Thanks useful information: 
I suppose there may even be an argument put forth in that the use of a 'protective filter' could lull the owner into a false sense of security (and not intentionally tho!) that knowing that the filter is there(subconsciously), they're more likely to get themselves into a situation where they place the gear in 'harms way' or may be more accidentally prone.Originally Posted by dtoh
I could be classified as one of the most accident prone individuals lucky to still be (only just)walking this planet, and yet one thing that's never happened to me(as a photographer) is a gear related mishap.
I don't think that I'm prone to being extra cautious over my gear in any way either.
I've always had thought of concocting a process of proving disproving the notion that a protective filter could provide any protection especially from flying debris, this thread has rekindled that interest, and I have a few ideas on how to best go about it.
But, in one of my former jobs as a courier, one of the most, if not the most, annoying aspect for me at least, has been the number of times I've had my hands cut open by flying stones.
Generally they seem to get wedged in between truck tyres, or within the tread pattern of the tyre itself, but over the years, I've had my hands sliced nicely by these flying stones(or whatever they've been.. as sometimes you never actually say the culprit.
But there is one thing I am sure of. If a stone was to hit my hand as whatever force, the impact may cut the skin and cause a bruise for a few days and you bleed profusely for a few hours, but I'd rather this one solitary stone hit my hand rather than a million shards of glass at a similar force(all impaling themselves as glass splinters), as well as a stone/rock or whatever debris is attracted to the front of your camera.
We know from common sense that a rock at a decent velocity is going to easily shatter through a wafer thin protective element.. this is simply common sense(except for this HD glass from Hoya .. we've seen the 'videos' but exactly how effective they are may still only be guesswork).
So, knowing that the flying debris is going to penetrate the wafer thin glass protective element, but also knowing that this glass element has some Newtonian level common sense about it, it will reduce the impact of the flying debris on the front of the lens. The amount of reduction power is probably only going to be a guess on almost anyone's part but we're confident that the debris will still get through.
The thin wedge of air between the filter and the glass provides zero impact protection from the protective filter, and we know that from physics that almost all energy is transferred from one body to another and that very little energy is lost in an impact such as this. So all that will happen is that the protective filter sill eliminates some of the force, and that percentage of (eliminated)force will be proportional to the original force of the flying glass crushing debris. So that a larger object will lose proportionally less energy than will a much slower and lighter flying debris.
If the larger object is the one to find your lens(as they tended to find my fingers and hands) then the point is moot.. your lens will get damaged, but you also lose the value of the filter too.
But if the object is slower and/or lighter which dramatically lowers the energy count of the debris, then again, the filter does provide for impact protection, but the quality of the protection is questionable.
That is, if the force is lower then the chances of damage occurring to the lens without the protective filter may not have eventuated anyhow!
It's all about common sense, avoiding flying debris and keeping your lens clean on a regular basis.
I think the one thing that a protective filter may also discourage, is more regular cleaning of the front element.
A soft piece of cloth is hardly to be considered as the arch nemesis of the hardy sturdy resilient glass front element(some consumer level lenses use plastic, so they may not even count, due to their prices).
A regular cleaning of the lens is something that should be done. The build up of dust and gump and goop and haze could end up being more of a danger to damage to front lenses than the cleaning process itself.
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