"Focal Distance" / "Depth of field" of endocsopic and (car) backup cameras

Question

In a nutshell, my question is:
Why is the depth of field for endoscopic cameras so limited? It seems it is generally around 3 to 8cm (some are around 3 to 40cm). Why aren't they similar to automobile backup cameras that seem to have an "infinite" depth of field (or, at least, about 4cm to really far from the camera).

Out of the netshell:
First, it seems that my question could have aspects that may not be strictly photography related. If so, should it be posted on a different Stack?

Also, I am not a photographer, so my terms may not fit precise, strictly photographic (or other technical) usage. But, I think they're clear enough. However, just in case I'm not as clear as I think I am, here are a couple of terms and how I think of them:

"focal distance": The distance from the camera that is in focus. E.g., 8cm.

"depth of field": The range of distances from the camera that are in focus. E.g. 3 to 40 cm.

However, in product descriptions of the relativly inexpensive endoscopic cameras that attach to personal computers & smart phones, is seems that either term, "focal distance" or "depth of field", may be used to mean "depth of field" as I defined above.

Anyway, I ask because when I need to look in a wall or other tight spot, I don't want to have such a limited range of focus (the ~3 to 8cm that endoscopic cameras have). I've used my android device (an otherwise inactive Samsung S3) when the wall opening was large enough, and it provided really good images in focus over, if not infinity, most of the interior of the wall. But, it is also cumbersome at best for such an application (and usually not possible at all due to hole sizes). I've considered MacGyvering up a backup camera for endoscopic use, but they are quite a bit larger in diameter, and would again limit usage in tight spots. Plus, I'm no MacGyver...

Answer 1

There's only ever one perfectly-in-focus theoretical distance from the camera. But, because perfect isn't necessary to actually appear in focus, there's a broad range — depth of field, as you correctly define it — which is "good enough" for the viewer in a given situation. Because this is perceptual, the exact limits are fuzzy — you can read more about it here. What exactly determines depth of field?

The nutshell answer to your question is: because of how this all works out, apparent depth of field is larger when the "exact" in focus part is set to be further out. When you focus very close, depth of field is minimized.

The endoscopic cameras you are looking at are meant to focus very close — sometimes right at the tip of the device. They're meant for working in small areas. By contrast, car backup cameras are meant to perceive things relatively far — usually multiple feet — from the camera. So, they can be set to provide more effective depth of field.

Answer 2

Only one distance can be "in focus." Everything else is blurrier than the in focus distance to one degree or another. Just how blurry it is is determined by several factors. Among them:

• Distance, relative to the camera, from the in focus position. The closer something is to the focus distance, the sharper it will be. The further something is, the less in focus it will be.
• Focal length. Longer focal lengths provide less depth of field at the same focus distance, so less is in focus. Shorter/wider focal lengths provide more depth of field at the same focus distance, so more is in focus.
• Aperture. The narrower the lens' effective aperture, the more depth of field an image will have. The wider the effective aperture is, the more shallow the depth of field will be.

When one combines a very wide angle of view with a relatively narrow aperture, the hyperfocal distance can often stretch from just a few inches/centimeters in front of the camera to infinity. That makes such a setup very useful for things such as backup cameras. But such a camera is not very useful if one desires to have something only 2-3 centimeters in front of the camera's lens in focus.

In the case of your two purpose built cameras with fixed focal lengths, apertures, and focus distances:

• One is designed to focus on things very close in a tight environment that is probably not that well lit. This requires a very close focus distance combined with a relatively wide aperture. Thus, the depth of field is shallow.
• One is designed to focus on things several feet or more away (even the pavement directly beneath the backup camera is usually a couple of feet below where the camera is mounted) with more light (either natural or perhaps infrared provided by an emitter included with the camera). The angle of view is wider, focus distance is longer, and the aperture can be narrower. Thus, the depth of field is much deeper.

To put it another way: If you tried to use the car's backup camera in a tight spot where nothing is more than 5-8 centimeters away, everything would be just as out of focus as the endoscope camera is when you point it at something a couple of feet away.