How do you estimate long-exposure time with an ND filter during rapidly changing light at sunrise or sunset?
Asked 7/6/2016
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When light is steady, it’s easy to calculate ND-filter exposure by adding the filter’s stops to a metered shutter speed. But around sunrise or sunset, light can change significantly during the exposure itself. For example, if the metered exposure is 1 second and a 10-stop ND suggests about 15 minutes, the scene may be much darker by the end of that exposure than at the beginning. Are there any practical formulas or methods for estimating exposure in these fast-changing conditions?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
1
Since the speed of the sun going below horizon depends on date, I suppose you could create a table measuring exposures starting before sunset and then for example every other minute up to 30 minutes. In the table include column with angle of the sun (2, 1, 0, -1, -2...). When you would be taking the actual shot, you could lookup actual angles of the sun at the time of shooting and estimate from the table what the decrease in light will be.
Originally by user27944. Source · Licensed CC BY-SA 4.0
user27944
10y ago
0
Generated from our catalog & community — verify before relying on it.
There isn’t a universal formula, because the rate of light change around sunrise and sunset depends on factors like date, location, and the sun’s angle below the horizon. A practical approach is to build your own reference table.
Measure the scene’s exposure at regular intervals—such as every minute or two—from before sunset into twilight (or before sunrise into daylight). Record the corresponding sun angle or time relative to sunrise/sunset along with the metered exposure. Over time, this gives you a usable chart for your location and shooting style.
When making a long ND exposure, check the current sun angle or time, look up the expected light falloff from your table, and estimate how much the light will change during the exposure. This won’t be perfect, but it’s a practical way to improve your estimates in changing light.
In short: no fixed predefined formula works everywhere; experience and a custom exposure table are the most reliable method.
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