How do I accurately measure the size of an object using CCTV lenses?

[bobjim]

[SOLVED] Understanding CCTV lenses

Hey guys, I'm trying to get my head around the way a lens works...

I've found the nodal points, and since the medium surrounding the lens is air constant refractive index (air) then they nodal points coincide with the principle planes, right?

Also the Focal length of the lens is 12mm, which I assume since it's a CCTV lens, is measured from the mounting surface rather than the last element of the lens... is that a fair assumption?

I've drawn a diagram of what I think represents the path of a ray going through the lens. (see attached) What I'm trying to do is accurately measure the size of something from it's size in pixels in the image.

 

[bobjim]

damn, I talk some rubbish sometimes! *think* I've figured it all out now... ignore everything above! If an admin want's to close/delete this thread, please feel free.

 

[astrokreng]

Hi there! Measuring the size of an object using CCTV lenses can be a bit tricky, but here are some steps you can follow to get an accurate measurement:

1. Determine the focal length of the lens: As you mentioned, the focal length of a CCTV lens is usually measured from the mounting surface. This is important because it will affect the field of view and magnification of the lens.

2. Calculate the magnification: Once you know the focal length, you can calculate the magnification of the lens by dividing the focal length by the distance between the lens and the object. This will give you a rough estimate of how much the object will be magnified in the image.

3. Measure the object in pixels: Use the CCTV camera to capture an image of the object you want to measure. Then, using image editing software, measure the size of the object in pixels. This will give you a baseline measurement to work with.

4. Use the magnification factor: Using the magnification factor you calculated in step 2, multiply the pixel measurement by this factor to get the actual size of the object in millimeters or inches.

5. Consider the distortion: Keep in mind that CCTV lenses can have some distortion, especially towards the edges of the image. This can affect the accuracy of your measurement, so it's important to take multiple measurements from different points of the image and average them out.

I hope this helps! It's always a good idea to double check your measurements and take multiple readings to ensure accuracy. Best of luck with your measurements!