I'm looking to buy a Skylight filter leave permanently attached to a lens for the primary purpose of protecting it. I don't really understand the difference between Skylight 1A and Skylight 1B filters. So, which of the two is the better choice for a general and always on filter?
The difference is that Skylight 1B has a slight pink tint, to add some warmth to the images.
If you have a digital camera and use automatic white balance, there will be no practical difference at all between the filters as the white balancing compensates for the color tint. I would choose 1A as it has no tint, so it affects the incoming light less.
The value of any UV filter for a dSLR is questionable. On the lens protection front, a filter that is broken, as noted, can scratch the lens, thus undoing any protection it may have offered in the first place. The other downside is that the filter can cause unexpected ghosting of light sources in your image as a result of reflection, something that will annoy the heck out of you.
In any case, cameras and lenses are not as delicate as some make them out to be. Photographers have been running around with these in everything ranging from extreme weather to extreme violence without destruction of their gear, so unless you're extraordinarily clumsy and likely to bounce your camera and lens off of hard surfaces on a regular basis, I wouldn't worry about it. Buy your filters for photographic effect, not protection.
UV (or any other clear) filters have a purpose, but as others have said, they are NOT generally necessary or especially useful to protect or shield the camera lens. I much prefer a good full round lens hood, preferably flexible rubber, to protect the front of the lens from bumps. Soft rubber is preferable to hard plastic or metal, as the rubber provides some "give" if the camera is bumped or dropped, thereby absorbing most of the forces generated. This protects not only the lens but also the lens threads and mounting lugs.
What are UV filters good for? Well, it depends... UV filters were originally designed to protect sensitive films (remember film?). As mentioned elsewhere, Digital cameras don't generally have this worry.
UV filters can also be used to protect human eyes from concentrated UV in a view finder. If you shoot a lot with the viewfinder constantly at the eye and are taking in bright outdoor scenes, particularly with sunlight reflections off of water, snow, bright sand, glass or metal (i.e. motorcar and architecture), there is a small chance of causing what amounts to a sunburn on the surface of your eye. UV filters will reduce this possibility. If you don't have or use a "live" viewfinder, this is, or course, moot.
1A is clearer than 1B, which will tint and add warmth.
Best filter to always have on is none.
Guffa's answer is incorrect. Both Sky-1A and Sky-1B filters block about 50% UV and are tinted magenta (by absorbing green). The letter indicates the strength of the filter, so the 1B would be expected to have a stronger tint. For a filter that has no color tint in the visible spectrum, use a UV-0 filter.
I know this is an ancient question but on reading the answers saying these filters are pointless I couldn't ignore it.
The benefits of filtering UV and/or IR is that camera sensors are sensitive to a wider range of light than is visible (that's why many video/cctv cameras can see full-colour at day and by infra-red illumination at night). Many sensors/imaging modules have an IR-cut filter built in that's physically moved in & out of the light path by a servo (Sony industrial camera blocks as used in most high-end CCTV are #1 example of this). The humble Raspberry Pi camera module is available with the filter removed "NoIR version" to enable night-vision.
Non-visible light picked up in this way can really screw with your picture - depending which pixels of the sensor are sensitive to which colours you can get an odd tint, colour shift, fringing, incorrect metering and confused auto-focus - we have seen all of these in the field.
With modern sensors with their amazing low-light performance you hit problems more quickly than before - with IR or UV wavelengths being focused differently through the lens you can end up with an AF system that's battling a picture with multiple focal-points in low-light situations. We had to prove this using a cold-mirror filter on one installation where bright IR illuminators were battling bright flood-lighting for dominance.
Personally I notice skylight filters knock the brightness of the sky down a tad, making it a deeper blue with better defined clouds rather than just a big bright area of little interest.
All that aside, a skylight filter is also an excellent and cheap way to protect an expensive lens. Being available for cheap (
For reference, here's the spectral response of a Sony HD industrial camera block that costs more than many entry-level DSLR's:
Notice how it's sensitive down into the UV end as well as way up into the non-visible IR end, and how the sensitivities of the three colours (RGB) do not just tail off outside of their target colour - this is how you get an odd purple-fringed light from IR as the blue + red cells pick it up first.