A few points on using protective filters on camera lenses. Here, I will be talking about using filters for protecting lens front elements, irrespective of other functions that filters may also perform (haze reduction, polarizing etc.)

There are two main schools: the filter-off school, including some prominent nature photographers, advocates not using filters at all, unless one can justify why a filter is (occasionally) necessary. The filter-on school says keep filters on. My personal view is that, in uncontrolled environments, for example outdoors, dusty factories, little kids running around with ice cream in their hands, it is better to keep filters on, unless there is a justifiable reason to (occasionally) take them off. For example, a deposit of dirt/dust/salt on a filter that is hard to clean quickly may justify removing the filter, taking a few photos and then putting the filter on until there is a chance to clean the filter properly. If continuing taking images in this specific situation is an overriding priority, it may even make sense keeping the filter off until the cleaning time and temporary protecting the lens front element by other means, for example a lens cap, a glove over the lens hood, a plastic bag etc.

I have had lots of various stuff deposited on my filters that I would not want to see on (and clean from) lens front elements. At least some modern lenses have special coatings on front elements that are quite easy to scratch (trust me on this one). While it might be possible to clean lens front elements without any damage in a controlled environment, trying to remove nasty stuff from the front element quickly in the field would be a risky business.

More expensive protective filters also have some coatings on, to reduce reflections; however, filters are usually much cheaper than lenses. If I scratch a filter, and the scratch affects the image quality, I just get a new filter keeping the old one for especially dirty shooting conditions.

By the way, I test my filters, especially if I suspect that the filter might be seriously degrading the image quality. One of the obvious signs is a lower contrast: the image may look washed out. Some years ago, I have tried one of my favourite super zooms from film days on a digital body and was very disappointed by the lack of contrast. I was almost ready to dismiss the lens as not compatible with digital cameras. Out of curiosity, I have removed the filter and taken a few images again: they all came out contrast and sharp (see below). The filter was at fault. In that case, cleaning the filter thoroughly has improved the image significantly. However, this specific lens still works noticeably better without any filter.

Image captured a few minutes later (cropped), after removing the filter

And this brings me to a very important point. Placing any protective filter on a lens, even the most expensive multi-coated, sparkle-clean and straight out of the box, will degrade the image quality. In controlled environments, for example a portrait studio or an office, it makes a perfect sense to photograph without filters, with the hood on, if available. In fact, hoods provide a certain additional level of protection in most cases and I prefer to keep them on even if photographing with protective filters.

In case of any concerns about the extent of image degradation when using protective filters, I suggest going to a store and trying several different filters on, writing down file names and filters used. Take images of the same object from the same position and in the same lighting environment keeping the framing unchanged. Then come back home and check the results on your computer screen. If you find a filter that causes no noticeable image degradation, you might have a winner! Various lenses may respond differently to the same filter: make sure you check all the lenses that you are planning to fit this filter on.

Finally, what about the image of a shattered filter at the beginning of this post? This one was on my lens that was attached to a camera and packed inside a camera bag. When jumping off a truck with my bag in one hand, it hit the ground, not hard but noticeable. Something inside the bag, or maybe a sharp rock on the ground, must have punctured the glass. The lens was OK: I only needed to blow off the glass fragments from the front element: no noticeable damage!

May 2014