Doesn’t that remove the narrow depth of focus the author is going for?
From an optical point of view, light does not bend differently just because you put a differently-sized rectangle somewhere in its path. Or to put it another way, if you cut the edges off your sensor, that won’t alter the image on the remaining area of the sensor.
Only in the sense that you generally use a smaller sensor because you want your camera to be small.
If you take a full frame SLR and attach a 100mm f1.8 lens to it, you’ll get a shallow depth of field. Now crop that image down to an area of the sensor corresponding to the size of a phone sensor, and the cropped image will have the exact same depth of field.
> Now, here's the kicker: the bigger the focusing lens is, the larger the cone of light rays is, meaning the the out of focus parts of the image will be more out of focus
From the page [0] it takes the depth of focus image from:
> [Depth of focus] differs from depth of field because it describes the distance over which light is focused at the camera's sensor, as opposed to the subject
[0] https://www.cambridgeincolour.com/tutorials/depth-of-field.h...
Depth of focus isn’t really relevant to the rendering of an image (except insofar as you want your camera to be built to sufficient tolerances that a sharp image can be obtained when desired).
I assumed you were using “depth of focus” to mean “depth of field”. If you really meant “depth of focus”, then I would say you are mistaken in thinking that the author’s goal is to obtain a narrow depth of focus.
FWIW, here is what Claude has to say:
>> Is depth of focus, as opposed to depth of field, generally relevant to practical photography?
> Not really, no. Depth of focus and depth of field are related but distinct concepts, and for practical photography, depth of field is what matters almost all the time.
> Depth of field refers to the zone in front of the camera where subjects appear acceptably sharp. This is what photographers think about constantly: choosing apertures to blur backgrounds in portraits, stopping down for landscapes to keep everything sharp, figuring out hyperfocal distance, etc.
> Depth of focus refers to the tolerance zone behind the lens, at the image plane (the sensor or film), within which the image remains acceptably sharp. It tells you how precisely the sensor needs to be positioned relative to the lens for focus to be maintained.
> For the overwhelming majority of photographers, depth of focus is invisible because it's a manufacturing and engineering concern, not a shooting concern. Camera makers deal with it when designing bodies and ensuring sensor flatness, lens mount tolerances, and autofocus calibration. You encounter it indirectly if you ever need to calibrate autofocus micro-adjustments, shim a lens, or diagnose back/front focus issues, but you don't actively manipulate it while composing a shot.