How to read light level with an SLR camera?

Advanced Techniques GrowFAQ
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Contributed by: ngc7579 submitted February 15, 2003

How to read light level with an SLR camera?

Using your SLR camera in manual mode, you can measure light level:

  • Set camera ASA speed to 200.
  • Set camera shutter speed to 1/125.
  • Hold a large, white card (or sheet of paper) in the proposed plant location so that it gets max illumination.
  • Be sure that nothing but the white card is showing in the viewfinder.
  • Adjust f-stop on camera (or lens) until camera meter reads "correct exposure"

  • f4 = 64 foot-candles
    f5.6 = 125 foot-candles
    f8 = 250 foot-candles
    f11 = 500
    f16 = 1000
    f22 = 2000
    f32 = 4000

    I use this technique to:

  • Determine "hot" spots in the room - I put my youngest plants there.
  • Determine "weak" spots - install reflectors as needed to bring up light levels.
  • Determine when bulbs are getting weak - instead of "it looks dimmer than before".
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