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Low diopter C-U lenses

Subject: Low diopter C-U lenses
From: Moose <olymoose@xxxxxxxxx>
Date: Mon, 23 May 2016 15:36:34 -0700
You've now seen some examples of what the Nikon 5T can do on my PLeica 100-400 
@400.

I love the performance of the 5T, and with the quick change filter holders, I can flip back and forth in a moment. Waaaaaaay quicker and easier than a tube. OTOH, the working distance is kinda close, about 68 cm from the front of the lens.

In my effort to find the best fit*, I've acquired a couple of lower diopter C-U 
lenses, for longer working distances.

The B&W 0.5 diopter, single element, may be an interesting special effects lens, for the kind of soft images I get at 400 mm. Lovely working distance, though. ;-)

Not so bad at the short end, but no point. The Minolta No. 0, 0.94 diopter lens is better, but I think not so very good. I suppose I may have to do a real test . . .

What's interesting, if not necessarily enlightening, is the curvature of the lenses themselves. I immediately noticed that the B&W has quite steeply curved glass, a real meniscus, like a piece cut off a large fishnet float. I have a mechanical diopter measuring gauge, so I checked. The front is +6.0 and the back -5.7 diopters. The difference of 0.3 is less than the strength of the lens, but I suppose that's because the refractive index is greater than what the gauge expects. It was made for eyeglasses.

The Minolta is less extreme, +3.8 & -3.1 diopters, but still fairly steep. Measuring a cemented doublet still works, as the inner surfaces cancel each other out. Overall refractive index depends, of course, on those of the two elements, and would seem to be slightly less than the B&W and Nikon.

The Nikon is way flatter, +1.25 & -0.3.

I suppose one wants a negative curve at the rear, to minimize the possibility of reflections. I wonder at the wide range of curvatures, the Nikon almost a slab and the B&W more like a contact lens. I know none of them were designed for such a long primary lens, Nikon says 80-200 and Minolta 100-135. I have no idea what the B&W was made for. It doesn't work well with long FLs, yet is large, 62 mm, and would have little effect on shorter FL lenses.

Dioptically Challenged Moose

* And possibly to have more crap than anybody, but I'm afraid I'm doomed to 
mediocrity in that contest. :-)

--
What if the Hokey Pokey *IS* what it's all about?

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