While photographing a mountain stream some days back, I realised the need for a dark ND filter. I was hoping to capture some motion blurs too. Unfortunately the darkest of the filters that I had were not dark enough for the purpose. Faced with a need to buy a dark ND filter, I started searching for one and in the process, came to know how overwhelming the choices are. Buying a simple filter can be a daunting task. There are a huge number of variables and various things to be considered. Filters have come a long way and now these are not just a simple piece of glass in a ring.

(Stack of Bamboo – Clicked with a Nikkor 105mm lens with polarizing filter mounted on it)

 

What defines a good filter?

How well a filter performs the function that it is supposed to do is what defines it. A colored filter should have a clear color and no other hues. A polarizer should polarize the light well without adding any color casts and do it without reducing the available light too much. ND filters should darken the scene without adding any color casts. Filters should be distortion free, reduce the chances of flare as much as possible, easy to clean and not cause any vignetting on wide angle lenses.

 

Filter Size

Every second lens that I own seems to have its own filter thread size. Damn these lens manufacturers. Even 50mm f/1.8 D lens from Nikkor from some years back has a 52mm filter thread and the new Nikkor 50mm f/1.8 G lens has a 58mm filter thread. What can I expect from other focal lengths and companies? Now I have lenses with filter threads starting from 49mm and going all the way to 86mm! Buy the largest filter thread that you intend to use. That is approximately what I have also done. I have finalized a filter for 77mm filter thread since that is the largest filter thread size in the lenses with which I will be using this ND filter. The important thing to evaluate is the use of the intended filter on the lenses that you own. I did not opt for the 86mm filter thread since that is a very long telephoto lens and I’m never going to use a dark filter on that. So for the lens with 77mm this filter will fit fine and for all the others I’ll use stepping rings. Stepping rings are simple and straight-forward adapter rings that let different filter thread sizes to be used on any lens. The quality of these varies as per the price. A good filter costs! For occasional use, buy an inexpensive one. Another small suggestion – buy a screw-on hood for the largest filter size that you are planning to buy. Many a times the bayonet hoods sold with the lenses can’t be used because of the large filters. This is when the screw-on hood for these large filters helps.

 

Filter Ring

The filter itself is mounted on a metallic frame. A good quality frame or ring can make the mounting and un-mounting the filter easy. A poor quality ring can make spoil the filter thread of the lens and sometimes even cause galling. With new lenses that have polymer filter threads or alloy based threads, this is not much of a problem but then it is always better to play safe. Brass rings are superior to aluminum rings. The same logic applies to stepping rings too. For wideangle lenses, thin profile filters (thin frame or ring) can help prevent vignetting. For long teles, the thickness of filters hardly matters. The ring itself should be easy to grasp, turn around and internal surface should be non-reflective.

 

Quality of Glass

Good filters use good glass. There are filters available now which are made up of polymers (read plastic) but the good ones are made up of glass. Schott glass is the in-thing now a days. This is high quality glass with no distortions. Inexpensive glass can have minor distortions and are therefore better avoided. Hoya had come out with hardened glass some years back (the HD series filters). This is supposed to be stronger than the conventional glass used in filters. In fact there was a video that went viral in photography circles, where a Hoya salesman was shown banging a HD filter on the edge of a table. If you are planning to buy a clear filter for providing some kind of protection to your lens, then go for this version (and no I am not going to debate the use of clear filters for protection here).

 

Quality of Filter’s Effect

Like I said before a filter is as good as it works. Polarizer should ideally polarize the light without actually adding any color casts and by minimal effect on the amount of light passing through. Good quality polarizers are lighter in color than the poor quality polarizers. Similarly a ND filter should reduce the amount of light by exactly the amount it is rated at, equally in all the regions and without adding any color casts. A colored filter should have only the color stated and at the strength stated. The filter factors should tally with what is expected from the filter. Filter’s quality also depends on if the effect is a part of the glass or the effect is created using a specialized coating or polymer film stuck to the glass. As you might have guessed filters with the effect built into the glass are better than filters where there is a effect coating or film attached to the glass.

Hoya once had two types of UV filters – UV(C) where a coating on the glass does the required work and UV(0) where UV reduction is built into the glass. The difference is minimal but I have always found the UV(O) version better. Investigate the quality of filter before buying.

With dark ND filters (anything above 9 stops of light reduction), do check the performance data in IR band as well. In very long exposures, the infra-red and deep red light can sometimes produce weird effects.

 

Coatings

As if the above factors were not enough to confuse a simpleton like me, filter manufacturers have a range of coatings as well. There are un-coated filters, filters with one or two coatings and then there are filters with multiple coatings. If the filters are going to be used where stray light or sun might shine on to the filter, it is better to invest in multi-coated filters. Same goes for filters which are to be used regularly on normal to wide lenses. With telephotos, a hood can be easily used to prevent flare. You can save money there and buy a single coated or even an uncoated filter.

For ND filters, consider the quality of coatings as well. The coatings ensure that there are no unsightly reflections. This is especially important since during the long exposure times, these minor reflections can create distracting and unsightly effects. Good quality coatings are also useful while cleaning the filters.

 

Brands

There are many many more companies now than I was familiar with when I started with photography. Some of these new companies are actually putting out really good filters. There was a time when I knew only Hoya, B+W, Cokin, Tiffen, Heliopan and a few others. There are many more companies now. Even Manfrotto is in the filters and so is Polaroid Optics. I had never expected to see these names in filters. Many more new names and a lot of inexpensive filter makers are also there in the business.

One argument says that using inexpensive filters on excellent glass will degrade the image to the quality of that filter. Another argument says that the filters are used very rarely and so shelling out huge amounts of money is not a good idea. If I keep the money factor aside, there is yet another line of thought that has recently emerged. One to one test of average filters and top of the line filters in real world scenarios do not seem to degrade photographs. Wow! Does that mean we can save our money and buy an inexpensive average quality filter for use on our super expensive lenses? In my opinion, check all the other factors as listed above and then buy an average brand. I have personally tried the most expensive filter that I own and a couple of generic filters on one of my sharp lenses. The generic filters did degrade the image quality a little. So, the age old wisdom of using good filters actually makes some sense. However the difference in the most expensive filters and some not so expensive brands is not much. For filters which are to be used occasionally, like ND filters, it’s fine to buy an average brand and not go for the top of the line filters. If you have really expensive lenses, then go for the best that you can afford. For polarizers, I recommend Hoya HD filters. These permit more light to pass through than many competitor brands. UV filters are a matter of debate but if you are investing in one, buy a good quality filter. Zeiss filters with T* coating control the reflections well. For ND filters, I prefer Hoya, Tiffen, Heliopan and B+W. Cokin and Lee have good range of graduated filters. Hitech and Singh-Ray are some more good brands to consider.

 

(Nikkor 105mm at f/5.6, 1/125 sec. Hoya polarizer with a little less than full effect. A full dialing in would have removed the shine from the surface of the water completely making the image bland. A little bit of shine on the surface with visibility of the sunken parts of the tree make this photograph complete.)

 

Square / Rectangular filters

Whole of the above discussion was focused around round filters that screw in into the filter thread of the lens. Rectangular filters a topic for an entirely different topic and in my opinion these are the best option for graduated filters. I’ll be writing about these in detail sometime soon.

 

Further Reading –
Polarizing Filters
Neutral Density Filters