photoshop basics: resizing the right way

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It’s Greek To Me – So Take A Minute And We’ll Explain

june, 2005 | art and copy: John Siebenthaler

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Nothing gets me crazier than a newly minted requirements sheet from a fresh faced production coordinator dictating images be submitted as an arbitrarily high DPI measurement. In this column we’ll run down the must have aspects for a successful digital print experience. Read why pro photographers are essential for more product photo background.

In The Beginning (Skip this section and head to the last paragraph if you just want to get down to cases.) To understand how we got to where we are today, let’s go back a few decades, when print publishing had yet to be affected by the technological conversion from analog (engraving) to digital (scanning), the forerunner to what’s refered to now as desktop publishing.

The engraver’s craft of halftoning was the reproduction of continuous tone artwork (usually a photograph) with only a few solid colors. Look at a photograph, and it’s impossible to determine where one shade of gray (using a black and white photo as a reference) leaves off and another begins.

Today’s digital software fixes the number of discrete shades at 256 levels, to give you an idea of how sophisticated our computers have become. Simply put, to reproduce the example, you’d need 256 shades of ink on press, ranging from dense black to nearly white. Which would also require 256 plates to carry the ink, 256 ink fountains to be filled, and on and on. You get the picture.

What’s All This About Image Resizing? Instead, a method of screening the original art was devised that basically said light areas of a print would get fewer tiny dots and dark areas of a photo would get more bigger dots.

Practically, this means a lot of ink in the shadows and not very much in the highlights. And this is how the dots per inch, or DPI, measurement came into being.

Halftoning originally required the photo to be rephotographed through a glass filter etched with varying line screens onto lithography film, which was then used to burn a metal plate and which then went on press.

Today we scan the art if necesary, then burn either to film or direct to plate. In either case the result is still referred to as a halftone, and in practice it allows one color of ink (black) to be deposited onto the paper in larger or smaller amounts, thus presenting the effect of darker or lighter shades of colors.

It’s the same process for color as it is for black and white. Extremes involve screen sizes of up to 600 dots per inch on waterless presses for really detailed printing, but whether it’s an ad in the classifieds, a billboard over center field, or your favorite centerspread, the process is basically the same. (I’m not going into stochastic printing, a different and much more confusing method.)

Desktop Printing: The Wild West of Publishing In today’s digital environment, pixels per inch, dots per inch, and lines per inch are all used more or less interchangeably, to everyone’s continuing confusion. The short explanation is that scanners (hardware) and Photoshop, Fireworks, etc. (software) have removed the middleman engravers and film houses of previous years, and replaced craft and skill with varying degrees of expertise and knowledge.

All in all the digital medium is fantastic, but unless the basics are understood it can produce downright weird results.

DPI – It’s Not The Measurement You Think It Is The heart of the problem, from where I sit, is the blind insistence by many publication and print production managers on an arbitrary and capricious measurement that has nothing to do with actual sized reproduction.

Traditionally, the higher the line count (55, 85, 133) the greater the detail. But since art is now produced digitally instead of on film or art board, there’s another critical piece of the puzzle that needs to be understood for DPI to have any meaning. And that’s content.

In the way old days, content was measured by film format or print size — thirty-five millimeter, roll film, four by five or larger sheet film. Often, and this still continues, artwork was submitted as an 8x10 or larger photo, regardless of the original format. Bigger was (almost) always bigger, and nothing’s changed in that regard.

Today, digital rasterized artwork, whether an original image shot with a digital camera, or a scan produced by desktop flatbed or lab drum, is measured in kilobytes (KB) or megabytes (MB) instead of film size.

What’s this got to do with you? Well, in practical terms, the usual minimum file size for an average letter sized (8 1/2 x 11) illustration is in the general vicinity of 20 MB. A quarter page (approximately 4 x 5) raster (bitmap) image should be around 5 MB. Only then can we decide on the DPI part of the equation, and for each of these it’ll be around 200 DPI.

My Megs Against Your DPI You say you want a 300 DPI full page raster ad? Than you’d better have an original file size that can support it. The reason is, the higher the screen count, the larger the file has to be to support it. That 20 meg file is plenty for letter size art at 175 DPI, but falls way short if you insist on the same dimension but at 300 DPI output — there’s not enough data to fill the dimensions at that reproduction rate.

At this point, I need to mention the variables that also enter into the equation. Photo subject matter has an enormous amount to do with file size and perceived sharpness and clarity.

A picture of a blue sky with just a few puffy clouds doesn’t require anywhere near the number of pixels a food shot for the cover of Gourmet magazine does. For the sky shot, you’d get virtually the same quality of reproduction at, say, 133 DPI as you would at 300 DPI.

The main reason is the huge difference in contrast between the object color boundaries, and the math required to interpolate the colors, but this is a discussion best left for another day.

In the early days, which ran from the late ’80s to the mid-’90s, a lot of folks used a weird math formula that when applied rendered a goofy one hundred something point something something DPI for a screen setting.

It was against the backdrop of really expensive storage, difficult delivery and really slow processing (RIPing) – forget e-mailing, because the internet was in it’s infancy then – that JPEG standards began to evolve, all because people had to think really hard about absolute file sizes more than screen settings.

So what do we know? Reproduction (DPI) settings determine the resolution an image is reproduced at, and how hight that can be set depends on the image’s file size. We can play with the DPI by adjusting it lower or higher, which usually (but not always) affects the amount of detail reproduced, but we can really only reduce the actual file size without affecting quality.

Trying to increase the file size to match the resolution through upsampling (interpolation) can work, but is very much dependent on the subject matter of the image. If fidelity is critical, it’s best left as an emergency option only.

I Really Didn’t Need To Hear Any Of This The bottom line is, your major responsibility is to understand a) the final size (width and height) the ad or p.r. image is going to be reproduced at, and b) making sure the file size will support the DPI setting that will be applied.

Remember, when we say how big, we’re technically referring to image file size, but we’re thinking image reproduction dimensions of width and height.

Basically, most of what we see in print can be accurately reproduced at 200 DPI. The reasons are mostly technical, but for the most part the original scene content isn’t that demanding, the paper stock is less than premium, the taking hardware isn’t that good, the shooting conditions are perhaps less than ideal, the photographer may or may not have a professional background, and frankly the reproduction size doesn’t warrant the (potential) detail.

In fact, it can be stated that reproducing less than perfect art at higher than necessary screen settings can actually degrade an image. Here’s a handy PDF that demonstrates the relationship between DPI and file size.

1. Here’s what’s never mentioned. All the absolute insistence on a 300 DPI setting totally falls apart if the art’s to be printed in very large format. Thinking about that centerfold? An 11 x 17, 200 DPI, RGB file is a tidy 22 meg. Bump it up to 300 DPI, and you’ve got a 48 meg file – before conversion to CMYK, which as you might expect further increases the bloat factor.
2. It gets worse if you’re thinking about, say, posters. A 40 x 48 poster at 300 DPI weighs in at a hefty 495 meg, before CMYK conversion. Which is why we image billboards at around 50-55 DPI, or the world would collapse under the weight of all those ones and zeroes.
3. In short, if you don’t have the original file size in megabytes, you’re probably better off at a lower DPI.

In a perfect world, all publications would specify size requirements in terms of megabytes, not screen settings, which as we’ve seen physically affect only the reproduction dimensions, and determine the final DPI only after the repro size has been set. Instead of saying all art must be 300 DPI, your half-page ad would instead require a minimum file size of 10 MB, optionally with resolution set at 300 DPI.

I hope these hints have helped. If not, give me a call and I’ll offer up even more confusion.