How to Use a Camera to Measure Foot-Candles of Light for Houseplants

Do you love both photography and houseplants? If so, did you know that you can easily use your camera to measure the intensity of the light falling on your plants?

In the United States, optimal light intensity for houseplants is often discussed in foot-candles (and for those of you more familiar with lux, 1 foot-candle is about 10.76 lux).

If you don’t want to buy a dedicated light meter for your houseplants (and haven’t needed one for your photography), you can also gauge light intensity using just your camera (if it has a built-in light meter and manual controls) and a sheet of ordinary white paper.

First, set your camera’s mode to aperture priority (Av), sensitivity to ISO 100 and aperture to f/8.

Next, hold the sheet of white paper right in front of a plant’s leaves so that the light is hitting the sheet of paper instead of the plant. Hold the camera in front of the sheet of paper (not further than the width of the paper) while doing your best not to cast any shadows.

Now meter with your camera and see what it recommends as the shutter speed for correct exposure. Use the chart below to convert the result to foot-candles:

These numbers are based on a handy online calculator by the store First Rays. You can use the calculator if you’d like to figure out foot-candles for different combinations of ISO, aperture, and shutter speed.

There you have it: a quick way to measure foot-candles using the camera you already have. Happy houseplant growing (and photo shooting)!

Image credits: Header graphic based on illustration in Foliage House Plants (1977). Paper photo by 1681551.