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It's a Plössl, it has two magnifications, but it's not a zoom; how is that possible?

Ben the Ignorant

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I have bought a few junk roof binocs and an old-fashioned Porro for spare parts, the kind that costs only a few coins. They have poor prisms but always good objectives, and sometimes good eyepieces, but what matters here is the (achromat doublet) objectives. Once they are assembled a certain way they make excellent magnifiers with a unique plus: when you flip them magnification is doubled. Rubber taken from the binocs gives the magnifiers a good grip, and protects what you look at against bumps.

Magnifiers 1.jpg

The top of the magnifier's frame is made different from the bottom so you know which magnification (and eye relief) you're using. Background left: 2x and 4x, background right: 2x and 4x, foreground: 2.5x and 5x. A classical (symmetrical) Plössl has two achromatic doublets facing each other, that makes very good telescope eyepieces but for a magnifier the doublets have to be stacked like this:

Asymmetrical Plössl magnifier.jpg

And the gap between the achromats has to be as wide as an achromat, or crescent distortion and edge softening will be very strong. Here is the result, center and edge:

Magnifiers 2.jpg

Magnifiers 3.jpg

As you see, edge sharpness is very nearly the same as at the center (with a field of about 50°, like a telescope Plössl). Some negative (crescent) distortion shows, but it's not too obvious, and maintains resolution near the diaphragm. Note there is no chromatic aberration, and no lateral color whatsoever, despite the hard black and white contrast (not everything inside a junk binoc is junk).

The magnifiers made from larger objectives have slightly longer focal lentgh, so they enlarge things a little less. My 35mm and 32mm diameter lenses magnify 2x or 4x, depending on how you hold them, and the 25mm is either 2.5x or 5x. For low power you bring your eye close to the lens, and get a 2cm by 3cm field of view:

Magnifiers 7.jpg

To switch to higher power, you flip the lens like so...

Magnifiers 4.jpg

Magnifiers 5.jpg

...and move your eye further back; magnification is doubled (or even more than doubled if you go further) and distortion is still quite moderate. If you don't hold the lens right, edge softening and crescent effect are so strong you'll know right away. You can use the magnifiers like this...

Magnifiers 6.jpg

...for basic macrophoto (sharp to the edge with dust specks and all!):

Magnifiers 8.jpg

Can they serve as telescope eyepieces? Sorry, but no. The magnifier has half the focal length of its doublets, that's between 65mm and 55mm for me, and when put at the scope's focus it works only if you hold your eye very far away for it, thus the apparent field is very narrow. But I tought do-it-yourselfers with junk optics in their drawers would like to find a use for the leftovers.

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I also enjoy experimenting with optical components. I frequently made approximately 100mm FL Plossl eyepieces from 50mm binocular objectives arranged front to front. These were for the optical image viewers for camera obscuras which typically had 4 - 6 metres FL. 30-35mm binocular objectives, remounted provide quite reasonable focal reducers, I made and sold quite a few cheaply years ago on a try before you buy basis.   :icon_biggrin:

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