1) Those who think that having a clear glass filter on the front of the lens is crucial for protection of the front lens element: If you accidentally bang the lens into something, the filter will take the blow, protecting the much more expensive lens underneath it.
2) On the other hand, there are those who think that the filter does little to protect the front lens element: It is much less strong, since it is thinner than the front lens element of the lens is. Also, if it does break, the shards from the broken filter could further damage the lens. The naked lens might not have been damaged at all by a similar blow, according to this group. This group also tends to propose using a lens hood for protection, and to say that any filter put outside the lens will degrade the image quality.
Now, it is hard to say which group is correct. I'm certainly not going to bang my lenses into a sharp object to see what kind of damage it takes.
But one thing that can be tested is if the filter degrades the image quality. I tried to use a cheap filter previously, and found that it did degrade the image quality significantly in high contrast situations, at night. This time, I use a somewhat more expensive filter, which claims to be multicoated:
Hoya HMC multicoated slim frame 58mm
I used it on the Lumix X 12-35mm f/2.8 lens. Here is a picture taken at 35mm at night, both without and with the protective filter:
|Without filter||With filter|
The only difference I can see here, is that the full moon has moved between the shots. It appears that the filter does not add any extra flare, unlike my previous test with a cheap filter.
I can compare the sharpness by looking at 100% crops from both images:
This comparison shows that there is in fact some loss of sharpness when using the filter. The image is not as crisp on pixel level. However, you can ask yourself if you really need that much sharpness on pixel level.
Here is another test with the lens at 12mm:
|Without filter||With filter|
In this case, there is a very subtle difference in the flare: Slightly more flare when using the filter:
But beyond this rather subtle difference, there is no significant difference between the image taken with and without the filter.
In daylight, when the contrast is smaller, there tends to be even less impact of using filter.
Based on this and my previous test, I can conclude that using a cheap, non-coated filter can seriously degrade your images, especially in high contrast. However, a reasonably priced multicoated filter can be used with little risk of degrading the image quality. If using a filter like this gives you peace of mind in terms of avoiding damage, then you don't risk damaging the images significantly either.
You can buy a filter like this on Amazon.
Appendix: Technical details
I took the images with a Panasonic GH2. I would have preferred to use my GH3, however, it is still with Panasonic for a repair. After some weeks of use, the automatic switching between the LCD and EVF stopped working.
The camera was set on a tripod, with OIS turned off, the ISO set to 160 (base value). I refocused for each shot, also after mounting the filter for the second otherwise identical shot. I used the self timer to avoid camera shake. The aperture was set to f/4, and the shutter speeds were around 1-2s.