[CyLith]

Optical engineer here. This is what they don't tell you about continuous/transition/progressive bifocals: optically they don't work. The lens design is an overconstrained optimization problem and the solutions they come up with end up compromising a lot on everything, to the point that it is practically useless.

[sokoloff]

I got my first set of progressive lenses in safety glasses earlier this year as a way to try them out. Based on that experience, I switched my daily glasses to progressives in May and love them.

Once or twice a day, I notice a visual artifact that I perceive to be in my left peripheral vision, and of course I sometimes need to move my head slightly to see something, but being able to see clearly at all distances with relative ease is the exact opposite of “practically useless” IMO.

[inejge]

> This is what they don't tell you about continuous/transition/progressive bifocals: optically they don't work.

I'll defer to your judgement re optical properties, but I want to offer a counter-anecdote about practicality.

I've had myopia and have been wearing negative diopter glasses for over half my life. I've never needed vision correction for reading. This is not an unusual combination, and if you want a celebrity example, watch some old Apple keynote videos with the late Steve Jobs, who would repeatedly lift his glasses to read something on the phone in his hand. This "works", but can be inconvenient in some situations.

A while ago, I started thinking about progressive bifocals. Cursory web searches told me that my particular combo was impractical. My optician didn't see a problem, so I decided to trust him and got a pair made. The TLDR is that they work for me much better than the old ones. There was a period of adaptation. Going downstairs and looking at the floor/ground were a bit disorienting for a while, but i don't notice it any more. Switching between the monitor and the phone or paper in front of me works, which is why I wanted the bifocals in the first place. I only use the old glasses for watching TV, probably meaning that my TV-watching posture sucks, but fortunately I don't watch a lot of TV. I still take off the glasses for sustained book reading.

[myops]

What's your lay of the land when it comes to different glass qualities and glass thickness options? Stores here (nordics) have around 4 different glass qualities for progressives, with varying and opaque names like "better" and "supreme". Thickness starts with 1.6 but store people push for 1.67 or 1.74. I get strong upselling vibes, but hard to say. (Is the "supreme plus" noticeably better than "supreme" or merely there for upsell anchoring effect, to make people pick "supreme" without feeling they overspent?) Are there high quality information resources for consumers on what the underlying lens material and processing technologies are?

[walterbell]

Search HN for "ABBE". The very cheapest material has the highest optical quality. Thinner ones are for weight, which matters if you have strong prescriptions. Weight can also be countered with a smaller frame height/width, which might require a vintage or designer frame, but that's a one time cost that can used with cheaper and higher optical quality lens replacements.

[nikolay]

Aren't the bifocals just a convenience over quality? It's inconvenient to wear two sets of glasses, but why would one wear bifocals while driving? This convenience can come at a high price!

[itishappy]

Bifocals in general are quite useful. It's nice to be able to see the road and the speedometer using the same lenses.

Traditional bifocals and progressives are different beasts. The hard outline on traditional bifocals means you get essentially two different lenses, both able to function as intended. The soft blend on progressives means you get essentially one big blurry lens that does not have well defined properties anywhere.

[EvanAnderson]

> The soft blend on progressives means you get essentially one big blurry lens that does not have well defined properties anywhere.

That seems to be exactly my experience with them, stated very succinctly. I've had these about 9 months and I'm still struggling with the ergonomics daily. I think I made the wrong choice.

[nikolay]

I used the wrong term, sorry. My concern applies to both bifocals and progressive lenses, though: aren't drivers trading off convenience over safety? Shouldn't we have at least two pairs of glasses - one for driving, and another for everything else?

[sokoloff]

When driving, I need to see things far away (mostly) but also on my dash/instrument cluster.

I am near-sighted overall and have needed distance glasses all my driving life. I got progressives last year and driving is safer now as I have a small area that I can use to clearly (and quickly!) read the instruments, the radio (read: map), defroster controls, etc.

In my case, not having multi-focal lenses was prioritizing convenience/laziness/cost over safety.

[itishappy]

No worries, I understood your intent!

The impact is a lot less than you may expect. Brains are ridiculously good at filling in incomplete information. For example, did you know that you have a roughly sun-sized blind spot slightly off center in each eye (where the optic nerve attaches)?

Now imagine needing to switch glasses to read your dash. Inconvenient and unsafe!

[MarceliusK]

But for actual use, especially screens and reading, the tradeoff can be pretty bad

[walterbell]

Search for "Chemistrie" magnetic lens attachment in +0.5 increments. They can even stack for different focal lengths, e.g. reading vs computer screen at 36". Magnets embed in primary glasses. Expensive via local opticians, they will retrofit via mail order and buychemistrie dot com.

[coldtea]

>to the point that it is practically useless.

And yet I know many people who use them and love them. So?

[walterbell]

The human brain mostly adapts. Big Optical makes $$$$ billions. A small percentage of human brains have persistent dizziness and depth perception errors, which can lead to physical injury and reduced quality of life. We don't have yet have a test funded by disablity insurance, which can determine which brains might fail to adapt.

The good news is that multiple glasses have never been cheaper or more accessible, so progressives are entirely a choice.

[coldtea]

>The human brain mostly adapts. A small percentage of human brains have persistent dizziness and depth perception errors, which can lead to physical injury and reduced quality of life

That's a fine argument, which I accept. Not the same as "practically useless", which was the original argument.