Wednesday, February 20, 2008

Camera Settings

I’ve been reviewing the keywords that have been used when people find this website and noticed that there are some questions that are presented in these keywords that have not been answered previously. I thought it would be a good idea to pick one from time to time and explain the answer.

Keyword Search: “Best F-Stop Setting for Taking Footwear Impressions”

For those who don’t know, there are three main considerations when adjusting your camera’s settings for each photograph. They are the shutter speed, f-stop, and the ISO.

  • Shutter speed is measured in either seconds or fractions of a second. Anything longer than 1/60th of a second needs to be mounted on a tripod to prevent blurring of the photograph due to camera movement. 1/120th of a second is preferred for freehand photography. Faster shutter speeds will make it possible to photograph moving items with clarity, although this is rarely a consideration for crime scene photography.
  • F-stop is a setting for the aperture, or how far the shutter will open. The larger the f-stop number, the smaller the aperture. This causes some confusion for new photographers, but becomes second nature fairly quickly. Aperture determines the photograph’s depth of field. A shallow depth of field is ideal for portrait photography where you want the model’s face to be in sharp focus, but the background to be blurred thereby focusing the viewer’s eyes on the subject. A low f-stop (f2.8) will create this effect. This effect becomes amplified as the subject moves closer to the lens. For crime scene photography, this causes a problem when taking certain photos. A fingerprint on a curved surface needs to have a greater depth of field so that all portions of the fingerprint are in sharp focus. For this reason, a higher f-stop (f32) should be utilized. The same is true for photographing footwear or tire impressions.
  • The ISO determines the camera’s sensitivity to light. On 35mm cameras, this is equivalent to the film speed. A lower number (100) means less sensitivity to light. A higher number (3200) means a greater sensitivity to light. Generally speaking, a lower ISO produces a photograph with finer detail although this is not always true. With today’s high-end digital cameras, the ISO is used primarily to help achieve the desired shutter speed and f-stop.
The trick to determining these three settings is to understand how they all work collectively rather than independently. It’s all about getting a sufficient amount of light to the sensor or film inside of the camera. When the aperture is small, less light reaches the sensor. This can be compensated for by increasing the shutter speed or ISO. Likewise, a larger aperture might allow too much light into the camera which would require a decrease in the shutter speed or ISO. Changing one setting affects the others.

I prefer to determine which setting is the most critical for each photograph and then adjust the others accordingly. When photographing evidence, the aperture is usually the critical component due to the desire for depth of field. Many cameras in use today have a number of composite settings that make this task fairly simple. Most people use the “P” (program) setting which adjusts the big three automatically. Some use the “M” (manual) setting where the user has to set each of the big three manually. But there are also “S” (shutter speed priority) and “A” (aperture priority) settings which allow the user to set one and allow the camera to determine the other two to produce the preferred balance.

So, to answer the question presented by the keyword string…
- Use the highest f-stop possible with the available lighting.

Remember to place the camera on a tripod perpendicular to the surface of the impression and place a scale next to the impression. Use a remote shutter release to avoid shaking the camera or moving the tripod. Take an initial photograph with available lighting. Then take four more photographs using a handheld flash unit. For these photographs, hold the flash on one side of the impression at an oblique angle so that the light casts shadows in the impression. Do this for each of the four sides of the impression. This will give the examiner detailed information of the entire 3-dimensional surface. After you’ve completed the photographs, you’re ready to make a cast of the impression.