Join Deke McClelland for an in-depth discussion in this video Using reliable zoom ratios, part of Photoshop CS3 Sharpening Images.
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Now based on the previous exercise, those of you who watched it, you might figure that given that your screen resolution is somewhere in the neighborhood of one quarter to one half of the output resolution, the resolution at which you're going to print an image, then you could just zoom out in order to gauge how your halos are going to look, in order to gauge the sharpness of the image. And that's true to an extent. But you have to be careful when zooming an image, and I am going to give you a sense of why you have to be careful with the help of this whacky image right here.
I know it's a little bit unpleasant to look at and I wouldnt stare it your image because its going to burn your retina, but what we have is a series of parallel lines, hence the name of the image, it's Parallel lines.PSD. It's found inside the 01 How It Works folder, and its a series of parallel lines that are formed by a combination of four pixel wide black lines followed by two pixel wide white lines, and they're alternating of course. Now currently I am looking at the image at the 400% zoom ratio. What were going to do though is were going to go ahead and crop the movie so that we can see these lines nice and tight.
Alright. Now I'll just narrate where we are in terms of zoom ratios. Now anything above a 100% is perfectly fine. Its not going to throw you off, as long as its in the even multiple of 100%. So 100%, 200%, 300%, etc. are going to work just fine for you. You're going to have an accurate view of the image, albeit you'll have a really big mass of pixels. Its when you start going below 100% the things get problematic. So here we are at 300%, the lines are still very uniform as you can see. Here is 200%.
Everything looks hunky-dory at this point right here. I am going to go ahead and scroll the image over a little bit, but the pixels look just fine, the lines look fine, that is to say. Heres the 100% view, things are still holding up very nicely, meaning that the widths of all the black lines is uniform and the width of all the white lines is uniform. OK. Now I am going to press, I am pressing by the way, Control + minus or Command + minus on the Macintosh side in order to zoom out. If I press Command + minus again, or Control + minus here on the PC, here is the 66.7% zoom ratio.
It's a terrible zoom ratio, and the reason is because it drops pixels. Photoshop just goes ahead and abandons pixels as it's rendering the screen. So instead of trying to resolve the entire image to this new zoom ratio, it just goes ahead and drops out pixels that it can't use. As a result, in the case of this particular image, we have a gigantic distortion midway through the image over here on the right hand side of the image. It suddenly looks totally wrong. We just have this weird disconnect between the right side and left side here.
that's not really part of the image, but it's showing up. The way it's going to resolve itself when you're looking at a continuous tone photograph, as you're going to see sharp, sometimes jagged transitions where sharp jagged transitions do not exist. So if anything the image is going to look sharper than it actually at 66.7%. Alright. If I press Control + minus or Command + minus again to zoom out to the 50% zoom ratio, everything is one again hunky-dory, and the reason is that Photoshop is now performing an interpolation, what's knows as the bicubic interpolation, meaning that its averaging the pixels in the real image in order to create this 50% view.
Now 66.7 isnt the only bad one, everything between 50% and 100% is bad. So everything from 50.1% to 99.9% has problems and you should not believe it. So you can believe 100%, you can believe 50%, you can't believe anything in between. Now lets press Control + minus or Command + minus. Again, this is 33%. Can you believe it? I don't think so. Something has gone terribly wrong with the right half of the image, it's turned solid black.
Meanwhile, Photoshop is doing a decent job over here on the left hand side of the image. Again, were going to see jag a transitions and sharp edges where none exist inside the actual photograph. Now zoom out another stop here, another increment to 25%, its another good bicubic interpolation view of the image. You cant trust anything between 25% and 50%, though you can trust those two. So we got 100, that's good, we got 50 that's good, we got 25, that's good and anything in between those guys, is bad.
Heres the next zoom out. This is 16.7%, also bad, and then zoom out again to 12.5%, that's good, and then we start getting so small that is not very helpful. So every other, if you're pressing Control + minus to zoom out or Command + minus on the Mac to zoom out or Control + plus or Command + plus to zoom in, then every other increment is good. So starting at 12.5%, I'll press Control + plus or Command + plus to zoom in, we've got bad, we've got good, we've got bad, we've good, we've got bad, as witnessed over here on the right hand side of the image, we have good at 100%.
So just something to bear in mind, some of the zoom ratios, 12.5%, 25%, 50%, and 100%, are good, and then even multiples of 100%, 200%, 300%, and 400%, those are good as well. But anything in between 12.5, 25, 50, and 100, consider those absolutely 100% unreliable where gauging, sharpening in Photoshop is concerned.
- Understanding the effects of sharpening
- In-depth examinations of Unsharp Mask, Smart Sharpen, Emboss, and High Pass
- Smoothing an image with the Surface Blur, Despeckle, and Reduce Noise features
- Working with smart objects and smart filters
- Creating edge masks and non-edge masks
- Sharpening for digital-image capture using Camera Raw
- Gauging and exploiting luminance frequency
- Exploring creative applications of sharpening
- Sharpening a multilayer composition
- Sharpening eyes, hair, and out-of-focus backgrounds
- Reducing noise in a high-frequency image
- Determining ideal settings for commercial and inkjet output
- Sharpening very large-format images
- Sharpening an image for the web or screen output