Is it still required/highly recommended that I buy a UV or similar filter to 'protect' the front element of my lens?
I ask because this advice has been around for 30 years and you would think that the lens maker would have solved this problem with a built-in protection or something like that. It's good to check these old assumptions from time to time.
The front element of a lens is still (and probably always will be) a lens element, usually the largest and most expensive single piece of glass in the system. If you need physical protection for that element, you still have to add it (where the lens design permits).
Every element in a lens system contributes its own refraction and reflection. When you put an extra element at the front of a lens, light that would have reflected from the "old front" harmlessly into space will now reflect off of the rear (and front) surfaces of the new element. Those reflections can be minimized with coatings, but they are still there, and they still contribute to a degradation of the image. And keep in mind that the new element, whatever its shape, will still refract light and add its own distortions.
So you have a choice -- use the lens naked for best optical performance, or add a filter. When the filter is used for photographic reasons (neutral density, polarizers, color filters) you have a trade-off that's worth the price -- you give up the ultimate performance possibilities of the lens in order to capture a picture that the lens alone could not have captured.
You may be of the opinion that protection of the front element is also a reasonable trade-off at all times. I'd only use a "protective" filter if I was working in an environment that's likely to cause damage (say a windy day on the sand dunes or photographing someone using a grinder). For knocks and bumps, a lens hood offers good physical protection without putting anything in the optical path. But it's your choice—and it should remain your choice.
I looked through the manual for my Dad's old film SLR, purchased in 1982: nowhere did it mention protecting the lens with a filter. The Accessories section, which offered everything from bellows to cases (including those cool, now-retro cases that screw in to the tripod mount, flip over the top of the camera, covering both the camera and lens, and snap shut on the back) to microscope adapters, didn't list any filters except for two close-up filters.
The manual for his third-party telephoto zoom, from roughly the same time period, doesn't mention filters at all.
I looked at my more modern telephoto zoom lens' manual, and while it recommends using the hood to "protect the lens from rain, snow, and dust," the only mention it makes of filters is that they can be attached to the front of the lens, and suggests that you "use a polarizing Canon filter."
I interpret this to mean that for at least the last 29 years lens makers have considered the problem solved, and feel that their lenses are sturdy enough to not require additional protection.
Since the front element of this lens moves when focusing (zooming), you need to attach a Canon PROTECT filter sold separately for adequate dust- and water-resistant performance. Without a filter, the lens is not dust or water-resistant.
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My short opinion: I prefer to use a clear (or UV) filter for protection on all lenses at all times, removing it temporarily only if I observe artifacts.
The chance of something flying at my lens during normal use is not actually a big concern in the environments I shoot. More of a concern is that if I accidentally ram the front lens element into an object (e.g. when getting too close in or shooting through a window or wire mesh), or if I drop it a short distance, the lens has a better chance of surviving, which allows me to shoot more adventurously and get better shots. It may also be accidentally scratched during regular cleaning, if you forget to brush it - a filter allows the lens element to be cleaned much less often. If you stick with high-quality filters (e.g. B+W MRC) you get very high transmission and not much glare in most situations.
In the end what it comes down to is because my budget for replacement parts is very limited, I'm willing to give away a little image quality for a little insurance. However, they are not at all necessary - front lens elements are specifically designed to be exposed to the elements and cleaned regularly, and some include additional coatings for this purpose (e.g. fluorite coatings on Canon lenses). If you can get an affordable insurance policy on your equipment, that might be a perfectly acceptable alternative of similar cost.
still? It never was.
required? It never was, never will be.
recommended? Depends on who you ask. I'll say no, my dealer says no, many other people making money selling the things will say yes.
caveat: in some environmental extremes, it can be beneficial to protect the front element, but in those conditions you're likely going to need to protect the entire camera assembly, an underwater housing is likely a better option.
I keep a clear filter on all of my lenses, where possible. These are high-quality multi-coated filters, B+W for the most part. I do this not to physically protect the front element from impact, but to keep the front element clean and avoid cleaning the front element directly, which can wear down the coatings (it's almost always much cheaper to replace a filter than replace a lens).
Of course, a filter introduces an additional optical element and corresponding air-glass interfaces that degrade image quality. A good filter will keep this degradation to a level that is not perceptible under normal shooting conditions. I explain this in more detail here.
Filters aren't the best option for protection against physical impact, although there are several well-known cases where a filter took an impact and broke where the front element could have sustained damage instead. If protection against impact is what you need, consider getting a lens hood instead. A hood shields off light that shouldn't be part of the image, increasing contrast, and extends the lens beyond the front element so that the front element is much less likely to be damaged on impact. A filter is useful for keeping the front element clean, but you'll need to decide whether this is worth the image quality trade-off.
However, some Canon L lenses with weather sealing (several non-white L lenses that are not internal focusing or internal zooming) require a filter to be installed in order to complete the weather sealing. For example, page 1 of the manual for the Canon EF 17-40mm f/4L USM lens states:
Since the front element of this lens moves when focusing (zooming), you need to attach a Canon PROTECT filter sold separately for adequate dust- and water-resistant performance. Without a filter, the lens is not dust or water-resistant.
You have a point, but lenses manufactures just provide the cap, there is no 0 element to protect the front element from scratches, fat and dust, all get in the way and if the front element is kept clean in an immaculate way you win.
Other answers here cover the protection issue fairly well — there are other ways to protect your lens which work as well or better in most cases, and the loss of image quality is very real. (See Are there any downsides to using a good-quality UV filter?). But I wanted to run some risk analysis numbers.
Let's say you have a fancy $1700 lens that you're worried about damaging. Prices for a UV filter in the right size range from $7 to $420. You decide you want to avoid the cheap end, but the top seems a bit much. You look at the middle of the list... the median is about $80. You want a little better than average, so you go for a $100 model. Seems okay for peace of mind, right?
But, what's the chance that damage will occur? Glass is actually pretty strong, and modern materials and coatings make front elements rather resistant to damage. And, although not great for resale value, tiny scratches or similar don't have meaningful effect on image quality — I'm sure everyone's seen the LenRentals smashed front element example by now. I'd be inclined to guess that the chance of damaging the front element in a non-cosmetic way is less than 1% per year (unless you're in extra-dangerous conditions), but for the sake of argument, let's say it's 5%. Over the course of 10 years, that 5% accumulates to an overall 40% chance of an incident (which, again, I think is very high).
But, the cost for replacing the front element for that lens is about $200. That means the expected value with a 40% chance of needing a repair is... $80. So, why spend $100 on a filter?
Or, let's say it's the kit lens that came with your camera. The image quality concerns of cheap filters don't go down for cheaper lenses, so we're looking again at least a $500 filter — but, never mind repair: you can almost certainly buy a replacement for the whole kit lens for $50 on eBay.
This is an example of loss aversion — a natural human cognitive bias where we fear loss more than we appreciate equivalent gains. I might go so far as to say that camera stores which push protective filters as a must-have are preying on this weakness for profit.
But, of course, if you're in a dangerous environment, a filter might make sense. And some lenses have surprisingly high front-element replacement costs. These things might change your math. LensRentals actually has a more recent post on all of this which isn't a comprehensive guide but gives the price for many common lenses (and is worth a read on this topic in general) — Front Element Lens Protection Revisited.
One benefit of taking the "protective" UV filter off the front of the lens:
When shooting in daylight, UV from the sunlight passing through your lens will inhibit the growth of fungal spores that may be (hint: are) in your lens¹. The filter stack directly in front of the sensor has a fairly strong UV filter anyway, so there's no real benefit to preventing UV light from entering your lens unless it is constantly exposed to sunlight 24/7/365, and there's a definite benefit to at least periodically allowing UV light through the lens.
¹ Fungal spores are everywhere on the surface of the earth, including the skin of most animals and the leaves of most plants. They require three things to multiply: Moisture, organic matter (provided by almost every kind of dust), and the absence of UV light.
I've not read the other answers. The lens IS THE camera. It should be pure. Without contamination.
Coatings or filters have two adverse effects.
Firstly it's an obstruction causing loss of some light through absorption by the filter. Or it's loss by reflection.
Better get a pure crystal lens without imperfection such that it doesn't require ar of any description.
In Catholic religious terms: I advise you get a perfect lens. That's "immaculate". Which means without defect masked by coating (as in marble statutes were said to be "immaculate" if they were free of defects filled with wax at the point of sale.
In short. Coating are to hide blemishes. Best have eyes/lenses free of missis.
Hope this helps. Applies to all models, years and makes.
Note. Make-up is good. Mask is bad. Make-over is good too. Not Take-over. But that's another question altogether. New Lens. Where to get and how to correctly focus it to the body.
For those into Bi Bull speak. The rock has to be as is, No grave tool. And not added to or subtracted from. Not EVE N a jot or tittle.
