Originally Posted by
prathmann
If the aperture is set to f/2.8 you won't get very much depth of field at all. You need a really small aperture (say f/16) to get good depth of field.
If Curtis has access to a variety of lenses one approach would be to mount a second lens in front to change the effective focal length so the current focus position works right for far-away objects. You'd want to increase the focal length, so the additional lens in front should be a negative one, i.e. concave instead of convex.
That's true of the larger film formats and digital equivalents, but not applicable to the typical digital action/documentation video camera.
Most action video cameras use a tiny sensor, typically 1/2.5" or thereabouts. Combined with an ultra-wide lens for a field of view around 170 degrees or so, depth of field is virtually infinite with an f/2.8 aperture at the hyperfocal setting. The hyperfocal setting may be only 12" or less than a foot away, to get everything in reasonably sharp focus for video purposes.
While the typical action cam has an actual focal length lens around 3mm or so this isn't as useful for fisheye lenses as for rectilinear wide angle lenses. So most manufacturers specify the FOV. You'll notice that most action camera output is non-rectilinear when uncorrected, a near-fisheye effect with noticeably barrel-distorted edges. While this can be corrected in editing, the rectilinear correction results in cropping the edges. This might be desirable for artistic effects, but might lose crucial information in a documentary traffic cam.
On a teensy sensor like that an f/16 aperture would have the effect of a pinhole lens camera on film, with so much diffraction the image would be softer rather than sharper. There are no practical gains beyond around f/5.6 with a teensy sensor and ultra-wide lens. Even by f/5.6 diffraction is beginning to compromise resolution on a typical 5-6mm lens on a tiny sensor, so while many P&S digicams with teensy senors offer apertures smaller than f/5.6, these aren't often practical to use, other than rare situations where we might want a flower or bug at macro distance in focus along with a distant figure or horizon.
The optimal aperture for these teensy sensors would be closer to f/4 with a 5-6mm lens (roughly equivalent to a 28mm wide angle lens on a 35mm film/full frame digital camera). However the gains in DOF would be lost to increase noise in dim light. So f/2.8 is a reasonable compromise for DOF and nighttime performance in the typical ultra-wide lens and teensy sensor used on affordable action cams.
With this combination of factors -- tiny sensor, ultra-wide lens, fixed aperture around f/2.8 -- only a tiny bit of lens rotation is needed to adjust focus. It's likely vibration from normal use of the OP's camera jiggled the lens slightly, enough to move it away from the hyperfocal setting.
While it would be nice to have, say, a one-inch (or CX in Nikon parlance) sensor, for better resolution and low light performance, the cost would be significantly higher, probably starting at around $500 and more likely much higher. Lenses would be larger and mounting this mass more critical for vibration resistance. Smaller apertures would be needed for hyperfocal setting. This would offset some gains in low light performance. There probably isn't much market for this among bicyclists and hobbyists. If I'm recalling correctly only one or two high end Samsung phones have incorporated the larger 1-inch sensor. Most still use tiny sensors and improved processing to retain fine detail along with more elaborate noise reduction.