How do sensors "focus" on the bottom of the lens (flange distance)?

Question

I know that when a camera sensor is used with a lens there is a specific distance between the sensor and bottom of the lens (aka "Flange distance" or "flange focal distance") in a C-mount for example, this distance is 17.526mm.

I am wondering how is it that the sensor "sees" the image when it's projected and focused on to it while it doesn't pick up a sharp image of a tiny object or my finger if I'll touch it (theoretically).

If there is a gap between the sensor and the lens when it's on, how come the sensor doesn't pick up light from all around inside and get a blurry picture as it does when it's without a lens?

I am thinking that even if the image is "projected directly" on to the sensor and the image is perfectly focused on the sensor, there should still light coming from all over inside that space into the sensor.

And why when holding a C-mount lens 17.5mm above the surface of a table there is no sharp image projected on it?

Many Thanks in advance.

Answer 1

I am wondering how is it that the sensor "sees" the image when it's projected and focused on to it while it doesn't pick up a sharp image of a tiny object or my finger if I'll touch it (theoretically).

The sensor isn't "seeing" an image at the bottom end of the lens. Rather, the lens "bends" beams of light so that they all converge on the sensor. The sensor then converts that energy into electrical signals. If the lens is focused improperly (in relation to the object being photographed), then the image is formed either before or beyond the sensor surface, and the result is a blurred image.

If there is a gap between the sensor and the lens when it's on, how come the sensor doesn't pick up light from all around inside and get a blurry picture as it does when it's without a lens?

Because it is, or should be, otherwise dark in there. This is why cameras are closed boxes and painted black on the inside: to discourage stray light and reflections. (The word camera itself comes from camera obscura, literally "dark room".)

With old film cameras one sometimes has the problem that the structures that are meant to prevent stray light from entering the camera have degraded. One then gets light leaks, a topic often discussed here.

And why when holding a C-mount lens 17.5mm above the surface of a table there is no sharp image projected on it?

Probably because the lens wasn't properly focused, or there was nothing to focus on, or there was too much stray light for the image to be discernible. But try this: hold the lens 17.5mm away from a wall and make sure there is some well-defined object on the opposing wall. With proper focusing, an image of the object will be projected on the first wall.

Answer 2

The flange to sensor distance of 17.526mm is unique to ensure the correct mounting of this lens for lens and camera bodies associated with a type “c” mount system.

One key point: Where are the measuring points located? Every complex camera lens has two cardinal measuring points. We use these to ascertain focal length and object distance. From the front nodal we measure object distance. From the rear nodal we measure focal length and distance lens to focused image.

These two cardinal points are chosen by the lens designer based on need. They may at the center of the lens array but more likely they are separated. They can actually be inverted. They may fall in air forward or rearward of the barrel. They are not normally published. Their location can be discovered but this will be tricky.

The space between the last lens element and the image sensor is called the “back focus”. The lens maker adjusts the position of the two cardinal points so that clearance is provided for swinging mirror and/or focusing mechanism. The length of the lens barrel can be artificially shortened by the positioning of the cardinal points. This makes the lens less awkward.

The lens acts like a projector lens in that it projects an image of the outside world onto the image imager. You can view this projected image if you hold the lens so that it projects an image of a bright object onto a white paper screen.

Stray light does bath the projected image despite the fact that the camera body is equipped with an image mask and image baffles. This stray light mainly reduces image contrast. This stray light can induce artifacts we call flare and ghost images. The lenses are coated with a thin transparent mineral coat that greatly reduces stray light creation.

You cannot produce an image on the image sensor by holding something in close proximity to it. You can produce vague mages called shadowgrams, and you can see artifacts which are shadows casts by objects on the cover glass of the image senior. Generally, these are nuisances rather than image that are art forms.