Friday, March 1, 2024

Calculating Your Low Level Lighting Exposure

LED Panel lights at Fairyland Point, Bryce Canyon

How to Calculate the Proper Intensity of Your Low Level Lighting

Calculating Your Exposure: Your exposure is actually determined by your sky exposure, which is often something like f/2.8 for 30 seconds at ISO 6400. Since you want your foreground landscape to match that exposure, what you are really trying to do is place an amount of illumination on the foreground that will look natural with your sky exposure. Because it is a night scene, your foreground should be about two stops darker than a daytime foreground.

LED panel lights are one of the most popular Low Level Lighting sources. Because of the “Inverse Square Law” of light, a non-focused light, like a panel light, will significantly lose illuminance (the amount of light reaching the subject) the further it is placed from the landscape foreground. For instance, a typical panel light might give you a 500 lux reading one meter from the light, but place it only two meters and the illuminance drops to only 125 lux, or 1/4 as much (500 divided by the square of 2). Move the light 328 feet away (100 meters) and the luminance drops to 0.05 lux (500 divided the product of 100 times 100). At this distance, you are going to have to rotate the power dimmer switch to about 1/4 power or less to get a proper exposure. A Low Level Lighting of about 0.01 lux illuminance (which is about halfway between 0.008 lux and 0.02 lux) is a good starting point.

Is there an exact method to getting the right exposure for each landscape location? Yes and no. Lux light meters that can take accurate readings in the 0.1 to 0.001 range are scientific instruments that can cost thousands of dollars. Even if you bought and used one in the field, each landscape situation is different and will still require some exposure adjustments to suit your tastes. Using one of the following three methods, I rarely have to make more than one or two light intensity adjustments before I am satisfied with my results:

Lighting Intensity Adjustment Method #1: The Visual Method. Once you have placed your lights at the proper angle and distance away from your landscape foreground, begin to lower the power adjustment switch until you can barely see features against the night sky (remember, you’re trying to mimic weak moonlight, not the light of a full moon). Due to glare from the light, you’ll want to shield your eyes with your other hand, or even better, stand about 30 feet (about 10 meters) to the side of the light and have a partner adjust the light intensity while you call out the adjustments. This visual adjustment is the fastest and easiest method, and you’ll find it to be quite accurate as you gain experience—so accurate that you’ll rarely have to go back and fine tune the intensity more than once or twice. 

Lighting Intensity Adjustment Method #2: The Distance Method. This method requires you to have had some verified experience with the visual method. Once you have had success with the first method, record the power setting and the distance. Keep a record of other distance and power setting combinations. Eventually, you will have power settings for several distances and can interpolate the settings for in between distances. For instance, if your first successful light set up was at 100 paces, and you used the 10% power setting, and your new foreground distance is 200 paces away, you would need four times more light intensity, or 40% power. Undoubtedly, you will still need to make one or two fine tune adjustments.

Lighting Intensity Adjustment Method #3
: Inexpensive Light Meter Method. There are inexpensive lux light meters (about $15-25) that are fairly accurate within to 20 to 2000 lux range. These can be used to measure the light close to light source (within the meter’s range of accuracy) and then interpolated for the distance at the landscape foreground. For example, let’s say your light is 200 paces from the foreground landscape, and you want to obtain a good LLL starting point of about 0.01 lux illuminance at the landscape. To get an accurate meter reading, I suggest you move about 1/50 of your total distance away from the light, which in this case would be four paces (200 divided by 50 is 4). Now, hold your meter in the light path, facing the light and check the meter value. Since the distance from the landscape at your meter reading is 50 times greater than the distance from the meter reading position to the light, the meter reading at four paces needs to be 2500 times greater (50 squared) than at the landscape or 25 lux (2500 x 0.01). In summary, to get that 0.01 lux value at the landscape, adjust your light’s power until your meter reads 25 lux at the four paces position. After making a test exposure, you can make any additional adjustments to your tastes.

Please note that you could have chosen a meter reading position that was only two paces from the light (or 100th of the total distance from the landscape). In this case, the meter reading would need to be 100 lux (100 squared [10,000] times 0.01).


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