So your theory is that bending down the temple tips does not really help in preventing slippage but rather the bending inwards helps for you. But in my opinion, i think temple tips should not be bent too much inwards as well. Rather, it should be bent at the correct angle inwards. If you bent too much inwards, you will leave a gap between the temple tip & the back of your ears, minimising the contact area & thus lessening the grip. In other words, I feel that the temple tip should be flushed against our skull behind our ears to provide maxiumu grip on the contact area.
More importantly, i feel that the length to the temple bend (the distance between the frame front and the place where the temple starts to bend down around the ears) is the more critical crux of the slippage problem. The bend should start 2mm behind the place where the ear joins the head. If the bend starts in front of the ear or more than 2mm behind the ear, the spectacles will slide forwards. The person will need to keep pushing them back onto their face with their fingers.
I am still rather perplexed by the fitting triangle theory. I am still wondering whether the side bow is the most crucial factor because that is the place which is supposedly to create pressure at the ears (two points of the fitting triangle) and keep the spectacles firmly positioned on the person's head.
bending in work on certain frames only and the wearer's sensitivity. as in all cases, enough is enough. it's like wearing clothes. unless you love nipples poking out. there's really no need to have them so thight.
not really what I call a theory, it's more about my personal exp.
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I think regarding your thoughts on the 2mm thing.. it's rather hard to maintain. I usually suggest it's best to use your natural wearing position and fit according to the user.
one of my friends believe very strongly in glass lenses due to the optical quality so she fitted all her kids with glass lenses.... while in theory glass does offer a better vision. fitting teenagers with glass lense contributed to them changing lenses often due to the material brittleness.
have also met those who just prefer the glasses to hover on the bridge or above the ears. it's against common sense to force adults to adapt and make the situation more uncomfortable.
fitting triangle theory... lol you know enough about glasses to sound like you're either obsessed with them or you're working in the optical trade.
it's like how a tripod works. you need all three legs.
and depending on how you adjust you can shift the weight around. and for those who don't mind it tighter it's possible to tighten the sides to make the glasses lighter
but the main idea is really just to make sure it's balanced