Three Important Filters

Once you get into photography for a short time, you will want to add filters to your arsenal. These filters are a UV filter, a circular polarizer (CPL) and a one or more neutral density (ND) filters. These screw onto the front of your lens. A second option for ND filters are filter holders that hold flat glass or plastic ND filters. This may be considered superior by some, but it is certainly less convenient. More about filter holders later. You may even acquire multiple ND filters of various strengths. What follows are some of the uses of these filters and some criteria for selecting filters for your camera and lens system.

UV Filters

A UV filter serves two purposes. It reduces unseen UV light and the visible haze that often accompanies it. It also protects the front of the lens from scratches, but it can be scratched easily itself. You should keep UV filters on your expensive lenses. Cheap lenses, not so necessary. Scratches on the coating on the front of an expensive lens greatly reduce the resale potential for the lens. That alone is worth the cost. UV filters will reduce light transmission by a little over one-half stop, therefore you might not want to use one in darker situations when no tripod is employed, such as night time street photography.

Circular Polarizer Filters

A circular polarizer takes some of the light that comes to your camera lens and filters it out so that glare and reflections can be reduced or eliminated. This is beneficial in photographing water when you want to remove glare or when you want to see the rocks under the water. It  will also reduce glare off wet rocks in the stream or on the shore. For street photographers, it can reduce reflections from storefront windows. A CPL filter can help you reduce reflections on pictures inside your home as well.

A CPL filter rotates, making you a light bender. You turn the filter until you get the maximum reduction of reflections or glare. The filters are most effective when shooting at a 90 degree angle to the direction of the light. They have little effect when shooting directly into or away from the sun. These are usually times you do not want a filter anyway, because the filter would reduce the sunset reflected in the lake or it would reduce the refracted colors of the rainbow. 

If you keep the CPL on your camera when shooting around waterfalls it may need to be wiped dry often. This is preferable, however, to wiping your more expensive lens so often. 

Neutral Density Filters

ND filters reduce the amount of light coming into the camera without affecting the quality of light. Some ND filters do impart some color changes to your image. These changes can usually be corrected in post processing.

The reason for neutral density filters is to allow you to use a longer exposure or a wider aperture. The brighter your scene, the more likely you are to use an ND filter.

A few uses of longer exposures are:

• Smoothing out oceans or lakes to give a flatter and less reflective surface
• Blurring waterfalls and rapids to give the sense of motion
• Blurring clouds to give them a dreamy effect; this works best on windy days
• Blurring moving people to give a sense of movement; with very long exposures they may appear as ghosts or may completely disappear the scene
• Keeping the lens open to capture lightning, especially during the day

A couple of uses of a wider aperture are:

• To create a shallower depth of field for a dreamier effect – useful for flowers and brides
• To isolate a portion of the scene from other foreground or background features – especially useful for outdoor video of a person

Why you might need more than one ND filter

ND filters come in various strengths reducing the light by one-half at each gradation. A two stop ND filter cuts the light in half twice, meaning two stops of change on your camera. A four stop ND filter cuts the light in half four times…that is, it cuts the light by 2x2x2x2 or 16 times. Most manufacturers refer to this light reduction in terms of stops which helps you relate it to the stops on your camera. But some manufacturesr use .3 as one stop, .6 as two stops, etc, all the way up to 3.0 which would be 10 stops. Adding a ten stop or 3.0 ND filter would take a 1/30 second exposure and turn it into a 30 second exposure.  Another system of measuring an ND filter’s strength simply states the multiplication factor you would use if you are going from your exposure without the filter to your exposure with the filter. A six stop filter would be labeled as a #60. If your standard exposure time is 1/125 of a second your new exposure would be 60/125 or which rounds to half a second. All this may sound confusing if you are new to ND filters, but you will quickly learn what works for your situation. And, there is an App for that! 

How to Decide What CPL filter to buy?

The simple answer to this question is to look in your wallet and see what you can afford. To a point, the more you spend will result in more desirable light transmission through the filter and better coatings with less color change. You can expect to spend $30-$150 for a filter that will fit most lenses under 100 mm in focal length. To evaluate further, look on a site like BH Photo & Video and read the reviews. Be sure to check whether the CPL also reduces the light coming in to your camera. Some CPLs reduce light anywhere from one-half to two stops. I prefer the least light reduction available, using ND filters when I need longer exposures.

How to Decide What ND filters to buy?

Depending on your budget, you will be purchasing either one ND filter or a kit of several filters. There is also the alternative to purchase a variable ND filter (VND). The VND changes through a range of ND values as you rotate its outer ring. The higher you go the more likely the VND is going to introduce some dark areas into your image. In my experience, a VND is most useful up to six stops, but not above. The VND is two polarizing filters that turn independently. As such, it can replace your CPL filter. I would prefer you buy a smaller quantity of ND filters. ND filters can be stacked. Therefore, if you purchase an ND 3 and an ND 6, you can make an ND 9 by combining the two. When adding  ND and CPL filters to very wide angle lenses you are likely to end up with useless corners on your images. This is exaggerated if filters are stacked.

If you are going to buy ND filters one at a time, first decide on what type of photography you are most likely do. If you want to take smooth ocean scenes, then a ten stop filter might be the way to start. This will allow you to take exposures of 8 to 30 seconds, even longer if in twilight. If you are into waterfalls, start with a 4, 5 or 6 stop filter. This will allow exposures of around 1/8 to 2 seconds. To keep your kit small, according to Photoboomer “go light” principles, stick with no more than three ND filters. My setup is a 3-stop, 6 stop and ten stop. But 2-stop or 3- stop, 5-stop and 10 stop also makes sense, as it would also give you a seven or eight stop alternative by stacking the filters. There are some ND filters that also polarize. Again, check out the reviews before going that route.

As mentioned in the opening paragraph, ND filters also come as 100mmx100mm sheets that slip into holders that fit to your lens. I love these, but have gotten away from carrying my set due to the additional space and weight they take up. They are available in glass of acrylic form. Acrylic scratches easily and usually has a strong color cast. Glass is far superior until you drop one, which is a quick way to spend another 100 bucks. On the plus side, the additional size of sheet filters helps reduce the corner darkening that comes from stacking round filters.

Choosing the size for your filters

Round CPL and ND screw-on filters are made in standard sizes to fit most lenses. With your camera system, look at what lenses you might eventually end up with, not counting long telephotos that tend to take huge (and very expensive) filters. If you are using a full frame camera, this size could be as much as 82mm. For APS-C cameras 72mm or 77mm would be a much more frequent largest size. These numbers are always on the inside outer rim of your lenses. Buy that size. Then look at all of your lenses and their filter thread sizes. Let’s say you have 49 mm, 63, mm and 72 mm with a largest size of 77mm. Buy three step-up rings- one 49-77, one 63-77 and one 72-77. Of course, it would be ideal to have identical filter threads on all your lenses, but you are unlikely to find that situation. However, you may want to take this into consideration when choosing between two equal lens options. It is helpful to reduce the number of step up rings in your bag.

Step up rings are relatively inexpensive. They are thin so your filter does not get too far off the lens which would create an even greater vignette or cut off corners of your image. That puts a premium on quality. Aluminum step up rings are notorious for getting stuck on the filter, rendering the filter useless except on one lens. Brass step up rings will cost two or three times as much. They can still get stuck, but not so much. A little candle wax ahead of time on the step up ring threads can help. A ring remover tool can help rescue the stuck situation. 

I’m happy to answer questions for any of you newbies to filters. Shoot me an email or use the contact form.

RS