The camera accessory market offers an abundance of filters that we can screw on, slide on or otherwise attach to our lenses. Along with it there is the never-ending discussion about their necessity. Filters certainly are not some frivolous item that sinister accessory manufacturers have dreamed up to get their hands onto more of our photography budgets.
For instance, there are color correction filters. These have lost a lot of their importance with the advent of digital photography where white balancing has virtually eliminated their need. But especially among Leica users, film and film cameras are still widely used and so are color correction filters. Anyone who has ever shot under fluorescent lighting appreciates the FLD and FLB filters that get rid of the ugly green cast common under those lighting conditions. We have the choice of daylight and tungsten film, but have the wrong film in your camera, and you will appreciate a proper color balancing filter to be able to keep on shooting without ending up with overly red or blue images. Excessive amounts of blue also occur when shooting during winter with snow covered ground on bright, sunny days. The blue of the sky reflecting off the snow will generally cause an excess amount of blue, something easily corrected with a skylight filter.
Then there are a myriad of special effects filters. These do apply to equally to film as well as digital photography. The need or value of them can only be assessed by the individual photographer. It’s an eye of the beholder thing.
Finally, there is the issue of lens protection. Many photographers have UV filters permanently attached to their lenses as a means of protecting them in case of a mishap. They certainly offer a certain amount of protection and the argument that it is a lot less expensive to replace a filter than a lens does make sense at face value.
However, the main reason for UV filters is the fact that some films are sensitive to UV light. This is out of the range of most lenses color correction with the result that the UV light will be recorded as an unsharp image, adversely affecting the overall image sharpness. UV filters are therefore designed to block any UV light from reaching the film and thus eliminate any of the UV associated unsharpness.
With Leica lenses such filters are unnecessary because Leica lenses are designed to block UV light from transmitting, making UV filters unnecessary.
During a visit with my father to Leitz Wetzlar we toured the entire facility with Rudi Kraut as our guide. He showed us everything from the camera production to the lens grinding department to the lens assembly. At one point he introduced us to a gentleman who was working in the lens design department. During our conversation the topic turned to filters. I simply asked what, if any, advise he had regarding filters. His face took on a rather stern look while he answered.
“If we had intended our lenses to have flat pieces of glass in front, we would have designed them that way.”
That caused me to research the topic once we were back home. After all, how bad can a flat piece of glass in front of a lens be? Flat is the keyword here. Unfortunately, some filters are less flat than others. Ideally, a filter is made of high quality, optical glass and ground from a blank, just like any lens element. The only difference is that the two surfaces have no curvature. The same precision and tolerances should be applied as with lenses. Only that will give the assurance that the two glass surfaces are perfectly parallel to each other.
Unfortunately that is not always the case. For one thing, there are two distinctly different production methods. One is the grinding process. This is an expensive process that is only applied by the top filter manufacturers. Unfortunately, the majority of filters are made in a much cheaper way. Here large, flat, narrowly rimmed surfaces are filled with glass granules and then heated to melt the glass into a large sheet. To make the actual filters, these glass sheets are again heated to the point where they become pliable and the filters are stamped in a process not unlike a cookie cutter. Cheap but not very precise. For one thing, the two glass surfaces are not nearly as parallel as can be assured with the grinding process. Secondly, the stamping does add a considerable amount of physical distortion to the edges of the filter which in turn does adversely affect lens performance.
Spectral transmission is another, important issue. Many filters need to be made with certain colorations to assure their proper effects. Here too we find considerable differences in accuracy. High quality filters are always dyed in the mass, meaning the glasses which the filters are made from receive the correct coloration during the process of making the glass. Unfortunately this process too is subject to considerable differences in accuracy.
A much less desirable approach is to sandwich dyed gels between two pieces of clear glass to achieve the proper coloration. Not only are there differences in accuracy regarding the spectral accuracy of the gels, but the problems of parallelism of the filter surfaces are doubled. This is actually an old, outdated approach and hardly any filter manufacturer outside of Tiffen still uses this process.
Another criterion is the thickness of a filter. Regardless how perfectly flat a filter is made, it will add a certain amount of distortion to any lens it is used on. The only variance is the focal length of the lens, with wide angle lenses being more affected by this than lenses of longer focal lengths.
The worst of all filters are the ones made of acrylic rather than glass. By nature these need to be a lot thicker to assure the desired effects. In addition, even the best acrylics are not nearly as clear as good, optical glass, thus adding to the undesirable effects of these less expensive alternatives.
The problem lies in the fact that when light hits the filter, it does not transmit straight through unless the light hits the filter in a 90 degree angle. There will always be a certain offset of the light path. The steeper the angle and the thicker the filter, the more pronounced this is. The only filters ever made to prevent this are curved filters. These are designed for certain focal lengths where the curvature is such that the light path through the glass is always reaching the filter in 90 degree angles. These filters are prohibitively expensive.
Finally, there are the filter mounts. Needless to say, we should stay away from plastic ones. They simply don’t offer enough precision to be worth any consideration. Most filter mounts are made of aluminum. However, most high quality lenses also use aluminum for the lens barrels. Aluminum against aluminum unfortunately has a huge amount of friction. This quite easily leads to filters being very difficult to remove. The best filter mounts are the ones made of brass. Brass against aluminum has a very low coefficient of friction and therefore brass mount filters are always quite easy to remove.
This brings us back to UV filters, permanently attached for protection. Do we really want this, do we really need this? Based on the flat glass comment at Leitz Wetzlar, I never use any filters unless absolutely necessary and I have done so for years. None of my lenses have ever been hurt because I take other safety precautions. The main one being that I always use a solid lens shade. That gives any lens a considerable amount of protection because the glass surface of the lens is recessed by a certain amount. This greatly eliminates the possibility of physical harm. Of course accidents can happen. I look at my insurance as a measure to protect my lenses in those cases.
Of course when shooting under condition where these measures are inadequate, a UV filter is definitely a good idea. For instance when shooting under extremely dusty conditions, or when wind whips up a lot of dust and fine sand, we should not subject our lenses to such ill treatment. That is where a high quality UV filter is definitely helpful. But personally, I leave it at that.
Should we all use just Leica filters? The simple answer is no. Leica is not a filter manufacturer. To my knowledge most of their filters are made by Schneider through their B+W division. B+W have proven to make some of the highest quality filters money can buy. Equal in performance are the Heliopan filters. Heliopan is owned by Zeiss. Staying with those two manufacturers will always give you the assurance of keeping the ill side effects of filters to a minimum. The top quality filters from Hoya could be added to that category as well.
Considering the overall performance of Leica and other high quality lenses it just doesn’t seem right to put flat pieces of glass in front of them except unless absolutely necessary. It especially doesn't make any sense at all to have the light pass through a cheaply made, low quality filter before it even reaches the lens just to save a few bucks. That approach has served me well over the years and will continue to do so.