I am doing some research involving geometric optics. I need to reduce an image in size so it fits a rotating mirror we are using.

I came up with the idea of projecting the focal plane on the mirror, since that would be very small. I could have used more lenses and projected the actual image on the mirror, however this image is later once again magnified in size and I want this image to have little to no distortion.

So yesterday I found that what I am looking at in my focal plane is actually the image of my light source. How is it possible that, even after the beam passes an 'object' (I used a resolution test chart), the light beam still has the information of the source 'encoded' in it, while only a fraction of the actual source passed through my object? How would one call this phenomenon (I'm having a hard time finding anything about this on the web)? And last of all: Would I need fewer lenses to reduce the image of my source in size if the source is a collimated beam?

To give some additional information: My setup consits now of, respectively: A source, an object, a 10x infinite corrected objective (these three to simulate a microscope), a lens, another lens a little distance away, and a CCD on which the image of the source is displayed. The CCD will be replaced by the small mirror at a later stage.

A horrible sketch of my setup

Might any additional information be required, please notify me and I will include it.

Thanks in advance,


  • $\begingroup$ I'm not quite sure what your setup and your actual question is. I believe a picture would help. However, I'm also not very proficient in geometric optics, hence I'll refrain from voting to close for now. $\endgroup$
    – Martin
    Dec 2, 2015 at 9:21

1 Answer 1


What you want to read about is relay lenses and the way to locate image planes. Basically, you want to set up the system so your res chart's object location maps thru the lens to produce an image which is at the rotating mirror.

Because you've got a transmitting res chart, there will be confusion about what you actually see at various places. If you simply illuminated a chart such that it reflected (diffusely) light into your optical system, you wouldn't see what looks like an image in the "wrong" places. But because the light passing thru the res chart is semi-collimated, you will see images of the source at certain locations (the source is at a different distance from the lenses than the res chart itself).


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