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Wednesday, May 23, 2007

Dealing With Viewing Distance

Creating accurate viewing conditions is critical when evaluating your images, and it's something no one seems to be talking about—until now

Tradition also has led people to view their images at 100% to determine how much sharpening to apply. While it's true that you'll need to view your images at 100% to accurately see the results of the pixel manipulations caused by image-sharpening routines, it's not true that viewing at 100% will give you any useful gauge of how much sharpening to apply. There's a useful trick to more accurately approximate the effective resolution of output on a display. Assuming halftone reproduction, where a 300 ppi image is being screened into a 150 lpi (lines per inch) halftone dot, the effect of the screening is four image pixels being formed into a single halftone dot. Ironically, I discovered that a Photoshop screen zoom of 50% will do the same thing—take four image pixels and dither them into a single display pixel. While still at an overly large physical dimension at 50%, it offers a better gauge of what image sharpening actually is needed for digital output.

At a 200% zoom, images look overly coarse and will show pixel artifacts that simply will be invisible to the human eye when reproduced. If you spend considerable time with your images at 200% and try to retouch each and every artifact, you're wasting your time.

The same image viewed at a 100% zoom will still look pretty ugly. A properly sharpened image may appear slightly to considerably oversharpened (depending on the output resolution or media) at a 100% zoom. This is as expected since you're basically looking at the image through a low-resolution 4x loupe.

The exact same image viewed at a 50% zoom will more accurately display both the results of pixel dithering as well as a closer (by one-half), more accurate physical dimension. I'll often even go to the effort of sliding back my chair and viewing the image at a greater than normal viewing distance just to better predict what the image sharpness will look like.

For printing even higher-resolution inkjet prints, say, 360 to 480 ppi, color-management guru Bruce Fraser recommends zooming out even further to a 25% screen as an aid to judge sharpness. It should also be pointed out that you mustn't use any intermediate zoom ratios such as 33% or, even worse, fractional zoom percentages. The blotchiness caused by Photoshop's arguably poor intermediate zoom ranges makes those viewing percentages useless for judging image sharpness.

The lack of proper image sharpening is one of the most serious reproduction problems facing digital photographers today. This issue is caused by low-resolution viewing devices and Photoshop's less than optimal method of displaying pixels on screen. The only truly accurate method of judging image sharpness is to print the darn thing and use your eyeballs.

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