How another Large Manufacturer makes Filters
Some filters are made with a thin gel or even colored glue laminate sandwiched between two pieces of regular clear glass, similar to the glass used in windowpanes. These types of filters are cheap to produce, but inferior for several reasons:
- Over time the expansion and contraction of the different materials can lead to de-lamination, which is a separation of the different materials. This will show up as bubbling, pealing, or discoloration, rendering the filter useless.
- The color of the gel can shift or fade over a relatively short period of time.
- If all six surfaces, three layers, two surfaces each, are not perfectly flat and perfectly parallel, the filter causes a “lens effect” which degrades the optical performance, or in extreme cases, shift or limit the focus of the lens it is used with.
How Hoya Makes Filters
To make colored filters, Hoya adds different raw elements, like gold, and chemicals to its optical glass while it is in a molten state. This insures that Hoya filter glass is uniformly colored all the way through. There is never any risk of uneven coloration, shifting or fading of the color, or delamination. The two surfaces are ground and polished for perfect flatness.
Hoya Coating and Multi-coating, the quality difference.
Hoya manufactures a full line of filters in both standard and Hoya multi-coated. The difference between Hoya’s standard line and that of other manufacturers is that Hoya standard filters have one layer of anti-reflective coating applied to each surface of the glass. Many other manufacturers standard filters are bare glass, and bare glass can reflect as much as 9% of the light hitting it. This greatly increases the risks of flare and ghosting.
This can be seen in a simple test. Take a bare glass filter hold it so that light reflection off the surface can be seen. Then take a long very thin object like a pin or the tip of a pen and hold it over the filter so that its reflection can be seen. There will actually be two reflections of the pin on the surface, one a little more pronounced than the other. The more pronounced reflection is from the front surface and the lighter one is from light reflecting off the rear surface.
Hoya’s single layer coating decreases light reflection off the surface from approx. 9% to an average of 4-5%.
To provide photographers with a higher quality professionals require, Hoya created the Multi-coated line of filters. These filters have a 3 layer coating system that further reduces light reflections off the surfaces of the glass, the average is only 1-2%. This means that 98-99% of the light striking the filter is going through it, and depending on the type of filter, into the camera lens and onto the film. These layers of anti-reflective coating are bonded to the surface of the glass in a furnace at a temperature of up to 800 degrees F.
You should beware! Some other manufacturers claim to have “coated” filters. But this coating is often only applied to the front side of the glass, not both sides like Hoya filters. Also, the coating on many filters is “painted” on or applied as a cold spray that wares off easily.
Best of the Best
In 1996 Hoya introduced the line of Super Multi-coated filters. Consisting of a Skylight, 1B, UV (0), ND 2X, ND4X, and a low profile circular polarizer, this line of filters has a 5+1 layering system on each side of the glass: 5 layers of anti-reflective coating and a transparent easy-clean top coat. This reduces light reflections off the filter surface to an average of just 0.3%. This is the lowest reflective rate on the market, from any filter manufacturer.
Lastly, how is the glass itself made? How do filters get their color? Some manufacturers simply take two thin sheets of regular glass and sandwich a colored gel or glue in between, as shown in the previous diagram. This process is called lamination. It is a very cost-effective process but not a high quality one. Remember, the disadvantage of this process is that over time the different materials can separate, causing bubbling or pealing (referred to as delamination,) rendering the filter useless.
Also the thin gel used can shift its color so that the filter does not yield the same color rendition over time. The last drawback of this process is that all 6 surfaces of the three layers have to be perfectly flat and parallel. If they are not, the filter will have a “lens effect” which can greatly reduce image quality.
To insure consistency in glass manufacturing, Hoya uses a furnace called an Automatic V blender to mix the different materials at a highly controlled rate. This process creates glass that is pigmented all the way through. With pigmented glass there is no chance shifting over time. There is also no chance of delamination. Also, the two surfaces of the glass are ground and polished for perfect flatness.
The only exceptions are Polarizer and Circular Polarizer filters. No matter the brand or quality, they all are made of a polarizing film, or a polarizing film and quarter wave plate in the case of the Circular Polarizer, sandwiched between two layers of glass.
Hoya believes the filter frame is an extremely important part of the filter as well. Hoya uses machined aluminum frames to hold their high quality glass. They prefer aluminum to other materials because it is strong enough to hold up to years of use. Some say that brass is the best material to use, however, Hoya doesn’t hold that view and here is why; brass is a far more rigid material than either aluminum or the polycarbonates that are being use in today’s lens barrels. This means that, should the front of the lens get hit, the rigid brass filter ring will transfer almost all the force of the shock to the lens barrels and mechanics. An aluminum filter frame will absorb some of the shock by bending and at a certain point the glass will chip or break, which is what the filter is supposed to do, protect the lens. Replacing a filter is always preferable to getting a lens repaired.
The Value in a Hoya Multi-coated filter
The wide aperture lenses of today are very expensive and all photographers want to get the most speed, optical performance, and dollar performance from their investment.
Say a customer pays $500.00 for a 28-70mm f/2.8 lens. Then, to protect this investment the customer buys a cheap bare glass filter, which has a light reflection rate of 9%. This filter is literally slowing the lens down by 9%, or effectively turning a $500 f/2.8 lens into the equivalent of a slower f/3.0 lens worth $455. The value of the lens drops 9% when you put the cheap filter on it. The cost savings of the less expensive filter do not off set the loss of lens speed.
Also, this does not address the loss of sharpness or focus shift, which can have a noticeable detrimental impact on picture quality. For these reasons, Hoya multi-coated filters present the best value on the market today.