Upscale and Out of Res! - Digital Photography School

Upscale and Out of Res!

Let’s trigger a stick of gelignite under our digital images. And maybe explode some myths.

Dots And Dots

You’ve got a digital camera. Perhaps you’ve got an inkjet printer – one with great photo quality reproduction. Now you’re ambitions are beginning to take wings and you want to get into print with your images.

The razzle with digital cameras is that you can shoot pictures in seconds, drop them onto your computer’s hard drive, dibble with them via your favorite software, add a little text – then output them onto a sheet of paper.

So far, so good, if you want only a few copies. But, as we all know, inkjet printers – for all their magnificent qualities – are still costly, in price per print terms, aside from being slow. If you have in mind a run of a thousand copies you have to knock on the door of a printer to arrange comparatively large print runs.

Without getting into the technical complexities, the printer will take your digital image, make colour separations (usually four – CMYK – Cyan, Magenta, Yellow and Black), then go through the process of making printing plates and finally arrange the impression of four colour inks onto paper.

Unlike traditional film, the printing process is not a continuously variable process when reproducing tones – instead, a system of line screens is used. These screens break up the image into a series of dots in C, M, Y and K. The merging of these dots in the eye produces the final semblance of full colour on the printed page.

Printed output can be made on differing qualities of paper – according to your needs. Newsprint is at the lower end of the scale, with paper of the quality used in this magazine at the higher end.

Most magazines use a screen ruling of 150 lines per inch which produces a very high quality reproduction of digital and traditional film images – along with clear, crisp text.

The only problem is that digital camera images have defined limits when it comes to reproduction on paper, using a half tone line screen.

Let’s explain the problem another way by quoting from an Adobe manual.

Bear in mind, a digital camera image is a bit map image, made of computer created bits of varying density. We’ll deal in measurements in inches, for that is how the publishing industry still works.

(Reprinted with permission from Adobe)

About resolution and image size

Several concepts are important when discussing the characteristics of bit map images: pixel dimensions, image resolution, output resolution, and screen frequency. Another type of resolution, called bit resolution or pixel depth, is important when considering how color is displayed onscreen.

Pixel dimensions

Every bit map image contains a fixed number of pixels, measured in pixel height and pixel width (the number of pixels displayed along the height and width of the image, respectively). The total number of pixels determines the file size, or the amount of data in the image. Pixel dimensions, along with the size and setting of the monitor, determine how large an image appears on-screen. A 13 inch monitor displays 640 pixels horizontally and 480 vertically. Larger monitors can usually be set to display a varying number of pixels, for example, from 640 by 480 pixels, at which setting the pixels maybe quite large, to 1920 by 1080 pixels, at which setting the pixels are small.

If you’re planning to display an image online (on a Web page, for example), your maximum image size is determined by the lowest pixel dimensions of the monitors used to display your image. For example, if your audience will view your image on a 13 inch monitor, you will probably want to limit the size of your image to 640 by 480 pixels.

Image resolution

The number of pixels displayed per unit of length in an image is called the image resolution, usually measured in pixels per inch (ppi). An image with a high resolution contains more, and therefore smaller, pixels than an image of the same dimensions with a low resolution. For example, a 1×1 inch image with a resolution of 72 ppi contains a total of 5184 pixels (72 pixels wide x 72 pixels high = 5184). The same image with a resolution of 300 ppi would contain a total of 90,000 much smaller pixels.

Because they use more pixels to represent each unit of area, higher-resolution images can usually reproduce more detail and subtle color transitions when printed than lower-resolution images. However, once an image has been scanned or created at a given resolution, increasing the resolution in Photoshop will not usually improve the image quality because in this case, Photoshop must in effect spread the same pixel information across a greater number of pixels.

The proper image resolution to use for an image depends on how you intend to display or distribute the image. Using too low a resolution for a printed image results in pixellation – large pixels that produce very coarse-looking output. Using too high a resolution (ie pixels smaller than an output device can reproduce) increases the file size unnecessarily and may increase the time required to print or distribute the image.

Monitor resolution

The pixel setting of the monitor along with the size of the monitor determines the size (and therefore density) of the monitor pixels. When converting printed images to on-screen images and translating image resolution into pixel dimensions, it’s useful to know that the default resolution of a Macintosh monitor is typically 72 dpi; the default resolution of a PC monitor is typically 96 dpi.

In Photoshop, image pixels are translated directly into monitor pixels. This means that when the resolution of an image is higher than the monitor resolution, the image appears larger on-screen than its specified dimensions. For example, when you display a 1×1 inch image with a resolution of 144 ppi on a 72-dpi monitor, it appears in a 2×2 inch area on-screen. Because the monitor can display only 72 pixels per inch, it needs two inches to display the 144 pixels that make up one edge of the image.

Printer resolution

Church Door 4

Church Door 3

Church Door 2

Church Door 1

If you’re preparing images for print, it’s important to understand that printer resolution – that is, the number of dots per inch (dpi) – is usually proportional to, but not the same as, image resolution – that is, the number of pixels that make up an image and that determine the size of the image on-screen.

65lpi: Coarse screen commonly used to print newsletters and grocery coupons.

85 lpi: Average screen often used to print newspapers.

133 lpi: High-quality screen typically used to print four-color magazines.

177 lpi: Very fine screen typically used for annual reports and images in art books.

* 85 lpi: With coarse screens, resolutions at the low end of the range can produce good results.

* 177 lpi: With fine screens, only resolutions at the high end of the range produce good results.

Screen frequency and image resolution

Many commercial and desktop printers use halftone screens, which consist of printer dots called halftone cells, to print grayscale images and color separations. Screen frequency, also known as screen ruling, refers to the number of halftone cells per inch in a halftone screen, and is measured in lines per inch (Ipi).

Comprtv Res

The relationship between image resolution and screen frequency determines the quality of detail in the printed image. As a general rule, to produce a halftone image of the highest quality, use an image resolution that is 1.5 to 2 times the screen frequency. In some cases, however, depending on the image and the output device, using a lower resolution produces good results.

Example: for a screen frequency of 150 lpi set the image to 225-300 dots per inch.

About file size and resolution

The file size of a digital image is measured in Kilobytes (KB) or Megabytes (MB) and is proportional to the total number of pixels in the image. Although images with more pixels may produce more detail at a given size, they also result in larger file sizes. A 1×1 inch 200-ppi image contains four times as many pixels as a 1×1 inch 100-ppi image and so is four times as large.

Read more from our Tips & Tutorials category.

Barrie Smith is an experienced writer/photographer currently published in Australian Macworld, Auscam and other magazines in Australia and overseas.

  • Jay

    “A 1×1 inch 200-ppi image contains four times as many pixels as a 1×1 inch 100-ppi image and so is four times as large.”
    The file size will be four times as large if it’s not compressed. But utilizing lossless data compression will usually significantly reduce that number.

  • http://danferno.deviantart.com Danferno

    The images could use a bit of explanation. What do they represent?

  • http://rfbfmike.redbubble.com Mike

    Great article. The only point I would dispute would be the idea that mac monitors are 72dpi and pc monitors 96dpi. This used to be true when we had set resolutions on small monitors in the early days. Now we have high resolution monitors, showing much more than 72 dpi or 96 dpi.

  • http://www.worldofapple.com Alex Brooks

    Factual article but seems completely pointless, who would want their images ruined by a four colour printing process at low resolutions.

    Photographs should be printed using an inkjet printer and most of these will benefit from being fed RGB images at the highest resolution possible.

    Why inkjet you ask? Well it might be expensive for large runs compared to some kind of press but frankly the conversion of your image to CMYK will saturate it and drop the overall gamut of the image considerably.

    May I recommend that if you’re looking for a one off image then inkjet is the way to go, if you’re looking for a large run then look at a digital press, they’ll accept RGB images, they often have high(er) resolutions and some even print using a six colour process to expand oranges and greens.

  • michael mckee

    Please do look at modern monitor resolutions. We haven’t seen 72 PPI for years. The majority of current LCD computer screens display over 100 PPI The 17″ MacBook Pro actually displays 133 PPI.

  • hagen

    I puzzled through the whole thing until the last line: the example stating 225-300 dpi.

    I would suggest starting the article at: why print at 300 dpi? here’s the technical reason.

    Hagen

    PS Mac and PC monitors running 1920×1200 on 24″ LCD

  • http://baby-safety-monitor.com Mick McCarthy

    I’ve enjoyed reading this post, thanks. We’ve justhad our first baby 8 weeks ago and thisis exactly what I was looking for, keep up the good work.

Some older comments

  • Mick McCarthy

    May 25, 2009 11:25 am

    I've enjoyed reading this post, thanks. We've justhad our first baby 8 weeks ago and thisis exactly what I was looking for, keep up the good work.

  • hagen

    April 24, 2009 05:47 am

    I puzzled through the whole thing until the last line: the example stating 225-300 dpi.

    I would suggest starting the article at: why print at 300 dpi? here's the technical reason.

    Hagen

    PS Mac and PC monitors running 1920x1200 on 24" LCD

  • michael mckee

    April 19, 2009 03:33 pm

    Please do look at modern monitor resolutions. We haven't seen 72 PPI for years. The majority of current LCD computer screens display over 100 PPI The 17" MacBook Pro actually displays 133 PPI.

  • Alex Brooks

    April 19, 2009 02:12 am

    Factual article but seems completely pointless, who would want their images ruined by a four colour printing process at low resolutions.

    Photographs should be printed using an inkjet printer and most of these will benefit from being fed RGB images at the highest resolution possible.

    Why inkjet you ask? Well it might be expensive for large runs compared to some kind of press but frankly the conversion of your image to CMYK will saturate it and drop the overall gamut of the image considerably.

    May I recommend that if you're looking for a one off image then inkjet is the way to go, if you're looking for a large run then look at a digital press, they'll accept RGB images, they often have high(er) resolutions and some even print using a six colour process to expand oranges and greens.

  • Mike

    April 18, 2009 09:37 pm

    Great article. The only point I would dispute would be the idea that mac monitors are 72dpi and pc monitors 96dpi. This used to be true when we had set resolutions on small monitors in the early days. Now we have high resolution monitors, showing much more than 72 dpi or 96 dpi.

  • Danferno

    April 18, 2009 07:47 pm

    The images could use a bit of explanation. What do they represent?

  • Jay

    April 18, 2009 09:58 am

    "A 1×1 inch 200-ppi image contains four times as many pixels as a 1×1 inch 100-ppi image and so is four times as large."
    The file size will be four times as large if it's not compressed. But utilizing lossless data compression will usually significantly reduce that number.

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