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jbeale
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Visual demo of lens focus difference vs. wavelength

Thu Nov 26, 2015 12:08 am

I thought this was an interesting demonstration of how different wavelengths of light come to a different focal point. This is using a 3rd-party RPi compatible M12 camera with 8mm f/1.8 lens. You can adjust the image to be in focus at IR (940 nm) or at visible light, but not both at the same time; there's quite a focus shift between the two. (I expect there do exist IR-corrected lenses, but this cheap item isn't one of them.)

https://www.youtube.com/watch?v=ASC_6Yce06k

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Dr H
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Joined: Sat Sep 12, 2015 7:58 pm

Re: Visual demo of lens focus difference vs. wavelength

Wed Jan 27, 2016 9:27 pm

I have been using one of these cameras with different lenses for a microscope project. While it works well, you can see a phenomenon called chromatic abberation, which results from the same effect, the dependency of the focus form the wavelength. In my case I get a yellowish spot in the center and a violet color in the outer areas if the image.

As far as I know there are no achromatic lenses available for the M12 mount, but if I am wrong, please let me know.

At the moment I set my hopes in a camera from ArduCAM available with a C or CS mount. I had ordered one in early December but I am still waiting for it. Has anybody got the chance to evaluate them?

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jbeale
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Re: Visual demo of lens focus difference vs. wavelength

Wed Jan 27, 2016 10:16 pm

The arducam works, I have one. But it is very limited (not enough onboard buffer memory to get the full resolution from the sensor) and it is hard for me to imagine a situation in which it would be preferable to the RPi camera.

The radial color cast you describe is often seen with the RPi camera when you use a different lens. It is due to a lens shading color pre-correction based on the "official" camera board with its built-in lens. The software correction for that lens will cause that yellow/magenta pattern when you use a different lens.

See for example viewtopic.php?f=43&t=126220

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