Micro Four Thirds Focal Reducer Shootout

Started Feb 20, 2014 | Discussions thread
OP brian Senior Member • Posts: 1,206
Re: Hot spotting - the cause?

This diagram shows how you can form a ghost image of the stop by reflecting off the sensor and then off of one of the lens surfaces.  This is just one particular example of how such an image is formed.  In general, there are hundreds or even thousands of possible ghost images formed in a complex imaging system, and you have to pay attention to all of them if you want to do a good job of minimizing ghosting.

Follow the green ray, starting at the aperture stop.  At the aperture stop, which is the object, the height of the green ray is zero.  Now follow the green ray to the image plane (surface 13).  At this point we have the first ghost reflection.  After reflectiing off the sensor, the green ray refracts through the rear surface of the lens and then reflects off the second-to-last surface of the lens.  The green ray then passes back through the rear surface to intersect the image plane at an image height of zero.

Since the ray starts at the middle of the aperture stop and intersects the image plane at the optical axis the ghost image is a true image of the aperture stop.  So, if the aperture stop is a triangle then there will be a ghost image in the shape of a triangle in the exact center of the picture.

The lens system consisting of the last three elements of this particular lens plus the flat reflective sensor comprises a catadioptric (mirror + lens) optical system with two mirrors.

The fact that the hotspot imaging in this case involves a relatively flat lens surface is a complete coincidence.  Often you get fairly steeply curved surfaces involved.  Ghost image formed by reflection off of front-mounted filters are completely unrelated to hotspots.  Such ghost images show up as sharply focused mirror images of bright areas in the primary images.

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Brian Caldwell

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