What is this interesting optical design? (2 Viewers)

[tenex]

I'll post my question here because some optical experts may not find the original thread on thermal (infrared) monoculars, which you can click here to find. This is the first description of how a new "zoom" (actually dual focal length) model works:

The ZH38's "zoom" lens is a very unique thing to be used by turning its focus ring only.

When in wide angle position [that's 19mm], you have to turn the image out of focus and keep turning, turn, turn... until the image gets in focus again, but with the 38mm field of view now.

What can be concluded from this about the optical design and how well it should perform? (Note that these lenses are gallium arsenide not glass, and I presume the number of elements is minimized.)

 

[fazalmajid]

I have one. I assume it works much the same way the Leica Duovid does, just with Infrared optics.

 

[tenex]

I have one. I assume it works much the same way the Leica Duovid does, just with Infrared optics.

The dual/multiple focal length lenses I'm familiar with don't use the same mechanism to change FL and to focus, and Duovid doesn't seem to either.

 

[fazalmajid]

Well, on my USB microscope there is one focus position, and if you keep turning long enough there is another position at much higher magnification. How they do it I have no idea.

 

[Eric_D]

Well, on my USB microscope there is one focus position, and if you keep turning long enough there is another position at much higher magnification. How they do it I have no idea.

I don't think it relates to the iRay ZH38 at all (since that has to handle objects at ~infinity) but I can explain the USB microscope.
You could ask iRay for the patent, or its number ...

You can consider the USB microscope optic as one simple lens (focal length F) that moves between object and image (sensor). The distance between object and image is constant. As long as the object-image distance is greater than 4xF, there are two positions where the object is imaged on the sensor. One magnifies it by a factor of xM, the other reduces it by a factor of x1/M.

and (Yes, I cheated by just rotating the diagram ...)

If the object-image distance is only just greater than 4F, there is nearly a zooming effect where you can vary magnification around 1x without refocussing.

If the object-image distance is less than 4F, then the focusser will never focus.
You can see negative reviews on Amazon, because the manufacturer doesn't prevent or explain this!

 

[Eric_D]

Oh, the Duovid seems to move two elements or groups along the optical axis, according to a cut-away diagram.

 

[John A Roberts]

Hi Eric,

The Duovid changes magnification by moving 3 lens groups in a non-linear way!
See post #32 at: Duovid Optical Construction


John