(Click to see a larger version of this image)

The above image started out life as a 3,072-pixel wide image from my digital camera. That's big. It's about what you get with a 6 megapixel camera. And you almost always will make it smaller. Certainly you want it smaller to email or use on the Web because smaller pictures mean smaller file sizes and faster downloads.

But there are two ways to end up with something smaller that should be obvious, but from what I see others doing, apparently aren't. (In other words, if what follows was obvious to you , my apologies.) The first way to make something smaller is to "resize" it - which is what I did above. I simply asked Photoshop Elements to reduce the original photo proportionately to 720 pixels wide. Then I reduced it even more - to 500 pixels - for this page. (You get the 720 pixel version when you click on it.)

But there's another way to end up with a smaller image that acts very much like a telephoto. You simply leave the image at full size - "show actual pixels" - and crop the portion you want. In Photoshop Elements you can set the dimension of a crop, so I set mine to 720 x 480 - a good size for display on the Web. Then I go and take a bite out of the picture of exactly those dimensions. You can see the result below.

(Click to see a larger version of this image)

In this case I wanted the best picture I could get of the Northern Pintail, a duck we seldom see around here - and sadly - one that is suffering a decline in populations across the country. In the original he was simply too far away for my strongest telephoto. But by setting my screen to display "actual pixels" and then cropping, it was like zooming in on him without a significant loss in quality.

And while we're on the subject of pixels I find that a lot of folks - including me, a few days ago - don't really understand the difference between what happens when you enlarge - or reduce - a digital photo as compared to what happens when you do the same to a photo taken on film.

Both photos - film and digital - consist ultimately of dots - or pixels, if you like. But when you enlarge a picture from a piece of film each dot gets larger. Eventually they get so large that the picture appears "grainy." We begin to see the dots.

That doesn't happen with a digital image. In a digital image the dots always stay the same size - well, for the purposes of this discussion at least. But if you've tried to enlarge a digital photo - in fact, sometimes even when you try to reduce a digital photo - you find the quality drops off. Why?

Start with the 3,072 pixels that were in the original photo above. Now reduce that to 720. To do so, the computer has to remove pixels. It did not make the 3,072 pixels smaller - it didn't squeeze them together. It removed ones it didn't think would be necessary. And were I to enlarge the image past the original 3,072 dots it would do so by adding pixels - inventing them on the spot and putting them where it thought they would look good.

When you understand this you can see there's a lot of room for things to go wrong. Bottom line: I take nothing for granted. I do very little enlarging because that almost always looks poorer. And when I reduce I look carefully at the results. Beyond that I have a lot to learn about the various ways and means of reduction. But this simple difference between reducing or enlarging the size of a dot versus using dots that remain constant and reducing or increasing their numbers - that was an eye opener for me. Hope it helps you in your digital experience.

Posted by Greg Stone at March 21, 2004 08:32 PM
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