A Question About Lens Selection and Camera Placement (Optics)

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A Question About Lens Selection and Camera Placement (Optics)

Postby BiggieCheese » Thu Jan 24, 2019 2:00 am UTC

Hello everyone. I don't have any experience with optics, but a project might be coming up at work that involves making a camera adapter and I want to get a head start and familiarize myself with the theory first.

Basically, I'd need to find a way to adapt a camera to take video through a borescope. Camera-borescope adapters exist, but I have some very strict regulations to deal with, so the adapter has to be custom made. The problem is that the camera and lens combo I'd likely have to use don't necessarily have the specs to make this work easily. The lens has a very long MOD (minimum object distance), too long to effectively see through the borescope. The adapter would have to be 40cm long, and the video feed would likely just be a tiny circle of light at the end of a long, black tunnel :?

Here are some specs:

Camera sensor: 1/4" CMOS
Focal Length: 3.4mm
Aperture: F3.5
Back Focal Length: 3.4mm
Lens Field of View (D x H x V): 108° x 84.1° x 62.3°
Operation: Fixed iris, manual focus
Distortion: -33.56%
Minimum Object Distance: 40cm
Probable diameter of image from borescope: ~10mm

Here's a possible candidate for the borescope. The eyepiece and focusing ring would unfortunately have to be removed for my application, and replaced with the custom adapter.

My questions:

How would one go about selecting a second lens to "zoom" in on the borescope and produce a clear image? I have a bunch of given info, and I bet there's a set of theories/equations out there that would point to a good choice. I've been smurfing around the web but I'm definitely missing some information.

Is the MOD value set in stone? I've played around with this camera before, and it was possible to see things with some clarity at distances less than the specified MOD of 40cm.

Am I over thinking this? Would placing the camera in such a way that its field of view is filled by the image from the borescope actually work, its technically focusing 40cm down the scope?

Thank you in advance kind strangers. If this project doesn't fall through, I'll still know more than I did before

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Re: A Question About Lens Selection and Camera Placement (Optics)

Postby DavidSh » Thu Jan 24, 2019 4:15 pm UTC

The borescope looks like a specialized version of a traditional compound microscope. There is an objective lens at the tip which casts an enlarged image, close to the eyepiece, of the object being viewed. The enlargement factor is the ratio of the distances from the objective lens to the image and to the object. One way of doing microphotography is to cast the image directly on the sensor. For example, if you had a film camera, you would take the lens off, attach the camera to the microscope, and adjust distances until the image is in focus. It sounds like you can't do that with your video camera.

A second way of doing microphotography is to directly take a photo of the image cast by the objective lens. It sounds like this image is also too small for your video camera -- you can't focus the camera close enough to bring the image close enough to the camera for the image to be of reasonable apparent size.

A third way is to place a converging lens between the image and the camera, creating a virtual image that is further away from the camera. You can even make the image appear to be at an infinite distance. Then the video camera will have no difficulty focusing, no matter how close to the device you place your camera. The eyepiece on the borescope can serve as such a converging lens. If, for some reason, you must remove the eyepiece, you should use a converging lens of similar diameter and focal length to the eyepiece. I don't have time to run the numbers/do the geometry now.

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Re: A Question About Lens Selection and Camera Placement (Optics)

Postby BiggieCheese » Thu Jan 24, 2019 8:14 pm UTC

Thanks for pointing me in the right direction!

I'm exploring the possibility getting the camera module without the integrated lens.

Assuming the size of the image coming from the borescope is 10mm in diameter, I searched online and found this lens with a 25mm focal length. Using these handy tables provided by the manufacturer, I figured that at a 1.6" working distance, it will be able to watch a .2" x .3" area (5.08 x 7.62mm), which almost perfectly scribes a rectangle in the 10mm image. And the sensor, of course, would be mounted at the 25mm focal distance.

Anyway, this is all purely theoretical, since I don't know what size image will be coming from the borescope, or where it would be located. I just wanted to go through the process of lens selection and distance determination.

If the image from the borescope is an order of magnitude smaller at 1mm, for example, would the correct choice be to use a 4x microscope objective? If the camera sensor is a 1/4" CMOS chip, the diagonal length of the sensor is 4mm, so 4x magnification seems perfect. Would a PLAN objective lens (not infinity corrected) be able to beam an image directly onto the sensor?

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