Filters after the fact: Digital split ND filters versus HDR
Ellen Anon | Software Techniques | Published Oct 6, 2011
A common dilemma when photographing landscapes is that some portions of the scene are far brighter than others. A prime example of this, shown below, is when photographing scenes containing both sky and reflection. The reflection is often several stops darker than the primary subject and of course, the sky.
|This is how the scene is rendered with no density adjustment. The sky is too bright while the geyser is too dark.|
The traditional in-camera solution is to use a split neutral density (ND) filter to balance the overall exposure. A split filter works by reducing the amount of light that reaches one area of the lens. Split ND filters have long been mainstays in the gear bags of landscape photographers. They come in various strengths (able to reduce exposure by 1, 2, 3 or more stops) and are available with transition areas (between reduced and normal brightness) that can either be gradual or abrupt. Yet there are many situations in which they are not ideal.
The most obvious is when the boundary between the area in the scene you wish to darken and the area you wish to leave unaltered is not a straight line, as in the picture of the geyser above. Split ND filters typically change density across half of the filter. Trying to compensate for a V-shape area as might happen if photographing between mountains, or any other non-linear shape will not be as successful. In such cases it’s better to apply a solution in the digital darkroom.
There are several digital solutions that address these issues. Programs such as Nik Software’s Color Efex Pro 4.0, Adobe Camera Raw (ACR), Lightroom and Aperture all have tools that can simulate the effects of a split ND filter, with the great advantage that the density adjustment can be applied to multiple and non-contiguous areas of the image. Alternately, using HDR (high dynamic range) software such as Photomatix or Nik Software’s HDR Efex Pro are options as well.
Whichever program you use, the first step is to look at the histogram of the best-exposed version of the image. The histogram for the image at the top of this article shows clipping in the highlights (notice the spike at the far right of the histogram). An ideal solution would be to combine two bracketed exposures (one containing full highlight data and one with rich shadow detail) into a single image.
|The histogram shows clipping in the highlights.|
If you are working with a Raw file, however, it’s possible that you may be able to reveal more detail in the highlights and/or the shadows from just a single exposure. Unlike 8-bit camera-generated JPEGs, Raw files, with their higher bit depth contain greater headroom. To determine the potential for your file, open it in a Raw converter of your choice. Adjust the overall exposure slider and check the histogram to see how much detail you can pull back in the highlights, even if the rest of the image becomes quite dark. Similarly, adjust the slider in the other direction to check for detail in the shadows.
In our image, decreasing the exposure by 2/3 of a stop regains the highlight detail, as shown below. Even better, in this particular image, I don’t lose detail in the shadows even though the overall image becomes too dark. Therefore I can expect that both single image HDR Toning and digital split ND filters will give good results. In addition, I've applied some fill light to create the best starting place for some of the other techniques.
|With a Raw file, we have brought out highlight detail that appeared to be lost, by applying an exposure value of -.65. Notice the histogram now tapers off at the highlight end.|
If you still see a spike at either edge of the histogram, then these techniques are not going to be able to restore full detail in the highlights and/or shadows - although the resulting image may still be quite pleasing. While our eyes more easily accept some loss of detail in the shadows, if there is significant clipping, the better approach would be to combine two or more exposures of the image into a single file, which I discuss a bit further on.
Digital Graduated ND filters
I tend to use the Graduated Neutral Density filter in Nik’s Color Efex Pro 4 plug-in, because it’s easy to use, you can customize the shape of the filter for each image and it's compatible with Photoshop, Lightroom and Aperture. Initially I set the Upper and Lower Tonality sliders to extreme positions (they determine how far to lighten or darken each section of the image) so that I can accurately place the Vertical Shift (specifies where the transition between upper section changes and lower section changes occurs) as well as the Rotation and Blend sliders which determine the angle for the transition and how wide a transition area there is between the upper and lower effects, respectively.
Then I adjust the Upper and Lower Tonality sliders to give a visually pleasing result. If I need to remove the effect from an area, such as the top of geyser in this image, I add a minus Control Point. Control Points are used in all Nik products to enable you to specify areas to apply, partially apply or disable the current settings. Control points employ auto masking which avoids you having to make time-consuming selections or create and edit layer masks.
You still have the option to tweak the final results using either the program in which Color Efex has been installed or a separate editing program such as Nik Software’s Viveza. Below I used Viveza to add an adjustment to lighten the geyser further.
Photoshop's ACR, Aperture and Lightroom provide adjustment brushes which allow you to simulate a graduated split ND filter with great flexibility. In these programs you create a brush to apply an exposure/brightness change to an area that you define by painting over the image. These programs use auto-masking as well. I do recommend zooming in to 100% magnification after applying the brushes to be certain the masks are accurate. If they are not you can easily modify them.
An advantage of the adjustment brushes in ACR and Lightroom is that you can have a single brush apply multiple changes, so that in addition to exposure changes, it can simultaneously apply clarity or saturation changes, etc. With adjustment brushes you can easily and accurately customize the area that’s affected as you paint with the brush.
In ACR and Lightroom you can also opt to use the gradient tool with the Exposure slider to emulate a split ND filter. Unfortunately you run into the same difficulties and limitations as with a split ND filter used while taking the image; namely that the transition area must be a straight line. It can be a little awkward to then try to add or remove the effect from an area that projects beyond that line. For example in the image above, the geyser would become even darker and would have to be readjusted using the Adjustment Brush. Of course the advantage of this approach is that the changes are made to the raw file itself rather than a converted file
- Realistic and natural looking results.
- Easy to do using several popular software programs.
- May require more steps to achieve final desired appearance.
- May have to check image at high magnification in some programs to ensure transition area is accurate and that any auto masked areas are correct.
An alternative solution is to use single-image HDR toning. To use any of the HDR programs, it’s best to begin with a Raw file or at least a 16-bit TIFF file. Most HDR software programs include options to modify the overall exposure, contrast and color saturation in addition to expanding the tonal range within the image. Some offer considerable choices in both the style of the algorithms as well as the strength of the effects.
Depending on the settings you choose, the results may be fairly realistic or quite surreal. It’s easy to get a bit carried away in some of the HDR programs and in most cases the image will be more dramatic, but potentially less natural looking.
I favor Nik Software’s HDR Efex Pro because of the unusually large number of choices in algorithms the program uses as well as a large selection of presets that serve as good starting points. The image below shows my original Raw file processed in Nik software’s HDR Efex Pro with settings chosen to keep the image somewhat realistic. You can see, however, that this result is different than the split ND version I created earlier, with increased image detail at the highlight and shadow ends and more saturated colors.
|The original image processed in Nik software’s HDR Efex Pro with settings chosen to keep the image close to realistic.|
If the initial histogram indicated significant clipping in your image file and you have several files of the exact same scene photographed at different exposures, combining them in the HDR program of your choice may create the best results. For the best results you want at least three separate image files, each captured at a different exposure. One file should contain no clipping in the shadow areas (even if this means the highlights are blown out) and another file should contain no clipping in the highlights (even though the rest of the image may be too dark) and one or more exposures in-between these two extremes.
|This image was created by combining three separate exposures in Photomatix Pro.|
A more tedious method of achieving a similar effect in Photoshop is to use the command File >Scripts > Load Files into Stack command to create a single image with each exposure as a separate layer. Then you can use layer masks to reveal the best parts of each file. This approach was more common before the arrival of effective HDR tools. Yet in cases where the tonal variation is clustered together in an area such as an overly bright sky or a distractingly dark subject, this approach is still effective and can lead to more realistic results.*
- After adjusting the sliders in a single interface, the image may not need any further work, particularly with programs such as Photomatix and HDR Efex Pro.
- Dramatic results are easy to create.
- Final image may not look as natural as desired unless restraint is used.
Both digital graduated neutral density filters and single image HDR toning, as well as multi image HDR processing can effectively even out the exposure within images that have areas that are far too light or too dark. They are the solutions of choice when the boundaries between the light and dark areas are not a simple straight line. HDR Toning is particularly well suited to images where the light and dark areas may be more scattered throughout the image, whereas digital split ND filters are preferable when seeking the most realistic results and there are discrete large areas of the image that are too light and/or too dark. The techniques discussed here work best when full highlight and shadow detail have been captured by the camera sensor in Raw format.
For full disclosure, I am a member of Nik Software’s Team Nik. However I write about the products I use and use those that are the easiest and yield the highest quality results for me. Of course you may prefer other software to achieve similar results.
*For more details on this see Photoshop CS5 for Nature Photographers; A Workshop in a Book. (Anon & Anon)