I'm having a hard time wrapping my head around this situation I've come across. I'm essentially recreating a homemade microscope that my understanding would suggest shouldn't work. This setup uses a 10X Olympus PLAN N objective attached to a beam splitter to provide illumination. Then, 55 mm worth of extension tubes connect to a camera (Point Grey Grasshopper3 with a 1/1.2" sensor). There is no tube lens between the infinity corrected objective and the camera sensor (other than the beam splitter).

My understanding of how infinity corrected microscope systems works would seem to suggest that this shouldn't work, but a correctly oriented and seemingly undistorted image shows up on the camera.

Presumably, we aren't actually realizing the actual magnification this objective is designed to provide (in fact, its about half what it should be the sensor is 8 mm tall and a ruler placed in the view shows about 1.5 mm across the short dimension of image).

Can someone clarify what the light path looks like here and how we are actually able to see an image? Is it possible that the image quality is actually really poor, but we just seem to be getting results that are more than suitable for how we are using it?


2 Answers 2


Are you sure it's an infinity objective and not an RMS standard 160mm one ;-)

It's also possible that you don't have it focussed at exactly it's working distance. Remember the objective is just a (complex) lens. It only produces an image at infinity when the object is at a particular distance. If you put the object further away from the lens the image will be formed nearer than infinity.

  • $\begingroup$ Haha... yes, it's an infinity corrected Olympus 10X Plan N RMS10X from ThorLabs. I did double check... the spec'd working distance is 10.6 mm but it produces an image at around 13 or 14 mm. $\endgroup$
    – comwiz0
    Jul 3, 2017 at 2:09
  • $\begingroup$ That's what you would expect, an object further away than the focal length produces a closer image $\endgroup$ Jul 3, 2017 at 3:36

Just like any other lens, Objective lens is a lens, without the Tube lens also it will form an image by following the Lens equations, Lens Equations.

It is just that if working distance of the objective lens would be changed, the camera can image at different distances.

The above link gives an equation for single lens but this can be extrapolated to a multi-lens system.


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