Sizing Digital Images
One of the wonderful things about digital images is the ability to crop and manipulate them in a variety of sizes and orientations with such relative ease. However, sometimes you need to be able to determine with some accuracy the size that a digital image will occupy when presented. For example:
- You want to print an image on an ink jet or laser printer;
- You want to publish (print) an image in a publication;
- You are developing images for a web site or CD-ROM and want to be sure that they will be displayed in a desirable manner in most web browsers or computer monitors;
- You are developing a slide show for a certain resolution LCD projector or monitor;
- You want to standardize on a size(s) for image archival or professional use.
I will outline some basic issues to consider and then present some sizes that I and others use.
These figures will help you estimate how large an image of a given resolution in pixels will print or be displayed in various media.
Resolution of digital images is measured in pixels per unit length; e.g. pixels per inch (ppi). Printer resolutions are usually specified in dots per inch (dpi). Sometimes “dpi” is used (incorrectly) when referring to a digital image without respect to an output device. If someone asks you for a 5×7 300 dpi digital image, they want a digital image that can be printed at 5×7 with 300 pixels of data per inch, or 1500×2100 pixels.
It is important to realize that image ppi and printer dpi are not the same. For a printer, it generally takes many dots to render a pixel. The higher dpi printer you have, the higher ppi images you can print. [Aside: for scanners, the dpi (optical) rating does correspond 1-1 with ppi; if you scan at 600 dpi, you are capturing 600 pixels of data per inch of photograph scanned.]
Ink jet printing
A good rule of thumb is to use 125 ppi for every 300 dpi of printing capability. Modern ink-jet printers can do an adequate job with a photo at 200 ppi and up.
Print shops and publishers
Printing in traditional media is done using halftone screens measured in lines per inch (lpi). You can estimate the pixels per inch (ppi) required in your digital images for these printing systems as roughly 1.6 to 2 times the lpi, depending on quality and other factors (so take this as a rule of thumb).
Different media have different printing requirements. A newspaper, for example, might use 100 lpi (160-200 ppi), while a magazine might use 133 lpi (120-266 ppi), and a book 150 lpi (240-300 dpi) or more.
The resolution of a computer monitor varies according to the size and capabilities of the monitor as well as the taste of the owner. Typically, the resolution can be considered anywhere between 72-100 pixels per inch (ppi).
Another important factor to consider is the aspect ratio of the images, which influences sizing.
There are two standard aspect ratios that are commonly used for digital images. Digital cameras will often use a 4:3 aspect ratio, which is the same as that used for most computer monitors, although many also use the 3:2 aspect ratio shared by 35mm film. Some cameras can switch between the two ratios.
There are a number of standards for digital image sizes, often relating to VESA monitor resolution/refresh standards, video standards, etc. For photographic-oriented images, the standard that has been widely adopted by graphics professionals and digital archivists is the Photo CD format developed by Kodak. This standard was developed with scanning 35mm film in mind (although it can accomodate, to some degree, medium and large format scans using the “pro” format).
Sizes for Digital 4:3
These sizes account for an aspect ratio of 4:3, which is what most computer monitors and many digital cameras use.
|Pro||6144×4608||N/A||TIFF, PNG, JPEG||85.33×64.00||61.44×46.08||30.72×23.04||20.48×15.36||Archive, Slide, Print||81MB|
|Huge||4096×3072||N/A||TIFF, PNG, JPEG||56.89×42.67||40.96×30.72||20.48×15.36||13.65×10.24||Archive, Slide, Print||36MB|
|Super||3072×2304||N/A||TIFF, PNG, JPEG||42.67×32.00||30.72×23.04||15.36×11.52||10.24×7.68||Archive, Slide, Print||20.25MB|
For slide viewing, the “slide” size is ideal for full-screen viewing on many of the newer XGA capable LCD projectors, and many desktops use an XGA resolution. if larger viewing resolutions are possible, the “large” or “jumbo” sizes are quickly downsampled to most desktops by a reasonably modern PC and image viewing software.
For album viewing, I find that the “album” size on a 1024×768 monitor is comparable to viewing a 4×6 print of a photo. The size will vary according to the user’s monitor resolution and dot-pitch; somewhere between the 72ppi and 100ppi sizes in the table above. The “small” size is also good, especially if you are designing for the web or smaller monitors (15 in), which typically run less than XGA resolution. The following table lists some comparable viewing sizes in terms of common conventional photo print sizes.
|Approximate viewing size in inches|
The “petite” and “tiny” sizes are good for web photo montages, where images are tiled in a grid. I personally prefer the 128×96 size for thumbnails on most desktops; it resembles a largish 35mm slide.
200 or 300 dpi is a reasonable value for photo printing resolution on modern ink jets. Using the “jumbo” size at 200 dpi results in a near 8×10, and at 300 dpi a near 5×7; the latter is almost indistinguishable from a conventional photo print if the appropriate paper and printer settings are used. The “slide” size at 200 dpi produces a decent 3.5×5 print.
It turns out that the 4:3 aspect ratio is better than 3:2 for making 8×10 prints, because the ratio is closer. The 8×10 format was designed for professional photographers doing portraiture. These pros generally use 4×5 cameras (perfect for the 8×10 format). 4×6 prints are ideal for the 2:3 (35mm) aspect ratio. The 3.5×5 and 5×7 formats are in between 3:2 and 4:5, the 4:3 aspect ratio fits about as well for these as the 2:3 does. On this basis, it makes more sense to shoot in 4:3 if you plan to print mostly 5x7s or 8x10s, or want to display without borders (landscape mode) on standard computer monitors. Some digital cameras have a user-selectable aspect ratio; you can switch to 3:2 if you plan to create a lot of 4×6 sized prints.
Sizes for Digital 3:2
These sizes account for an aspect ratio of 3:2.
|Pro||6144×4096||base*64||TIFF, PNG, JPEG, PhotoCD||85.33×56.89||61.44×40.96||30.72×20.48||20.48×13.65||Archive, Slide, Print||72MB|
|Huge||4096×2720||N/A||TIFF, PNG, JPEG||56.89×37.78||40.96×27.20||20.48×13.60||13.65×9.07||Archive, Slide, Print||31.87MB|
|Super||3072×2048||base*16||TIFF, PNG, JPEG, PhotoCD||42.67×28.44||30.72×20.48||15.36×10.24||10.24×6.83||Archive, Slide, Print||18MB|
|Large||1536×1024||base*4||JPEG, PhotoCD||21.33×14.22||15.36×10.24||7.68×5.12||5.12×3.41||Slide, Print||4.5MB|
|Tiny||192×128||base/16||JPEG, PhotoCD||2.67×1.78||1.92×1.28||0.96×0.64||0.64×0.43||Web, Thumb||72KB|
For slide viewing, the “slide” size is ideal for full-screen viewing on many of the newer XGA capable LCD projectors, and many desktops use an XGA resolution. if larger viewing resolutions are possible, the “large” and “jumbo” sizes are quickly downsampled to most desktops by a reasonably modern PC and image viewing software. On 4:3 computer monitors, the 3:2 aspect ratio will result in a “letterbox” display format for both landscape and portrait orientations.
For album viewing, I find that the “album” size on a 1024×768 monitor is comparable to viewing a 4×6 print of a photo: a pleasing size for photo album viewing. The size will vary according to the user’s monitor resolution and dot-pitch; somewhere between the 72ppi and 100ppi sizes (roughly 5×7 down to 3.5×5). The “small” size is also good, especially if you are web designing for smaller (i.e. average) monitors.
For printing, using the “slide” size at 200 dpi results in a near 3.5×5, and at 300 dpi a “jumbo” produces a near 5×7; the latter is almost indistinguishable from a conventional photo print if the appropriate paper and printer settings are used. The “large” resolution results in a near 5×7 at 200 dpi and a near 3.5×5 at 300 dpi.
See the remarks under the summary for digital 4:3 aspect ratios.
The “tiny”, “small”, “medium”, “large”, “super” sizes correspond to the ones standardized by Kodak for the PhotoCD format. The “pro” PhotoCD format includes also the “pro” size. Kodak claims that the “super” size is capable of holding most of the reasonable scan information from 35mm film; the “pro” size is designed for medium and large format film.
Here are some examples of the standard sizes listed above.
medium 4:3 768×576
medium 3:2 (PhotoCD “base”) 768×512
album 4:3 512×384
album 3:2 512×340
small 4:3 384×288
small 3:2 (PhotoCD “base/4”) 384×256
petite 4:3 256×192
petite 3:2 256×170
tiny 4:3 192×144
tiny 3:2 (PhotoCD “base/16”) 192×128
thumb 4:3 128×96
thumb 3:2 128×85
icon 4:3 96×72
icon 3:2 96×64