Relay lenses and optical formulas

Started 3 months ago | Discussions thread
alanr0 Senior Member • Posts: 2,234
Re: Focus breathing

bclaff wrote:

alanr0 wrote:

bclaff wrote:

tonipuma wrote:

first of all, I'm not sure (but I hope it) that this is the correct section for this not usual argument:
If I want to use a relay system (composed by the primary lens and the relay lens), how can I calculate (which formula) the effective focal length (EFL), the depth of field (DOF) e the field of view (FOV) of the complete optical system?
I'm using a relay system to increase the image circle of a small CCTV primary lens to use it on a MFT body.
Thanks in advance.

To measure the focal length determine magnification with and without the 20mm extension tube.

focal length = delta_x / delta_magnification = 20mm / delta_magnification

This clearly works for a single thin lens. It also applies to a compound lens forming a real inverted image, provided the internal separations of the lens elements do not change.


No. It works for any lens.
I have used this technique many many times.

I don't doubt it. As I said, "it also applies here too".

The point I was trying to make is that removing the 20 mm extension tube is likely to push subject focus distance beyond infinity. If you adjust the inter-lens distance so that subject distances between 100 mm and infinity are focussed with the extension tube removed, then the subject distance will be much closer (between 6 and 7 mm from the CCTV lens' first principal plane) with the extension tube replaced.

The 24 mm relay lens' focus movement is not coupled to that of the primary CCTV lens. The composite assembly suffers from focus breathing. Changing the inter-lens separation changes the focal length of the assembly. What you will measure is the focal length when the assembly is configured to accommodate subjects less than 10 mm from the CCTV lens with the extension tube in place. The apparatus as described by the OP will have a longer focal length when focussing on subjects between 1 m and infinity.

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Alan Robinson

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