Article by: Robert Fisher
Acronymical criminality aside, there is a role for your Tiffen Variable Neutral Density Filter that you probably haven’t considered before: High Dynamic Range Imagery.
Yes, that’s right, HDR with a VND filter.
When we’re shooting brackets for HDR merging, typically we shoot in Aperture Priority keeping the aperture setting – and depth of field – constant while varying the shutter speed. In many cases this works fine. There are some instances, though, where a different approach is better.
Using a VND filter for HDR bracketing has a couple benefits. First, it allows you to get around any manufacturer-imposed limitations on in-camera Auto Exposure Bracketing. Some cameras only allow for three shots and even if you can go up to two stops apart, that may not be enough. The other benefit is in the motion or blur characteristics of your scene.
In the case of a static scene, the standard approach works fine. What about where you have motion in the scene? Like moving clouds or water. When you combine shots with different shutter speeds and you have motion in your scene, you can get some unpleasant effects as the motion is combined in the various shots. There can be odd choppiness to water and undesirable ghosting in clouds.
The scene below is a perfect example. This is at the very picturesque Elora Gorge in Elora Ontario and shot behind the historic mill in the town. The rapidly flowing water creates a problem when merging in HDR software. And can present a problem when merging manually depending on how the layers are masked together.
Zooming in on the cascade area of the water, we can see how the motion is affected using both methods.
First is the water from the bracket created with the VND filter. You can see that it has the nice, smooth look that we typically want from flowing water like this.
Next is the water from the sequence made using the typical bracketing methodology. You can see that it has choppy look at is not overly appealing.
Why is the VND version better? The answer to that is in the caption of the last two pictures. It has to do with the shutter speed. Using the VND filter, you are going to keep your shutter speed constant for all the shots and the ‘bracketing’ is going to come from adjusting the amount of filtration on the VND filter between shots. This use of a constant shutter speed keeps the blur effects consistent through the entire series.
The Tiffen VND filter allows for eight stops of light attenuation, ranging from two to nine stops, allowing for the bypassing of any AEB limitations in the camera.
You do have to make some adjustments for exposure when using this approach. You need to adjust for the minimum filtration of the VND and you need to adjust for the total number of shots to create your ‘overexposed’ and ‘underexposed’ source images. The adjustments aren’t overly difficult though.
I meter using the matrix meter in my cameras because it gives a good ‘average’ starting exposure. The typical caveat for meters being fooled by an abundance of dark or bright area still applies, so keep that in mind too. With my starting exposure determined and focus set, I put the filter on the lens and open up my shutter speed two stops to compensate for the filter. This gets me back to the ‘zero’ starting point. Next, I open up the shutter speed 3 ½ to 4 more stops to compensate for the dark end of the filter and give me my entire range.
I’ve produced a 15 minute tutorial video on this approach as well that you can see on Vimeo.