Digital cameras and scanners have certainly revolutionized photography. They have brought thousands of photo enthusiasts and pros back to the world of do-it-yourself printing. A major factor in this revolution is the ease with which you can improve or radically modify your original images in real-time and see the changes without making expensive prints between alterations
I don’t claim to be an expert on digital editing like Rick Sammon and Joe Farace, but I can provide useful information by presenting software filters from a photographic and engineering perspective. (I will be using Adobe Photoshop Elements 2.0, along with nik Color Efex Pro 2.0 in all my examples, except as noted.)
What’s really going on in your computer?
All software-based filtering and image processing is based on mathematics. Your digital photo files, consisting of vast arrays of numerical data, can be subjected to clever mathematical manipulation to make the resulting images look like you want them to look. Fast personal computers with adequate memory make the manipulation of photographic images quick work.
Basic image processing involves adding, subtracting and multiplying the values in an image file. Contrast, brightness, color balance and fill-lighting functions are all based on this type of math. To show some before and after examples of what you can achieve, I’ll start with some amateur astronomical digital images of the ring nebula in the constellation Lyra.
Fixing a pain in the astrophotography
Two large problems plague the astrophotographer. The worst problem, particularly for those who can’t escape to really dark sky areas, is light-polluted skies. A typical unprocessed astro image file shot under such conditions will likely have few, if any, visible details.
By using a function much like “levels” in Photoshop and similar packages, moving a slider will clip the image-degrading skylight and darken the sky so the stars will show. Mathematically, you’re subtracting the skylight from the image. This works because the CCD sensors respond linearly to light levels. Subtracting the skylight will leave the brighter light from the stars intact! Of course other corrections, such as increasing contrast, are often needed as well.
Hey, what’s my camera doing in the fridge?
Additional astrophotography problems are noise and possible unevenness in the CCD array. These are corrected in two ways. The first solution is via hardware. By refrigerating the camera before taking the shot, the noise output of its CCD sensor greatly reduced and its sensitivity to light is increased. The second method of reducing noise is to shoot several blank exposures and combine these files with the exposed-image file. Statistically, adding these “pure black” images will reduce image noise even more.
Here are a pair of ring neubula shots, before and after digital filtering, taken by Scott Jamieson. They were shot under very light-polluted skies in the Milwaukee area. I did a 1-minute fix with the level functions in Elements to get the improved photo.
Visit altered universes!
Much of the more sophisticated image-altering software is based on spatial filtering. Formerly the bastion of scientists, engineers and the space program, the benefits of this technique are now widely available to photographers in the form of image optimization programs. The most widely used example is the sharpening function.
In a nutshell, spatial filtering, whether done with hardware or software, involves analyzing the image for certain characteristics, such as its frequency components, and then selectively increasing or decreasing certain quantifiable elements to get the desired effect. If you’re a math buff who wants a detailed account of spatial filtering, click on this link.
http://academic.mu.edu/phys/matthysd/web226/index.htm
Spatial filters, both optical and electronic, are frequently used for examining aerial photographs in order to make selected colors or shapes stand out more clearly. This makes it easier for the interpreters to find what they are looking for.
Spatial filtering and magic wands
How can photographs have frequencies (like the numbers on a radio dial)? Here’s how it works. Density changes in an image, like those that occur at the edge of a fence or person’s outline for instance, can be mathematically represented by how strongly they change and over how short a distance they change. A change over a short distance has higher frequencies than does a more gradual or softer change. Spatial filtering makes such useful functions as the magic wand selector tool possible.
A sharpening filter does what’s called a “Fast Fourier” analysis of the digital image to determine the distribution of these frequencies. You can then, via sliders, select how much to increase or decrease the high frequencies to get the effect you want. Anyone who has done any digital processing at all will have used a sharpening filter many times. As many of you have already seen, too much sharpening will give you images that look like those on a screwed-up TV. Here are before sharpening, typical sharpening, and way too much sharpening examples scanned from a single slide of an adolescent raccoon.
Can we graduate yet?
Other filtering functions, such as the graduated neutral density filter in Nik’s Color Efex Pro 2.0, use simpler mathematics to do their job. Assuming that you can get all of the desired detail into your digital file without over- or underexposing key portions, changing the relative density of the sky is easy with digital filtering.
Actually, digital filtering excels over optical filters in this application. In the before-and-after farm scenes below, I used the magic wand to select only the sky, and then added the darkening. With an optical filter, the barn would have been darkened too.
Nothing but blue skies
The Nik polarizing filter does a good job of darkening blue skies while keeping clouds bright. Since it relies only on color and density and not the degree of polarization needed by optical filters, it will improve blue skies in photos taken where the view was too close to, or directly away from the sun.
In these cases, an optical polarizing filter would have little or no effect. You can also use the magic wand to limit the filtering effect to specific areas of the photo. By the way, if you want a more vivid effect, apply the filter a second or third time. In this type of situation, software filters out-perform optical filters in terms of flexibility.
The two photos of a farm scene show how the Nik Polarizer filter helps the original image.
However, the Nik polarizing filter does have important limitations. While it can darken some reflections—the reflection of the sky in water, for example–it can’t remove reflections or glare off non-metallic surfaces, or reveal details the glare obscures. To demonstrate these limitations, see the photos of a custom motorcycle. The first photo is a straight, unfiltered shot taken indoors. The second photo was taken using a Tiffen circular polarizer on a Nikon D70. The third photo shows the result of maximum filtration using the Nik polarizing filter.
Thanks to Milwaukee Boss Hoss Motorcycles for providing the ultra-cool custom cycle.
Variety? Packed!
There are countless varieties of other filters available in the Nik Color Efex lineup, such as the black-and-white conversion utility, which really does an excellent job of converting color images so they look like they were originally shot in black-and white. Most provide (surprise, surprise) “special effects” rather than filters for optimizing your photos.
There is plenty of room for both optical and software filters in the new digital universe. Some applications do favor one type or the other, but you really shouldn’t exclude either optical or software filters from your working tools. If you prefer previewing your effects before your press the shutter, optical filters still have an edge, but for maximum flexibility in enhancing and creating images in post-production, plug-in filters are hard to beat.
I certainly intend to use both going forward–and I’ll bet that’s a good approach for you to take, too.
