I am preparing a submission to LensWork. The portfolio submission guidelines indicate a pixel width or height size. I did not see a dpi size. Do you prefer 72dpi, 240 dpi, or 300 dpi?
— Michael
This is a common question and needs some clarification.
First, the term dpi is "dots per inch. " This is a hold-over term from offset printing days which refers to the dot density in a half-tone images, like in the newspaper. Lay down a ruler and count the number of dots in an inch; no more sophisticated than that. In recent years, this has fallen out of favor for computer images for a more up-to-date term, PPI which is an acronym for "pixels per inch."
Both of them are meaningless in some regards.
The PPI is not really a measurement of the resolution of an image, but rather the density of pixels when spread over a given linear space. Output devices require a sufficient density of pixels for their needs. Greater density is discarded to no advantage; lesser density will result in a compromise in image quality — often called "pixelization," a blockiness to the edges and smooth tones.
Inkjet printers typically require a pixel density of around 300 pixels-per-inch, give or take a little. Most people use somewhere between 240ppi and 360ppi; there is some debate about the optimum.
The resolution of most monitors is 96ppi, older ones as low as 72ppi.
We produce two different publications with the images that are accepted for publication.
- LensWork (our print edition) is printed on an offset printer that requires 400ppi for the best reproductions. Because of the page size, the largest images can be as wide as 5.5" or as tall as 7". A little math will show that a landscape orientation image must have at least 2200 pixels horizontally or we won't be able to provide the necessary pixel density. Portrait orientation images must have at least 2800 pixels vertically. Hence our submissions guidelines specify these as the minimum dimensions — 2200 pixels wide or 2800 pixels tall, depending on the orientation of the image. More than 2200 pixels in width or more than 2800 pixels in height is not a problem, but not needed for our use. Fewer pixels in either dimension mean we run the risk of compromising image quality on the press.
- LensWork Extended (our multimedia edition on computer disc) requires 96ppi for display purposes, but we produce it at 200ppi so readers can zoom in on the details of an image without pixelization. This also accommodates large monitors and HDTV presentations. Based on the size of the InDesign layouts we use to create LensWork Extended, we need images that are at least 2222 pixels wide or at last 1666 pixels tall. This is within the parameters of the print dimensions we need for LensWork, so the 2200x2800 pixel images in LensWork will reproduce beautifully in LensWork Extended, too. In essence, we simply resample the images to the required pixel density — PPI — as needed for each publication.
So, it makes no difference to us whether you provide 10 PPI or 10,000 PPI images as long as they are at least 2200 pixels wide or not less than 2800 pixels tall. We'll arrange the pixel density as needed for each use.
For your own use, the PPI can make a difference in this regard: Placing images into some other software application, e.g., InDesign, will cause the receiving application to size the image base on the PPI specifications in the image file. Drop a 2200x2800 image set to 72 PPI into InDesign and the image will be huge. Change the image (in Photoshop, for example) to be a 2200x2800 image at 5000 PPI and the image place in InDesign will be tiny. Both images will be 2200x2800 pixels, they'll just render in your layout at different sizes based on the PPI specification. You can, of course, resize either of these images inside InDesign and as long as the final pixel density meets your output requirements there is no functional difference between the 72ppi and the 5000ppi image in the output from InDesign. The only thing that counts is the final output pixel density — which is a function of the available total number of pixels in the originating file and the output size in the InDesign layout.
Whew! Make sense?