Optics - Lens Selection / Geometry

In summary, the conversation discusses the issue of blurry edges in images taken with a RaspberryPi camera using a CS lens from the security camera industry. The individual has tried various methods to improve the focus, including modifying the lens mount and comparing it to a camera with a similar lens. They are also seeking advice on improving the image sharpness and considering the possibility of using a different lens.
  • #1
ElectricPaint
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0
Hello everyone!

I recently bought a small raspberryPi board, with its small "camera board".
(link for camera reference)

I want to use a CS lens on this camera (CS lens from the security camera industry).
I have been able to attach the lens, and it focuses very good, yet not "excellent".

Specifically, the center of the image is focused, yet the edges are blurry.
I will go down the list of things I have done to provide some background:

- The raspberry pi camera alone works awesome. Very good image quality.
- There is a seller in eBay that sells his own "rPi camera board" with a close range CS lens.
- I ordered this camera board and took it apart.
- The CS lens mount is made out of metal.

When I installed a longer range CS lens on this mount, the image was blurry. I could not get it to "focus" with sharpness.
Trying things out, I removed some material from the top part of the mount - this makes the CS lens sit CLOSER to the image sensor.
With this setup, I was able to get the long range CS lens to focus very good.
Yet, as best as I could get it, the "edges" of the image are out of focus.

Now is where I have doubts:
What else can I do to make the image sharp?
I don't think it is any changing the distance between the image sensor of the camera and the CS lens.
(Is that the "focal point"?)

As a side-note: I have an Axis 211M camera. Excellent unit.
In between the CS lens and the image sensor, Axis has setup a small "pane" of transparent material.
I don't know if it is glass, acrylic, or some "boro-silicate" glass type.
Its shape is the same exterior dimensions as the image sensor... yet it is thick.

I was wondering if this somehow helps the image sensor receive a focused image from the CS lens?

I can upload photos of the components if so desired by someone.
All help and direction is greately welcomed.

Thank you!
 
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  • #2
ElectricPaint said:
... Specifically, the center of the image is focused, yet the edges are blurry...

... When I installed a longer range CS lens on this mount, the image was blurry. I could not get it to "focus" with sharpness...

... Trying things out, I removed some material from the top part of the mount - this makes the CS lens sit CLOSER to the image sensor. With this setup, I was able to get the long range CS lens to focus very good. Yet, as best as I could get it, the "edges" of the image are out of focus.

It sounds like a couple of things are going on here, one is that the quality of the lenses you're using aren't all that great and your system's optics probably have some aberration content which makes the image quality suffer at the edges of the FOV. This, or the sensor you're using is larger than the lens was optimized for (which might also cause vignetting/blurring at the corners). The camera you spec has a 1/3" sensor which isn't huge, so that might point to a quality problem in the optics, maybe spherical aberration or field curvature both of which depend on distance from the image center.

Do you have some sample pictures? What lens are you using, and what imaging distance are you aiming for?

ElectricPaint said:
What else can I do to make the image sharp?

There's a lot in an optical system to consider, but to put it simply my guess is you need higher quality (and more expensive) optics. You need a lens designed for your sensor size, with appropriately-designed focal range, and minimum aberration within the FOV. Are you using a lens recommended by the camera manufacturer?

ElectricPaint said:
I don't think it is any changing the distance between the image sensor of the camera and the CS lens. (Is that the "focal point"?)

You might call it the "back focal length" or effective focal length of the lens. BFL is the distance from the lens's last surface to the focal point (which is where the focused image will be and therefore where your sensor should be). Typically BFL isn't something given as a spec for CS lenses, but nicer scientific-grade lenses might have it.

http://en.wikipedia.org/wiki/Focal_length

ElectricPaint said:
As a side-note: I have an Axis 211M camera. Excellent unit.
In between the CS lens and the image sensor, Axis has setup a small "pane" of transparent material. I don't know if it is glass, acrylic, or some "boro-silicate" glass type. Its shape is the same exterior dimensions as the image sensor... yet it is thick.

It's likely a protective window to prevent dust or particles from getting on the sensor, image sensors are very fragile and can be easily scratched by contamination. Probably simple optical glass.

ElectricPaint said:
I was wondering if this somehow helps the image sensor receive a focused image from the CS lens?

The camera manufacturer should have taken into account the "optical thickness" of the glass when determining the axial position of the sensor w.r.t. the lens mount. In any case you should leave it in place, it's strange that you had to modify the mount to make your "long focal length" lens able to focus, but I suspect that's a defect of the lens not the camera.

ElectricPaint said:
I can upload photos of the components if so desired by someone.

Sample pictures would be a big help, maybe of a printed optical target like this or this. Also read through this article on testing photographic camera lenses: http://grayfoximages.com/Pages/LensTesting.html
 
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  • #3
Mech_Engineer,
Thank you for your awesome reply.
Very, very informative.

I am in the process of gathering the images for comparisson.

I am also printing the USAF Resolution chart to test...
And reading up on that blog post "Testing Camera Lenses".

Amazing.
I knew it wasn't easy the problem... yet very interested in figuring this out!

I *think* the lenses I am using are "ok quality", because used with the Axis 211M unit, they produced very sharp and good images.
Same lens I used in the rPi camera, and that's where I am getting the bad quality.

Anyway, I will post up photos and more information ASAP.

Thank you!
 

1. What factors should be considered when selecting a lens for a specific application?

The main factors to consider when selecting a lens for a specific application include the desired focal length, aperture, and image quality. Other important factors include the type of material used for the lens, the coating on the lens, and the compatibility of the lens with the camera or instrument it will be used with.

2. How does the shape of a lens affect its performance?

The shape of a lens, also known as its geometry, can greatly impact its performance. For example, a convex lens is thicker in the middle and thinner at the edges, and this shape allows it to converge light rays to a single point, creating a magnified and focused image. On the other hand, a concave lens is thinner in the middle and thicker at the edges, and this shape causes light rays to diverge, resulting in a smaller and less focused image.

3. What is the difference between a prime lens and a zoom lens?

A prime lens has a fixed focal length, meaning it can only capture images at a specific distance from the subject. On the other hand, a zoom lens has a variable focal length, allowing the photographer to adjust the distance between the lens and the subject to capture images at different magnifications. Prime lenses are typically smaller and lighter than zoom lenses, but zoom lenses offer more versatility in terms of focal length.

4. How does the material of a lens affect its performance?

The material of a lens can greatly impact its performance. Different materials have different refractive indices, which determines how much light is bent as it passes through the lens. Materials such as glass, plastic, and quartz can all be used to make lenses, and each has its own unique properties that affect factors like image clarity, light transmission, and durability.

5. Can lenses be used for applications other than photography?

Yes, lenses have a wide range of applications beyond photography. They are commonly used in scientific instruments, such as microscopes and telescopes, to magnify and focus images. They are also used in everyday objects like eyeglasses and magnifying glasses, as well as in industries such as medicine, physics, and astronomy.

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