We have compiled a list of the TOP50 questions that we hear on our workshops. From gear, to settings, to composition, and shooting styles, we have decided to post three questions and answers a week till we get through them all.
We hope they answer some of the questions that you may have. Here is today’s question.
Question: Why are my images “blotchy” using a variable neutral-density (ND) filter?
Today's answer from David Topping: A neutral-density (ND) filter is great for adding motion effects to your images and it lets you create shallow depth-of-field by using large apertures, even in bright light. A variable ND gives you the added benefits of convenience and versatility by offering a range of densities (typically 2 to 8 stops) in a single filter. However, there are issues unique to these filters that you need to consider, whether you already have one or are thinking of getting one.
A variable ND filter works by using two stacked polarizing filters to control the amount of light allowed to pass through; by rotating the outer ring, it’s possible to vary the density of the filter. But there’s a limitation with this cross-polarization system that can result in uneven light-reduction. This can appear in the image as an “X” pattern or as “blotchy” vignetting, and it is most prominent at wider focal lengths when using the filter at the dark end of its density range.
To avoid this problem, it is important to stay within the maximum and minimum settings marked on the filter. However, you may still experience this problem within the filter’s range at or near the maximum setting. A slight adjustment to reduce the density of the filter should eliminate the problem, though it may also be necessary to change the focal length in conjunction with the density setting to get acceptable results.
Also, be aware that the dots printed on the filter ring between the “MIN” and “MAX” indicators are simply for reference to help you return to a particular setting; they are not f-stop increments. It’s up to you to determine the appropriate setting by taking a test shot and checking your histogram to ensure you're getting a correct exposure. This would be the time to look for any negative effects caused by the filter and adjust as noted above.
Another thing to consider with variable ND filters is the physical size. Because they are much thicker than a regular filter, there is more chance of vignetting with wide-angle lenses; this will limit your choice of focal lengths. Watch for this in your test shots, as well.
Ultimately, you should ask yourself whether you really need the versatility of a variable ND filter. You may find that having just two regular ND filters (for example, a 6-stop and a 10-stop), used individually or in combination, is all you really need. However, if you feel that the range of an all-in-one variable ND is what you require, you’ll need to be aware of their limitations and how to use them properly.
If you like using ND filters, or want to learn to use them better, why dont you come to Vancouver Island with us in August. all the details are here. http://northof49photography.com/vancouver-island-workshop