Autofocus and Snow

[VidThreeNorth]

Sometimes I am using a camera with autofocus in snow and the focus jumps around somewhat randomly, and often too close for a particular composition. The obvious solution is to switch to manual focus, but in some cases, you don't have that choice. What I have found is that it helps to find a large object to include in the composition. If this works then the autofocus will stay on the object and ignore the snow. I run into it mainly for video, but it can happen in stills. I have made a video demonstration:

"240215 Toronto Snow Test Clips",
posted Feb 28, 2024, [length 2:36]
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[cgw115 140]

The missing word here is "contrast."

 

[wfooshee3 2]

Also you need to consider that completely different systems are used to AF video compared to the AF system in normal photography. The "good" AF system for photography is up in the pentaprism area of the camera, and those sensors are blind during video shooting because the mirror is up. The contrast-based AF used by video is purely in software, with no sensors involved, and is not as accurate nor as fast as the phase-detection system used by the sensor up around the pentaprism for normal photography. Using continuous-AF in video will result in focus hunting.

Some cameras, like Sony, use a mirror that is not 100% reflective to send light into the pentaprism, but allow exposure on the sensor simultaneously. The benefit is quieter operation, but the downside is reduced light in the pentaprism and sensor both.