Do hydrophobic filter coatings help prevent lens dew or condensation?
Asked 10/29/2022
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Do hydrophobic or "water-repellent" coatings on filters, such as Hoya HD3 or EVO, reduce or prevent condensation/dew on a lens or filter? I'm especially asking about night shooting/astrophotography, where the front element can fog up. Do these coatings meaningfully help, or do they mainly just make the surface easier to clean?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
3y ago
2 Answers
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Since you placed an astrophotography tag on your question, I'll assume you are experiencing fogging while shooting the night skies.
You'll get fogging/beading even with hydrophobic coatings as Tetsujin stated. Problem is, the front lens element is being cooled by pointing it up at a cold sky (radiative cooling) which can cool the front lens element below the dew point. This is the same reason why you get frost on car windscreens that see the sky even when the air temperature is a few degrees above freezing.
Common solution to this problem is place a warming jacket around the front of the lens to keep the front element temperature above the dew point. This can be a purpose-built battery powered unit or wrapping a couple disposable hand warmer packets around the lens (hold it in place with a repurposed sock that has the toe removed).
Originally by user98537. Source · Licensed CC BY-SA 4.0
user98537
3y ago
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Not really. Hydrophobic coatings do not eliminate dew or condensation when the glass cools below the air’s dew point. In night shooting, especially astrophotography, the front element can radiatively cool while pointed at the sky, making fogging likely even if the air temperature is above freezing.
A water-repellent coating may change how moisture behaves on the surface: instead of a uniform smear, it may form more distinct beads that can shed more easily with airflow. But it can still fog or bead, so it’s not a reliable prevention method.
In practice, these coatings are mainly beneficial because they make the surface easier to clean and may help water bead rather than smear. They are not a substitute for anti-dew measures.
The usual solution is to keep the front element slightly warmer than the dew point, typically with a lens dew heater/warming band or even hand warmers secured around the lens.
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