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Reading Light Levels with an SLR Camera

Understanding how to read light levels with an SLR camera can be crucial for photographers, particularly when it comes to assessing your photography environment or optimizing your growing space. This guide will walk you through the process of measuring light levels using your SLR camera in manual mode.

Step-by-Step Guide​

  1. Prepare your camera: Set your camera's ASA speed to 200 and the shutter speed to 1/125.
  2. Position the white card: In the area you want to measure, hold a large white card or sheet of paper to receive maximum illumination. Ensure that only the white card is visible in the viewfinder.
  3. Adjust the f-stop: Change the f-stop on your camera or lens until the camera meter indicates a correct exposure. The f-stop will correspond to a specific light level in foot-candles, as follows:
  • f4 = 64 foot-candles
  • f5.6 = 125 foot-candles
  • f8 = 250 foot-candles
  • f11 = 500 foot-candles
  • f16 = 1000 foot-candles
  • f22 = 2000 foot-candles
  • f32 = 4000 foot-candles

Practical Applications​

By following the steps above, you can use this technique to:

  • Identify "hot" spots: Place your youngest plants in these areas for optimal growth.
  • Locate "weak" spots: Install reflectors as needed to increase light levels in these areas.
  • Monitor bulb performance: Determine when bulbs are losing strength, rather than relying on subjective visual assessments.
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