Are there any real downsides to using a UV filter?
I know that a poor quality UV filter can effect image quality ... but, assuming I use a good quality UV filter, will that actually be an issue? Is there any other reason not to use one?
Whether the IQ reduction is noticeable and/or whether this is a good tradeoff is another question. :)
Cost of the filter is a very real downside. In case of a consumer-grade lens and a good UV filter, you might find yourself spending something around 1/5 of full replacement cost on "insurance" against dust (hardly affects image quality, removable), dropping and scratches (worse, but happen seldom).
If you use the lens in good conditions and/or rarely, you have certainly paid more than the risk is actually worth, while you could have used the money towards some other piece of equipment that would actually broaden your photographic abilities (e.g. a polarizing filter).
A lens hood would give you most of the same "insurance coverage" plus better optical quality for less cash. It makes access for possible scratchers and stainers harder; and in case of a fall, I would expect a hood to absorb the shock even better than a filter. And instead of degrading image quality with extra glass, it will enhance it by keeping stray light away.
In some environments with air pollution (sparks, paint, chemicals, oil, sandstorm, salty water, smoke), lens hood does not provide enough protection and using a protective filter does make sense.
When the lens are designed they usually try to minimize the number of optical elements as each extra element will affect the image. Good filters make no noticeable changes, however it's still an extra element which can, for example, add a reflection in some rare cases.
Also, if you are using a wide angle lens, then choose low profile filter, otherwise you may get a vignetting.
The use of any filter will result in the reduction of image quality. The extra air-glass interface that a filter introduces, no matter its quality, will degrade image quality. However, a high-end filter, such as one made by B+W or Heliopan, will keep this degradation to a level that is not perceptible under normal shooting conditions. Furthermore, if a clear or UV filter is attached to the lens, you'll need to remove it to use a polarizer, ND, or other filter, which can be inconvenient.
Light bouncing around between the filter and front element (or any other two air-glass interfaces) can degrade image quality by causing flare and reducing contrast. High-quality filters minimize this degradation by using anti-reflective coatings that reduce the amount of light reflected by the glass and increase light transmission. Low-quality filters have poor or no AR coatings and therefore can visibly degrade image quality.
I personally do keep clear (not UV) filters on my lenses (mostly B+W), so that I don't need to clean the front element of the lens directly, which can wear down the coatings on the lens--I'd rather replace a filter than replace the whole lens. You'll need to decide whether this protection and cleaning advantage outweighs the aforementioned loss of image quality and inconvenience. To me, it does, but you need to decide for yourself.
Another commonly cited reason for using a filter is that it can reduce the chance of the front element becoming scratched or broken altogether due to impact. However, a lens hood is generally more effective at providing this kind of protection, and this isn't the reason I keep clear filters on my lenses.
Others have already discussed most pros and cons, but I'll just have to add this one thing: Some lenses don't like any filters, no matter how good or expensive.
For example, Canon EF 85/1.8 USM with an UV filter will create tinted reflections all over the picture from bright lights. This especially bad when shot wide open. I've tried and experienced this with two of these lenses and with high-end coated filters from B+W and Hoya. Ultimately I just gave up and never use any filters with this lens.
I guess this probably happens because the front elements of the lens throw some kind of reflection out of the lens in an angle small enough that any additional glass surface will reflect it back in.
Mutli coated UV filters are intended to reduce sun flare - so if thats the effect you are going for, then you may want to consider losing the filter.
Some anecdotal evidence, I (read as: my wife) dropped my 50 mm f/1.4 on a vacation. We had a uv filter on it, and it cracked it in several places, and jammed the lens cap into the filter threading. After some work with a pair of needle nose pliers, we were able to pry it of. Thankfully the lens itself was fine.
Another downside is that filters tend to add artifacts and reflections to an image due to the different sensitivity to light (than film). As a result, if you have a filter on, even a high end one, you can get weird artifacts. I have only found this to happen when taking night shots of bright lights, and then it is unmistakeable.
Here is an example of a forgettable image I took, but one that shows the artifacts being produced by the large porch lights on the house.
The nice thing about a filter is that its removable, nothing is permanent. I leave B+W UV filters on all my lenses, as it protects them and also keeps greasy fingers from the elements. When taking night shots or critical shots that don't need the filter, I take it off. Its easy to do.
Yes. Adding two additional refractive interfaces will always affect image quality to one degree or another. Putting a flat filter on the front of a lens system will always increase lens flare caused by reflections. The best multi-coated filters will greatly reduce those reflections, but they will not fully eliminate them. The quality of the filter material can also add distortion to the light that passes through it.
One must always consider the optical penalty imposed by adding two more air/glass interfaces into the optical path. Just how detrimental that will be depends on the specific shooting conditions as well as the overall quality of the specific filter and its coatings. For instance, the optical penalty imposed by placing a filter on the front of a lens is more of a consideration when shooting into strong backlighting than it is when shooting with strong lighting sources behind the camera and there are very few specular highlights in the scene. Shooting a night scene with small but bright light sources in the scene will cause ghosting. The extent of the reflections will depend upon the coatings on the filter, the lens' elements, and even the front of the sensor's filter stack.
Whether a UV or other protective filter actually provides a protective benefit is a hotly debated topic. Because a thin flat filter is more likely to shatter than a thicker and differently shaped front element made of different materials, there are cases where a shattered filter may actually increase the amount of damage to a lens' front element by causing multiple scratches.
For more about the overall subject of To filter or not to filter (for lens 'protection'), that is the question, please see the following questions here at Photography at Stack Exchange:
Beyond our site here, there is a good series of blog articles by Roger Cicala, founder and chief lens guru at lensrentals.com, that addresses the issues surrounding using filters for protection. They are presented below in chronological order.
I have never looked at a picture and said to myself: "Oh wow this picture was so shot with(out) a UV-Filter..." because no one has ever seen the difference. It is just a tool like any other tool. Use it against scratches and if it makes trouble for you in a certain situation remove it from your lens and shoot without it. You won't use your flash every time just because it is attached to your camera, right?
You are usually on the right track using a good UV-Filter because it
But as aforementioned it is not a flawless tool because
After all I am shooting with UV most of the time because it protects my lens when I am outdoors and some punk runs into me at the traffic lights. Again, when I am shooting pictures I have to think about more crucial things of my composition than the UV filter, because a high quality filter simply does its job - keeping the aforementioned bullet points in mind.