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I have a Canon EF 17-40L, and I am seeking to buy a UV filter solely intended for the protection of the front glass of the lens. I know that I have a hood for protection, but it makes the equipment bulky and it doesn’t fit my current bag, so I think that a UV filter is also a good solution.

I have seen lots of cheap and expensive UV filters, but I don’t understand the difference between them. What would be the possible downside of using a cheap UV filter in this case? Does it cause the lens flares, and/or reduces the light? I have read some related posts from SE, but they are more general, rather than explaining the differences between filters.

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    \$\begingroup\$ Lens hoods cut extraneous light from the sides that cause flare, a UV filter isn't going to do that for you, it's arguably going to make it worse. \$\endgroup\$
    – Joanne C
    Commented Mar 29, 2014 at 13:09
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    \$\begingroup\$ This: photo.stackexchange.com/q/35052/15871 \$\endgroup\$
    – Michael C
    Commented Mar 29, 2014 at 14:16
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    \$\begingroup\$ Basically, all of the answers to Are there any downsides to using a good-quality UV filter?, but amplified. \$\endgroup\$
    – mattdm
    Commented Mar 29, 2014 at 14:30
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    \$\begingroup\$ @kmonsoor Reversing the hood of the EF 17-40mm f/4 L doesn't save a lot of space due to the nature of the wide angle lens. Most of the bulk of the hood is an increase in the diameter of the lens barrel rather than an increase in length. See the-digital-picture.com/Reviews/… \$\endgroup\$
    – Michael C
    Commented Mar 29, 2014 at 16:23
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    \$\begingroup\$ @MichaelClark If you have an APS-C-format camera, it's much better to ditch the lens hood supplied with the 17-40 and get an EW-83J. It fits perfectly, doesn't vignette (because it's designed for a 17-something EFS lens) but gives much more protection and shading. It's probably narrower than the supplied hood, too. \$\endgroup\$
    – David Richerby
    Commented Mar 29, 2014 at 17:10

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Cheap is not necessarily cheap - you can get a cheap filter that is good, but if you get one that is bad, the following are side effects that have been reported/observed by other users:

  • focussing problems with cheap UV filters
  • loss of sharpness in the image
  • flares/weird reflections

Add to that, a front lens element is a lot stronger than most people realize it and a lot harder to scratch than most people realize too. Unless you will be shooting in an environment where you need a filter to seal the lens or where you know you will have dirt on your lens (say a beach/desert with sand, a rally event where mud may hit your lens) it is not advisable to use a filter for "protection" unless you desire the effect of the filter.

A broken filter may also scratch your lens - which leads to the question whether they really offer protection (they shatter first but the shards could damage the lens).

If you are worried about the "odd bump", a lens hood is the better option.

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    \$\begingroup\$ Since hoods for wide angle lenses are much more shallow than hoods for longer lenses, they tend to protect less in terms of bumps than longer hoods. Although I never use "protective" filters and almost always use a hood on most of my other lenses I rarely place it on my 17-40 due to the limited protection it provides from both extraneous light and physical bumps. \$\endgroup\$
    – Michael C
    Commented Mar 29, 2014 at 16:27
  • \$\begingroup\$ Thanks for the reply. I will not resort to a UV filter, but I won't use the hood either for the reasons Michael stated. \$\endgroup\$
    – user26804
    Commented Mar 29, 2014 at 17:08
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    \$\begingroup\$ Don't forget to use a lens cap when not shooting with the lens. \$\endgroup\$
    – Michael C
    Commented Mar 29, 2014 at 17:16
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    \$\begingroup\$ I use clear filters for situations where the lens might get unexpectedly dirty -- say I'm photographing a waterfall (or, most recently, a geyser) and the wind shifts, drenching my camera with spray. I can simply switch filters and clean the dirty one at my leisure. \$\endgroup\$
    – Mark
    Commented Mar 30, 2014 at 0:33
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I've been a mostly 35mm SLR (now a no-film-but-still-35mm-style DSLR) photographer for about 45 years. Every photography book I read, or course I took; as well as my photo editor at the newspaper at which I would later work, and all my fellow photographers there and at the AP all recommended either a UV or Skylight filter to protect the lens. Back then I was shooting with a pair of CONTAX RTS bodies, and four Carl Zeiss lenses; and so I had really good stuff, and wanted to protect it.

I first tried a Skylight filter, but it removed a little too much blue for my tastes (warmed things up a tiny bit); and also reduced the amount of light that hit the film plane by every bit of a tenth of a stop... maybe a tad more.

So I went with a UV filter and, honestly, I could see little difference. Even the amount of light hitting the film plane seemed almost the same. Only if I used a loupe, and looked really hard, could I detect, in the images taken with the UV filter on the lens, such things as a virtually undetectable, otherwise, reduction in the details of things like the branches of trees in the far distance. Seriously, the UV filter didn't make enough of a difference to matter.

So I settled on a UV filter, and never looked back. Every lens I've had for nearly half a century has had one -- and a good one -- on it. Any other filters I might use get screwed atop it; and I'm careful not to accidentally remove the UV filter when I remove whatever other filter (always keep a pair of plastic filter wrenchs in your bag, so that you can grip one filter while you turn the other if you ever have to; of course, no filter should be on so tight that you ever need one, but I'm just sayin').

The UV filter, though, must, indeed, be a good one. Actually, better said: The filter must be really, really clean so that not a single spec of dust or a smear or fingerprint or anything can, in any way, redirect light. That's the key.

If the UV filter is a cheapo, then the statistical probability will be higher that its glass will have some kind of tiny imperfection in it; which imperfection basically achieves the same thing as dust or a smear or something. And so it's essential that oone starts with a high-quality UV filter; one that truly meets the definition of "optically clear."

Then, beyond that, it's just necessary to keep it very clean...

...starting with ensuring that not a single spec of dust is between the UV filter and the lens. If you can keep that part clean, then keeping the outside of the UV filter that's in contact with the world is a cinch.

But keeping the dust out from between the UV filter and the lends is actually trickier than you might think. You should see me put a new UV filter on a lens: first getting just the inside of the filter so clean you could operate on it, and then protecting it from dust while I do the same to the lens glass (being sure to get all dust out of the metal bezel around the lens glass, too); and then keeping dust out of there while I retrieve the filter and get it screwed on. Then I inspect and inspect and tap things to see if any dust dislodges and ends-up on the inside. Yikes! It's quite a little undertaking... and a sight to see, I'm told. It gives the phrase "anally retentive" a whole new meaning. [grin]

If you can get that part right, though, then, out in the field, you simply make sure that the UV filter is always clean, by normal means, which is easy. And the beauty of even having the UV filter to protect the lens is that if any of said normal means just happens to somehow scratch something, it's the UV filter, and not the lens, that gets scratched.

I'm sorry, but I wouldn't take any camera even out of the house without a UV filter protecting all lens fronts. Ever. No matter what.

Others, here, can disagree; but my pushing a half century of experience says that one should be using UV filters on all of one's lenses. Just make sure they're really good ones; and, of course, that they're clean, clean, clean.

Hope that helps.

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    \$\begingroup\$ Wow. That all sounds like a lot more work than not using a filter and not stressing out if there is a little dust on it. \$\endgroup\$
    – mattdm
    Commented Mar 30, 2014 at 0:05
  • \$\begingroup\$ MATTDM WROTE: Wow. That all sounds like a lot more work than not using a filter and not stressing out if there is a little dust on it. MY RESPONSE: It's not just about dust. It's about things stratchging the multi-coated glass of a $900 lens. Not a trivial problem, no? \$\endgroup\$
    – Gregg DesElms
    Commented Mar 30, 2014 at 1:32
  • \$\begingroup\$ It takes a lot of dust and/or scratches to affect image quality to the degree a mid-grade UV filter can. See lensrentals.com/blog/2008/10/front-element-scratches and photo.stackexchange.com/questions/26965/… \$\endgroup\$
    – Michael C
    Commented Mar 31, 2014 at 1:12
  • \$\begingroup\$ I personally know a few photojournalists whose photos are carried by the wire services (AP, Getty, Reuters, etc.), magazines such as SI, and even a front page on the NY Times. I've not seen any of them with a protective filter on the front of some very expensive lenses in well over a decade. Things were different in the film era with respect to expectations regarding image quality than in the current digital environment. Many of the most expensive telephoto lenses don't even have threads for filters on the front of the lens. If a filter is required an internal drop in type must be used. \$\endgroup\$
    – Michael C
    Commented Mar 31, 2014 at 1:18
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As long as you take if off between shots, you will be OK :)

Cheap UV filters are worse than expensive ones but both destroy image-quality. Only keep them on the lens when you absolutely must like when around splashing salt-water or a sand-storm. Otherwise you will have flare and unwanted ghostly reflections.

For the case when you need the protection, it is best to opt for a high-grade multi-layer design which has a tendency to flare less and usually does not affect AF and sharpness.

If you want to see what these artifacts look like, there are plenty of examples on this QA site from people asking what is wrong with their images, with the answer mostly being the use of a UV filter. Think about it, adding a low-cost flat glass surface in front of high-quality optics is unlikely to preserve image-quality at its fullest.

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There can be issues with autofocusing, though it is my experience that more expensive cameras and lenses negate this so using a cheap UV filter with something like a Nikon D600 is not really an issue per say. Others may have different experiences.

In generally I would avoid using them in situations such as lowlight and reserve them for outdoor shooting in environments like the woods or beach.

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