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Photographic Filters as Lens Protectors

by Roger N. Clark

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The use of filters to protect lenses is controversial. The following is my opinion.

I use Hoya super coated UV filters on my lenses except super telephotos. I used to use tiffen. I have never seen an issue with degradation of image quality, except for the following: 1) Some 35mm lenses have a pretty flat front element, and 2) large aperture, telephoto lenses. The flat front element on some lenses can result in a reflection between the front lens element and the filter (any filter). I have found reflections in images with highly transmitting filters. (I have not seen the problem with circular polarizers). The Hoya super coated filters do not have this problem, and their optical quality is high, although not necessarily high enough for big telephoto lenses, like 300 mm f/4 or 400 mm f/5.6 and larger.

Filters on large telephoto lenses require the highest optical quality, so only use the best. The larger apertures of telephoto lenses, combined with their high magnification means any imperfection in the filter could degrade image quality. Indeed, see this article for actual demonstrations of such degradation: Evaluating Filter Quality.

The advantage of a filter is that it protects the front lens element. For example, when you work outdoors you will get dust on the optical surface then humidity/water spray/rain moistens the dust and glues it to the optical surface. Water and/or humidity partially dissolves the minerals/chemicals in the dust, which if not completely removed, will later precipitate onto the optical surface as the surface dries, cementing the dust particle to the surface. You can clean it carefully, but over time the optical surface and coatings degrade. One can simply throw away the filter an put on a new one when you get to that point. High humidity environments will do the same.

Figure 1. Dirty filter after a day at an ocean beach. Wind and salt spray result in salt crystals and dust on the filter. Many such sessions degrade the optical surface, requiring replacement of the filter. That is better than replacing your lens.

Examples: Hiking in the mountains: dust gets on the camera, you approach a waterfall and spray wets the dust. Ocean spray of salt water is very damaging. Snowy day: snow wets the lens, gluing the dust down. etc.

If all your work is in a clean room, you don't need a filter.

While a lens hood protects against some of the dirt problems, wide angle lenses are more of a problem because the hood can't provide a tight opening. My 500 f/4 L IS lens has a great hood and does a very good job of protecting against dust, and I do not use a filter on it (they are not made that big).

For my smaller lenses, I use step up rings to bring most lenses to a common size. That way I only need a couple of filter sizes. For example, my largest lens (excluding the 500 mm) uses 77 mm filters. But most of my lenses use 72 and smaller filters. So I bought step-up rings to bring those lenses to 72 mm. I use 72 and 77 Hoya super coated filters on my lenses and with digital I only carry 72 and 77 mm circular polarizers. On my large format system, I also use 72 mm filters, and use Hoya super coated UV filters on them and carry 72 mm circular polarizers and 81A and 81B filters.

Because I have found image degradation with high quality filters on telephoto lenses of 300 mm and longer, I no longer use front protective filters on telephoto lenses requiring larger than 72 mm filters. The long lens hoods on telephoto lenses do very well in protecting such lenses except in the harshest environments. I carry a "rocket blower" as small rubber air bulb that when squeezed ejects a fast air stream that blows off dust and water drops. Such a device works well on lens surfaces as well as the camera and lens bodies.

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First Published 2005
Last updated December 26, 2009