Ruined said:
Yes, you might get a small (VERY small) additional amount of flare with a B+W XS-Pro Clear multicoated 007m filter. There really is no other tangible impact on IQ.
But, in exchange you get:
* Weather-sealing: both the 16-35 f/4L IS and the 16-35 f/2.8L II are *not* weather sealed without a filter.
Yup. there is this. However most people give up due to weather before their cameras do.
* Ease of cleaning: Got some crud on the clear filter? Spit shine it with your shirt! If you scratch it, the investment to replace is minimal. Busting out a microfiber cloth in the field isn't always practical.
Hint: UV filters and protection filters scratch more easily than the front element of your lens. This means that your on the road cleaning of the filter with your tie can introduce a mark the results in flaring and needing to remove the filter anyway.
* Virtual lens cap: sometimes during sessions you need to swap between two cameras quickly. Would you want to throw a camera in a bag - or have it in a holster - with no lens cap? Well, assuming you don't drop your bag on concrete, for these fast swaps there is little risk if you put a capless lens/camera in a bag if it has a filter. One shouldn't make a habit of this, but its an option you generally would not have without a filter.
If you're careless with your equipment then bad things will happen sooner or later, irrespective of whether or not you put a filter on the front of your lens. Maybe one day when you throw it in the bag, the lens mounting will break, who knows.
* Riskier shots: 16mm on full frame may require you to get VERY close to what you want to photograph in some cases. Do you want to risk your unprotected front element in these cases? How about action shots, if you are photographing in harsh elements, or just greater confidence in general since you don't have to worry about your lens?
When I take risky shots it isn't the front element that's at risk, it is the entire rig and/or me. And I've taken some risky shots. Balanced on a rock in the middle of the Merced, if the camera falls and gets wet, the filter on the front of the lens ain't going to make any difference to anything except the front element of the lens and that's likely to be the least of my concerns. The occasional drop of water spray hitting the front of the lens is of little concern and a minor risk in comparison to everything else.
* In reality, it does cost a lot to fix the front element of a lens, and you will lose use of the lens while its being repaired: While some blogs have pointed out that the cost of a front element is not always that much, the labor to replace it usually is large amount and it involves your lens being out of action while getting repaired. If you are getting paid, this is not a situation you want to be in.
By all means keep a protection filter on if it makes you feel better/good but there is no reason, optically speaking, for it to be there today (unlike in the past.)
The problem is that people used to put UV filters on lenses to deal with film problems and habits are hard to break so people have kept doing that and in the absence of needing to do it for film, have come up with new reasons.
In this list of 5, there's one real reason (the weather sealing.) Everything else has just been hired in as supporting cast members.
