To use a filter or not to use a filter on a lens?

[Aldowski]

Yes. Few months ago I had to clean sensor (on A3000) with the swab, as photos had few small dots at the top. I had to make test with white paper, so next step was sensor cleaning. Those dots were visible only at narrow apertures. Lost at F3.5, for example.

[craig50]

I don't use UV filters. Yeah, they can offer some protection but chances are if it's enough to break the filter, it's gonna do some damage to the lens (I've had a lens tip over with a filter ad the lens was damaged anyway. UV filters introduce another element in the signal (light) path that can degrade the image. I had a once (twice) in a lifetime opportunity to travel 14 hours to shoot the northern lights a few years ago. Thank God I shot with two cameras. One was ruined due to the filter and introducing Newtonian Rings.

That said, I do used filters on a regular basis when required, such as ND, polarizers, solar filters (I do solar photography), infrared filters, red filters for black & white work. 

[Pieter]

8 hours ago, craig50 said:

Yeah, they can offer some protection but chances are if it's enough to break the filter, it's gonna do some damage to the lens (I've had a lens tip over with a filter ad the lens was damaged anyway.

The filter won't protect your lens from breaking in case of a full frontal collision, but the metal ring of the filter (and also the lens hood when used) will absorb a lot of impact energy when the lens is hit along the outer rim (which is usually the case). The filter is also there to prevent abrasion of the front element during cleaning and to prevent salt spray etc from eating into your lens coatings.

8 hours ago, craig50 said:

UV filters introduce another element in the signal (light) path that can degrade the image. I had a once (twice) in a lifetime opportunity to travel 14 hours to shoot the northern lights a few years ago. Thank God I shot with two cameras. One was ruined due to the filter and introducing Newtonian Rings.

If you buy good quality multicoated filters, the effect on image quality usually is imperceptible (less than 0.2% light loss, no additional ghosting/flare/coma). A weathered front element / coating on your lens will also degrade image quality. A filter is easily replaced once it starts to show wear, unlike a front lens element.

Due to the extremely monochromatic light emitted by aurora borealis, this is indeed a very specific case where the use of filters might do more harm than good. Especially when cheap filters without proper antireflective coatings are used.

[bwana]

On 11/26/2020 at 8:31 AM, Steve Waldman said:

Do you always put a filter on your lenses to protect them or do you never put a filter on your lenses because it makes the photo less sharp?

I've never liked filters.  I've had nothing but problems with lens flares, etc.  I find a lens hood is just as good at protecting the front element and doesn't mess with image quality.