The Low Down on Choosing ND Filters

I had a question on my Facebook page today about Variable ND (Neutral Density) filters and my answer turned into a full on guide to selecting and understanding ND filters which deserved it’s own blog post. So thanks to Michele for suggesting the topic and I hope I answer your question 🙂 History Back in the days of film photography a professional photographer would have had a wide variety of filters in their camera bags but these days Photoshop has replaced nearly all of them, thankfully! Pretty much the only filters a professional photographer will carry these days are a Polarising filter and a set of ND filters. I won’t go into Polarising filters here except to add that with the popularity of ultra wide angle lenses and images stitching they are becoming less and less used. The Options The effect of using ND filter is to increase or equalise the exposure time for an image by darkening all or part of the camera’s view, it’s a bit like having sunnies for your camera 🙂 ND filters are generally used to either lengthen shutter speed to smooth out motion or reducing the dynamic of a scene by darkening part of the view. Another use of these filters is to allow you to decrease depth of field in very bright situations. A word of warning when purchasing your ND filters, to work effectively an ND filter must be a true neutral gray otherwise a colour cast will be introduced so avoid cheaper filters. For example I have read that the ND filters in the Cokin filter system are not at all neutral an introduce a pinkish colour cast 😦 There are two options for mounting ND filters to your lens:

1. Buy 1 or more glass filters to screw onto the end of your lens
2. Buy a filter system and a collection of ND filters to slot into it

Additional there are 4 types of ND filters available:

1. Solid ND filter
2. Hard graduated ND filter
3. Soft graduated ND filter
4. Variable ND filter

Finally, ND filters come in a variety of strengths, indicating how much light they block out. Unfortunately filter manufacturers have several different ways of measuring the light reduction values which can be very confusing, check out this wikipedia page and scroll down to “ND filter types” for a comprehensive table of light reduction values. Graduated ND Filters Graduated ND filters are used where there is a large dynamic range between the brightest parts of a scene and the darkest part of a scene. For example a sunset or a scene taken towards the sun where the sky is very bright compared to the land. Without a graduated ND filter you would have to select either the sky or the land to be properly exposed and sacrifice the other to be blown out (pure white) or under exposed (black). So a graduated ND filter will allow to get a much better sunset shot in a single frame, the alternative being to take multiple exposures and blend them together afterwards in Photoshop or Lightroom. Graduated ND filters come in 2 types, soft & hard, the difference between them is the transition from the clear part of the filter to the gray part. As the names imply soft grad’s fade smoothly from clear to gray whilst hard grad’s transition abruptly along a hard line. Soft grads are much more popular and a lot easier to use as they are more forgiving whilst hard grads are really only suitable in certain scenarios such as oceanic scenes where you can align the transition line along the horizon. It is important to be aware that ND graduated filters are a compromise, very rarely will the transition occur exactly where you need it, so you will get trees that are darker on the top than the bottom or sand dunes with a similar problem. Horizons are rarely flat in well composed images and nature has a way of doing things outside our plans, as always it’s about finding the right tool for the job at hand 🙂 Solid ND Filters Solid ND Filters serve a different purpose to graduated as they affect the whole image. Typical uses for a solid ND filter would be when taking waterfall scenes when you want to smooth out the flow of water and can afford a longer exposure time or when you are shooting on a very bright & evenly lit day and want to decrease your depth of field by opening up the lens. Variable ND Filters Finally we have variable ND filters. These tend to be more expensive and are usually created by having 2 specially designed circular polarising filters placed next to each other with one of them fixed and the other rotating. Simply rotating the outer filter ring will change the effecting light reduction of the filter within a set range. The benefits of Variable ND Filters are:

• Flexibilty, you only need 1 filter for a variety of scenarios
• No need to keep changing filters (which is a pain with screw-on filters)
• Easy to experiment with different settings

The drawbacks to the Variable ND filter approach are as follows:

• Very limited brand options with conflicting reports on image & build quality
• Expensive if you mostly need a limited range of light reduction
• Expensive if they don’t provide the image quality you are looking for
• You cannot get graduated Variable ND Filters
• Only come as screw-on filters (that I know of) so see below for the pros and cons of screw-on vs filter systems

Filter Systems By far the most cost effective and flexible way to purchase any type of filter is to buy a filter system which consists of a filter holder, an adapter ring for each of your lenses and a set of filters. The benefits of using a filter system over screw-on filters are:

• You only need to buy one of each filter
• Filters can be rotated and slid up and down to allow for ideal positioning
• Lens vignetting can be avoided with the right system for your lens collection
• Filters can be quickly & easily installed and removed
• Easier to attach multiple filters

The negatives of a filter system over screw-on filters are:

• Higher start up costs as you need to buy the holder and attachment rings just to get one filter
• There are some low quality systems out there which should never be put in front of decent lenses!
• Variable ND filters are not available in this format

If you do go down this path then it is important to make sure you get a good quality filter system as all you future filters will come from a single manufacturer and what’s the point buying quality camera equipment and putting a piece of cheap plastic in front of it! Personally I would recommend the Lee Filter System which is used by most professional photographers and they provide the highest quality products. Screw-On Filters The simplest way to get filters in front of your lenses is to buy a circular screw-on filter. There are many filter companies out there that sell single screw-on ND filter of varying quality, basically you get what you pay for so don’t skimp. The benefits of screw-on filters over filter systems are:

• Low start up cost
• Less bulky
• Plenty of options

The negatives of screw-on filters over filter systems are:

• Wide angle lenses can produce lens vignetting as the filter edges are so close to the lens, especially if multiple filters are stacked
• Need to purchase a new filter for each diameter lens in your collection
• Bulky to carry around large filter collections
• Graduated ND filters are not available as screw-on filters
• Much harder to attach & detach filters

Michale’s Answer As far as Variable ND Filters go (to answer the original question from Michele) no I have never used one and in my opinion they have limited use and would have to be proved to produce a quality result with professional build quality before I would consider buying one myself. That doesn’t mean they don’t have a place for certain photographers but as a screw-on only solution that wouldn’t integrate with a filter system they would not work well if you also needed to use Graduated ND filters. As far as I can tell the Singh-Ray Vari-ND Filter is one of the best around but I’ve also read some bad reviews on it and at #350+ USD (depending on filter diameter) it’s not cheap. They are also quite thick so the chance of vignetting is increased over standard screw-on filters, in fact Singh-Ray themselves have a disclaimer about vignetting on their Vari-ND product page lol. Also, I gather some portrait photographers are using Variable ND filters to achieve certain unique lighting effects, so intended use always plays a part in such a decision as well. If anyone has any good / bad experiences with Variable ND filters then I would love to hear them 🙂   Summary So to sum, up ND filters are one of only two types of filters you still need in your camera bag but that doesn’t make choosing the right filter easy. Personally my approach to filters is to avoid using them whenever possible to enable me to produce the highest quality image possible from any scenario. I will always opt for multiple exposures of a scene over a filtered version, sure it takes a bit longer to blend the images to get a finished result but I’m much happier with the quality I end with. On the other hand there are always situations where an ND filter is the only way to achieve the desired result. I currently own 2 x ND screw-on filters which I rarely use, if I ever found myself using them more frequently then I would without doubt purchase a Lee Filter System with a set of ND filters and ND Soft Graduated filters. Of course whenever I approach a waterfall the ND filters come out of the mothballs to produce the ol’ clichéd “Smooth Flowing Water” look :wink:. Note: All of the images shown in this post were taken without the use of any kind of ND filter using HDR techniques to show the full dynamic range of the scenes. These images are all for sale on my web site, simply click on any image to get more information.