At long exposures, film needs more exposure than your meter suggests due to reciprocity failure. Calculate the corrected exposure time for your film stock.
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corrected exposure time
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extra stops
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factor increase
Correction Table for Selected Film
📷 What is reciprocity failure? The "Reciprocity Law" states that exposure = intensity × time. At very short or very long exposures, this law breaks down. At long exposures, film sensitivity effectively decreases — you need significantly more time than the meter indicates to achieve the same exposure.
🎨 Color negative vs slide film: Color negative films (Portra, Ektar) are more forgiving of reciprocity failure. Slide films like Velvia 50 have severe failure and often require correction even at 1 second. The purple/yellow colour casts at long exposures on Velvia are famous.
📱 No reciprocity failure in digital: Digital sensors do not suffer from reciprocity failure. A 2-minute exposure on a digital camera is exactly 4× the exposure of a 30-second shot. Use digital for very long exposures without correction concerns.
🔬 Velvia note: Fuji Velvia 50 has an exponent of ~1.73, meaning a metered 30s exposure needs ~150 seconds corrected. It also develops strong magenta or blue colour casts at long exposures. Many film photographers avoid Velvia for long-exposure night work.