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|| SportsShooter.com: News Item: Posted 2004-05-02
Pushing Pixels: Sharpening and Resampling - Why Not?
By Reed Hoffmann, Blue Pixel
Pardon me while I rant a bit in this month's column. Since I haven't received any good questions lately, I've decided to whine a bit about two of the cardinal sins photographers make that end up hurting them: resampling and sharpening. I know of no photographers who sit down at Photoshop and say, "Gee, I think I'll make it harder to reproduce my photo," but that's what many do when they resample and sharpen. A bit of explanation is in order.
First, resampling is when you choose to crop your photo and change the number of pixels at the same time. Instead of simply cutting away the pixels you don't care about (outside your crop marks), you set numbers in the crop options box for dimension and PPI.
Most commonly I see people setting the tool for 10-inches maximum height or width and 200PPI. When they crop the image, Photoshop will then be forced to either add (most of the time) or remove (occasionally) pixels to match those dimensions.
For instance, say you have a D1H file that starts as about 7.5MB. You crop into the heart of the action, say about 50% of the frame. Cutting away those pixels you don't care about would leave you with a 3.75MB file. If, however, you set 10-inches by 200PPI in the crop options, you now have a file of around 8MB even though you've thrown away half the image.
Voila, suddenly you've got a "better" image, right? Wrong.
Photoshop has to take those remaining pixels (the 3.75MB worth) and now fake a bunch of pixels around them to make the image the size and resolution you've requested. While it can add pixels of similar density and color, it can't add detail. So you've made the image larger, but at a cost of sharpness.
Also, if there are any problems with the image (such as noise), you've just made those larger as well. So what you've got now is a larger (longer to transmit, larger to store) file that's softer. It's a lose-lose situation. So why do people do that? Blame film.
Back in the old days of scanning film (yeah, think back all the way to the 90's), the problem was too much information, not too little. A good film scanner could easily produce a 15 - 20MB or larger file out of a piece of 35mm film. That's great if you want to print a poster, but for newspapers it was overkill.
Time for some guidelines about scanning. Since most of us were making 8 X 10 prints, we soon realized that an image scanned at 8 X 10 inches at 200PPI would create an 8MB file, more than enough for good newspaper reproduction. That was fine for film. Enter digital.
Early digital photojournalists started with the NC2000, a 1.3 megapixel camera that created a 3.7MB file. Newsrooms and prepress were accustomed to 8MB files. People were afraid that 3.7MB (or less if cropped) wouldn't have enough resolution for printing. "Why not," they said, "just resample the file up to about 8MB, using 10-inches by 200PPI, and have a file that's about the same size as we're used to. What could be wrong with that?" As I said before, plenty.
Some people and organizations still have that rule. It's well past time they changed. There is a time for resampling , though, and it's been in place a long time.
When an editor decides to use a photo in the newspaper, they have to decide what size it's going to run. Once that decision's been made, the order goes back to the prepress department, whose job it is to prepare images for printing. One of those steps is to resample the image to the size requested, at the resolution required for the presses. This is the ONLY time an image should be resampled.
When prepress resamples it, they'll also sharpen it to make up for the fact that the ink will spread a bit when it hits the paper. So if they're going to resample it anyway (and they will, either up or down to match the size needed), then why are you taking the time and trouble to resample before sending it in?
Not only is it a waste of time and file size, but you're going to soften the image as well. Leave resampling to the prepress folks, who know what they're doing. Among the many things we've learned in the last few years working with digital images is that they upsample much better than film did, so what would have been a small size for a film scan can look very good in the paper as a digital image.
All right, enough about resampling. You won't do it anymore, right? Now on to sharpening.
So if I've convinced you not to resample because you're softening your image, you may be thinking, "Well, why don't I just sharpen it a bit before turning it in, to make up for it?" Another bad idea.
Sharpening an image adds contrast, and can shift some colors. This can create a real problem for prepress in trying to make the image reproduce well. Remember, they're already going to apply some sharpening to the image based on size and content. All you can do by sharpening beforehand is to make their job more difficult and again, harm your photo.
For most people, you're better off leaving your camera set to Auto or Normal sharpening and concentrate on getting your focus right. There are times, though, where you may choose to turn off sharpening and that changes things.
Okay, I've just said, "don't sharpen your photos." Now I'm going to tell you there are times where you might, but in a very controlled and specific way. And this part is something that your photo department will want to have a policy on, to make sure everyone's doing the same thing.
For most cameras, when you shoot at high ISO, in-camera sharpening can make noise worse. There are definitely times, and cameras, where to get the best quality image you'll want to turn off sharpening. If you do this, then you're doing it to reduce noise, which means you'll need to deal with the noise before doing any sharpening.
Quantum Mechanic Pro by Camera Bits (www.camerabits.com) has been the industry standard for noise removal for many years. Newer generation cameras, though, are giving us a different sort of noise, and a new product, Noise Ninja (www.picturecode.com) looks to be a great solution. Rob Galbraith's written about it at www.robgalbraith.com., and when it releases as a Photoshop plug-in it will probably be a must-have tool. So your first step with an image from a camera with sharpening turned off would be to reduce the noise. If you're not fixing the noise, then forget turning off in-camera sharpening. Next you'd deal with the sharpness.
Since you turned off in-camera sharpening, your images will be a bit soft (it varies from mild to noticeable, depending on the camera). You can now bring that detail back with a low-radius Unsharp Mask. That would mean numbers somewhere in the range of Amount 200%, .3 Radius (that's point-three, not three) and Threshold of 2 for a telephoto lens.
An image shot with a wide angle lens would probably need a bit more sharpening, with the Amount bumped up to around 300%. These numbers will change based on the type of camera. The most important part of this step, though, is what comes next.
When you sharpen an image with Unsharp Mask, you sharpen both the detail and the color. It's the detail you care about, and the color that can create problems for prepress.
To avoid this, you can do one of two things. One, you could choose to change your image to Lab color, sharpen only the Lightness channel, and then convert t back to RGB. I prefer to skip the conversion steps, and after running Unsharp Mask, choose Fade Unsharp Mask. Once that dialog comes up, set for 100% and Luminosity. This is essentially an undo of the sharpening, then applying it only to the detail. If you choose to turn off in-camera sharpening and sharpen later, you must finish with this step. Otherwise look for your prepress department to come hunting for you.
Okay, I feel better. Now when you see another photographer resampling their photos, you can ask why they bother shooting with expensive lenses if they're just going to soften their images? And you know not to sharpen, unless your workflow includes that after noise removal. Remember, the computer gives us as many ways to harm our pictures as help them. Find the ways that help and make them an efficient part of your workflow.
(Reed Hoffmann is a former newspaper photographer and current member of Blue Pixel (www.bluepixel.net), a digital photography and consulting group that's worked with over 30 newspapers around the country to help them improve image quality.)
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