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I'm trying to set up a tiny camera for a tiny science experiment. The camera is an OmniVision OVM6211:

OVM6211

It's pretty much a 3mm cube, containing the image sensor and all the lenses. It has a 90º FOV and can focus from about 100mm.

What I need for the experiment is a much shorter focus distance of about 10mm - 20mm, and I thought this might be possible to do by adding another lens.

Tiny science experiment

However, I don't know a lot about lenses.

My questions:

  1. Is it possible to reduce the focus distance by 5-10 fold by using a single lens?
  2. What type of lens would I need? (What keywords should I be searching for?)
  3. Where might I get such a lens?

Notes:

  • In the experiment, the focus distance will be fixed.
  • I realise that the lens wouldn't plug into the camera, and I'd need to manufacture tiny parts to hold the lens in place accurately.
  • I'm not too worried about things like distortion or colour separation, although I would like the image to be in focus.
  • The camera needs to be able to see the whole of the object, and so needs most of its 90º field of view.
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  • \$\begingroup\$ Personally, I would use the terms "focal length" and "focus distance". If you start talking about "focal distance" it's less clear to me what you are referring to. \$\endgroup\$
    – osullic
    Commented Jul 6, 2018 at 12:45
  • \$\begingroup\$ What is the motivation to shorten the focus distance? My guess would be that there's either not enough space to place the camera 100mm away from the object or that you need to focus this close because you want more resolution. \$\endgroup\$
    – null
    Commented Jul 6, 2018 at 16:09
  • \$\begingroup\$ @null - It's going to go into a small chamber, where it's going to watch what's happening. \$\endgroup\$
    – Rocketmagnet
    Commented Jul 6, 2018 at 16:28

4 Answers 4

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You can mount a positive lens (convex) before the camera lens. Such a combination will allow close focusing. You can experiment using inexpensive reading glasses purchased from a drugstore. Yesterday I saw a display at my local dollar store. They were priced at $1.00. You can just hand hold them before your camera’s lens to test.

The power of reading eyeglasses is the Diopter unit of focal length. A lens labeled +1 is 1d (1 diopter). The diopter is easily converted to millimeters. 1d = 1/1 X 1000= 1000 (1000mm). If you mount a 1d before your camera, the maximum subject distance becomes 1000mm. Mount a 0.5d and the maximum is 1/0.5 X 1000 = 2000mm. Mount a 2d = 1/2 X 1000 = 500mm. Mount a 3d = 1/3 X 1000 = 333mm. Mount a 4d = 1/4 X 1000 = 250mm Mount a 5d = 200mm.

You can search the web for camera close-up lenses. You will find what you need at Edmund Scientific.

Because a +10 close-up will only get you to 100mm, you can use the more powerful magnifying glasses. A 10X magnifier has a focal length of 1 inch = 25mm. In diopters that’s 1/25 X 1000 = 40d. Mount a 10X before you camera lens and the far focus point becomes 25mm. Mount a 20X, this is a 50d. The far focus distance becomes 1/50 X 1000 = 20mm.

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Any converging lens will bring you closer to the target, you need to find a small magnifying glass or loupe (available online everywhere for a dollar or two).

In fact, the conditions are equivalent to a human eye trying to focus at close distance. Test the lens by placing it between your eye and the subject, see if you can focus from 10-20mm.

Small, good quality lenses can be found in some optical accessories, here I'm disassembling the SkyWatcher telescope eyepiece (10mm focal length Plossl, consists of 2 tiny achromatic lenses).

disassembling the SkyWatcher 10mm Plossl eyepiece

Now about the image quality... I doubt it can be perfect using such simple setup, try and see if it is good enough for your purposes.

btw. I'm doing this all the time, doing macro shots using my old fixed focus webcam.

common jeweler loupe

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    \$\begingroup\$ Thanks for the answer. Perhaps I should have emphasized the word "tiny". That loupe is gigantic compared the the size of the camera. And it's thickness takes up pretty much all of my 10mm - 20mm focus distance. Also, won't that narrow the field of view? \$\endgroup\$
    – Rocketmagnet
    Commented Jul 6, 2018 at 13:25
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    \$\begingroup\$ Decreased field of view is the price to pay for decreasing focus distance. You can search for "biconvex lens" or take the camera apart and increase the distance of the existing lens from the sensor. \$\endgroup\$
    – xiota
    Commented Jul 6, 2018 at 14:09
  • \$\begingroup\$ the apparent FOV is not necessarily reduced, simply make sure your additional lens diameter is large enough. single lenses, however, are known to limit the image quality further from the center. for both good quality and large FOV you need more complex optical system, which is probably not practical in your case. it would be nice to make your camera lens focusable, as advised by xiota, but the camera assembly seems so tiny and integrated that it is probably not practical, either :-( \$\endgroup\$
    – szulat
    Commented Jul 6, 2018 at 14:28
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You might be able to wrap the 100mm focusing distance around the object with mirrors.

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I've had good luck with molded plastic aspherical lens. Edmund Optics is an excellent source:

https://www.edmundoptics.com/f/Precision-Molded-Aspheric-Lenses/14752/

They have lenses as small as 1 mm diameter although there are more options around 5 to 6mm.

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