure.
  • Center-Weighted Metering:  The light is measured by averaging the tonal values around the center of the subject matter and adjusting to a middle gray value in order to create the proper exposure. Most often the metering will measure 75% of the center of the image and 25% of the outer area. Center-weighted metering is used most often when the subject matter fills a majority of the space in the image area.
  • Matrix or Multi-Zone Metering:  A metering process which electronically analyzes data regarding brightness, contrast, distance, and focus in order to adjust exposures to provide improved accuracy for color reproduction.

Exposure Correction

Auto exposure systems can become unreliable in some settings where extreme brightness (direct sun and reflections on water) or very low light can cause the automatic features to over compensate for the measured light. To override the automatic exposure controls, some digital cameras have an exposure correction feature (+/- EV compensation) which can be used for manual settings that are required in low-light situations. With exposure correction or compensation, the camera can be set to allow more or less light, depending on the required setting. For someone who does not have much experience with correcting for low light or bright light situations, some cameras have an image display feature allowing the user to see the effect on the image as the settings are manually controlled.

White Balance

Light coming through the lens needs to be balanced in order to achieve natural or accurate coloration. Colors differ under various lighting conditions. Some colors appear brighter while others appear dull in the same setting. Cameras using roll film adjust for color conditions (interior, outdoors, bright sun, etc.) with different types of film or by using lens filters. On digital cameras, color adjustment occurs through the use of white balance adjustments. Proper white balance is achieved by extracting white light from the light that comes through the lens. By adjusting the white light in a setting to compensate for the ambient lighting, all of the colors that make up white light (the additive primaries red, green, and blue) are adjusted accordingly.

Tone Compensation

To insure that natural tones are reproduced accurately, some digital cameras provide a tone adjustment or tone curve control, which compensates for the brightness and contrast of the subject matter. The tone curve control feature determines the optimal tone curve for natural coloration and natural tones in order to produce the highest quality image possible under the lighting conditions of the setting.



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