Written by Matthew Wronka
A group of photographers meeting for drinks in a bar are unlikely to jump into debates about what filters they use in day-to-day conditions; yet on the Interwebs, the debate on whether a UV filter should be left on your lens at all times for protection—or whether doing so will degrade image quality—appears on any highly trafficked photography site.
It goes without argument that putting a bad piece of glass (or worse, plastic) in front of an expensive, sharp, lens is a bad idea. However, what about top quality glass? Is buying a $100 filter that will suffer wear and be replaced in two years a good investment to protect that $1300 lens, or would you be better off just buying a $600 lens? What's the difference between a cheap UV filter and an expensive one, anyway?
There are a few filter brands I trust for quality glass, and one of those is Hoya. When I recently built-out my new Micro Four Thirds system, I decided to get a UV-cut filter. Looking at what Hoya offered at 58mm, I came across a $16 UV(c) filter and a $40 UV(0) HD filter (where the HD stands for "High Density" rather than "High Definition", although marketing would probably want you to think of the latter as well).
What's the difference? A couple of things jump-out right away. The UV(0) HD filter, which is made in Japan, comes in a very nice box, with a portable plastic container. It's very nice. The UV(c) filter comes in just a plastic container which is slightly more difficult to manage—but it's unlikely that you'd be trying to carry the filters in these cases anyway as you'd either leave them on the lens or have a multi-filter wallet, so the point is moot. The UV(c) filter was made in the Philippines.
From the spec sheet, they're nearly identical. The High-Density filter has a thinner glass element at 2mm compared to 2.5 mm for the cheaper filter; both have a 4mm frame which look near identical to me. The HD element is supposedly more resilient and coated to be scratch, water, and stain resistant. It's possible that the thinner glass and coatings also help reduce flare in harsh light. The UV(0) HD filter advertises 99.35% light transmission, while the UV(c) lists "over 97%". In a controlled studio environment the camera's metering reacted the same for both filters, and the case of no filter. Likewise, the image quality was indistinguishable, as you can see from the 100% crops: