With the extraction of the eye's natural lens, goes the ability to adjust focus to accommodate for distance. While corrective lenses can bring things into focus for any fixed distance (e.g. reading glasses), a fixed correction can't work over a broad range of distances. As depth of field increases as a function of distance to subject, it stands to reason that most accommodation issues occur when working with nearby objects. An older person feeling the effects of presbyopia can probably say a thing or two about the gradual frustration of needing bifocals.
This isn't about age-related problems or old people things. This is about my dumb broke ass trying to find a means to get by with my pointless daily existence. Anyone else would probably just buy some magnifier lamps or something, but those cost double-digits kind of money. If you've been paying attention, you'll know that I'll end up cobbling shit out of $0.99 Ebay garbage and then sitting in the dark at midnight writing rambling stories to myself about an experience that even I can barely care about. That's what I did. That's what I'm doing. Let us continue.
Presbyopia is common though. Where do most people start? Bifocals and reading glasses simply offer an increased optical power (increased lens convexity) so that focusing on near objects is possible. While bifocals are convenient in that they're always at hand, their use requires a strong downward gaze which I can't effectively accomplish -- a consequence of an entirely separate ailment.
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| A drift of reading glasses and very uncomfortable Optivisor clone |
Drug store reading glasses aren't meant to be used with prescription glasses, though it's ... possible. Prescription reading glasses are a simple addition to the SPH portion of the prescription (or specification of a +ADD). Of course, if you know your prescription and can use cheap readers to get an idea how much correction you need for a particular distance, you can do the math yourself and order any sort of odd double-reader bifocals or "computer glasses" you want.
For instance, let's say I wear plain single-vision glasses and I pick up a pair of +2.5 readers and slap them over the top. "Hooray!" I say as I can again read my own handwriting at 12". I can either add 2.5 to the SPH portion of both OD and OS lines on my prescription, or if it's appropriate, I can just use the +ADD entry on the order form. In this way, you can order your own readers or bifocals online without dealing with extra expense.
Say I have an existing bifocal prescription and I want to make computer glasses that can focus at 30", but without changing the power of the secondary lens I normally use for reading at 15". If I can determine that a +1.50 correction allows me to see at 30", I can just add that to the SPH section of the prescription and then subtract it from the existing +ADD which specified the original bifocals. Zenni Optical actually covers these sorts of prescription adjustments in their FAQ here and here.
Granted, buying glasses from Zenni beats paying $200 for glasses, but I'm not made of money. Not only that, but like I mentioned, depth of field is a function of distance. With glasses configured to focus at 20', you can focus at 200', but with glasses configured to focus at 2', you probably won't be able to see much at 20'. I went though the motions I describe above when I got my glasses for the computer. In fact, I collected the optimal focusing distance for a range of positive correction powers, as well as the minimum and maximum distance I could reasonably make out detailed edges in some text samples. It seems subjective at a glance, but when plotted, the trend starts to reveal a nice rational relationship.
With some use-cases such as the computer where distance is fixed and repeatable, it's simple to come up with a correction that's tailored to the task. Otherwise, shop work involving operations up close (checking pitch of a bolt with a thread gage) and at a distance (finding the bolt after you drop it) is hard to correct with a single prescription. At some point one has to accept that there is no single solution, and that carrying numerous solutions around is impractical.
So what's left? If I can't accommodate and I can't effectively use bifocals, I'm pretty much doomed to do half of my work in a blurry world unless I'm constantly swapping glasses or fumbling a magnifier. Some work patterns are harder to deal with than others. Automotive work requires focus between about 12" and 60", and the area of focus is typically changing regularly. On the other hand, lathe operations cover a smaller range of distances and most importantly, the critical areas of focus don't change as much. It's simpler to try coming up with task-oriented magnification tools for cases like this. ... simpler to try.
So I embark on a journey to come up with some positionable magnetic work magnifiers that I can stick down on the lathe or drill press or vise when I need them. There are lots of options out there already if you want to spend money. Certainly, I'd like a lighted magnifier on a Noga mag base, but that's not going to happen.
I found a MagniStitch magnifier that my late grandparents had purchased in the 80's. The lens is well-shaped and wide. The ball-jointed arm is much less useful than one might think. It tends to pop apart when positioning. I glued it to an old speaker magnet (because double-sided foam tape is bullshit). These can still be purchased for about $13-$20. I've considered buying others if perhaps I can CAD up a more appropriate articulated arm for it.
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| MagniStitch stuck on the drill press vise |
The MagniStitch didn't work well for me on the lathe simply because the arm is too short to reach around the tool post or to reach around the cross slide to see the dial. I considered making my own from an inexpensive pocket magnifier and a flexible arm of some sort. The trouble with buying cheap magnifiers is that a lot of them are completely useless shapes. That is to say that they are neither spherical or hyperbolic lens profiles, but they're molded or ground to some freehand wavy biconic shape that makes them about as useful as a fishbowl. I found some glass pocket magnifiers on Ebay that have proven to be good. I just popped the rivet out of one and made a wire arm for it. It works well enough, but it tends to wobble if it's on a machine that vibrates. It needs a stiffer wire or some dampening ... or maybe I just need to replace the bad belts in the lathe. It's a promising candidate though.
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| Pretending to put a shear cut on some trashy porous cast aluminum bar |
In the course of scouring china-mart for lens-shaped objects, I saw this thing and thought I had a brilliant idea. Increasing illumination helps a lot, partly because pupilary contraction increases depth of field. I could slap a USB power bank of some sort onto that and have a little portable magnifier light. Well, not really. The lenses in these things are absolutely useless. Luckily, it wasn't molded directly into the lamp.
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| Freehand wavy biconic lens-shaped object |
I ended up taking another one of my glass pocket magnifiers and grinding the lens down to replace it. The gooseneck is barely able to hold the lamp up anyway, and the USB connector really is a lousy mechanical support. By the time I had the replacement lens fitted, I had pretty much condemned the USB power bank and the whole idea. I glued it to another magnet and figured I'll use it until I come up with something better.
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| It's easy to fix the lens. Just inherit an old lapidary wet grinder... |
These aren't the best solutions, but they'll keep me busy for now. I plan on whipping up an articulated arm system in CAD. I'd like something simple that I can machine, but it might be nice to come up with a printable modular system.
If you're looking for a more substantial conclusion to this rambling post, there isn't one. These are not unique problems. Although it might be difficult for someone in good health to understand how frustrating and disempowering the experience of poor vision can be, plenty of people have it worse than I do.
