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Tuesday, May 22, 2007

The Art Of The Up-Res

Getting beyond the limitations of your camera's native resolution is an art and a science


Figure 4 














These images are cropped enlargements of the image taken with the two cameras. You can immediately see that the image from the 16-megapixel camera (Figure 4a) is superior in sharpness and textural detail to the image from the 6-megapixel camera that has been resed-up (Figure 4b). The limitations of a magazine's printing process are significant. Digital Photo Pro is no exception so the differences you see here will be more dramatic if you were printing the same image files on a pro-level printer or having them output by a lab.

Comparing The Results

The 16-megapixel native resolution image has more inherent textural detail, but the image from the 6-megapixel up-resed image does contain usable resolution that will reproduce well upon output (Figure 4). To compare the results, I've done screenshots of both images at a Photoshop zoom of 50%, which represents essentially four image pixels to one screen pixel, a result that basically simulates sending 300 ppi to a 150 lpi (lines per inch) halftone screen.

Does this mean a 6-megapixel camera can be as good as a 16-megapixel camera? Well, yes, with some limitations, notably final output size. While the 16-megapixel image is clearly sharper and contains more textural information, the up-resed 6-megapixel image looks good as well—at this reproduction size (Figure 5). What if you need a bigger image? Well, arguably, go much further with the interpolation of the 6-megapixel image, and the detail will fall apart. At 11x16 inches, the 16-megapixel capture isn't really being tested while the 6-megapixel capture is near maximum. How far can you go, and what do you need to do to make it bigger? That's next—how to take digital captures to the max!

Resolution Is Resolution Except When We're Talking About Resolution

Before we go there, there's some fundamental stuff I wish to point out regarding pixel resolution and what it means. There are three types of resolution: real resolution, native resolution and interpolated resolution. It might surprise you to discover that the “native” resolution of a Bayer CFA sensor isn't “real” resolution, but is itself "interpolated" resolution. A Bayer pattern is generally a grid of photosites with a red, two green and a blue photosite that's de–mosaicked (interpolated) to create the image data for a single pixel. So even before you start in on interpolation in Photoshop, your “native” camera resolution isn't “real” resolution.

Figure 5











In an effort to mitigate the difficulty in making comparisons in the printed magazine, Schewe made these screenshots at a zoom of 50%. The net effect is to simulate sending 300 pixels per inch to a 150 lpi halftone screen. FIGURE 5a: The image from the 16-megapixel camera. FIGURE 5b: The resed-up image from the 6-megapixel camera. Both images look good at these moderate sizes, which suggests that if your final output is expected to be moderate, you can do fine with resed-up files from a lower-res camera.

For the sake of argument, let's call a camera's de–mosaiced image its “native” resolution. By far, native resolution is much better than up-resed or interpolated resolution. Many people don't realize just how far their image's native resolution can take them.

Even a 6-megapixel capture is extremely flexible in the manner in which you choose to distribute its resolution. At a native resolution set to 180 ppi, a very reasonable 11x17-inch print can be made on an inkjet printer—particularly, if you're printing on matte or textured media (Figure 6). Conversely, if you were to change the resolution ratio to 480 ppi (what testing has determined to be the maximum useful resolution for today's inkjet printers), you could achieve an incredibly detailed 6x4-inch image on a premium glossy paper. The key is to use the image's native resolution whenever possible and sharpen the image for the specific output resolution and media. Glossy papers need a different sharpening than matte papers. An image at 180 ppi will need a different sharpening than one at 480 ppi.

Figure 6





For inkjet printing (based on 720/1440/2880 dpi), the optimal image resolutions are 180 ppi, 240 ppi, 288 ppi, 360 ppi and 480 ppi. While this is a contentious statement, it comes as a result of considerable testing by PixelGenius and others. The “native” (if there is such a thing) of a current pro Epson inkjet printer is arguably 360 dpi—meaning the physical resolution at the printer's ink nozzle—so some may argue that sending anything over 360 ppi is a waste. In fact, many argue that sending more than 240 or 300 ppi is a waste, but not in my experience. Even at 480 ppi, an easily discernable improvement can be seen in most images—if the resolution you're printing at is native or not interpolated resolution and the images are properly sharpened for both the image output resolution and the output media. 



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