4

I hope this doesn't cover too much of the same ground as the typical DPI question.

I am a technical writer working with my company's marketing staff on developing an image archive. Because most marketing content will be printed they want to institute a 300 dpi requirement on any image submitted to them. The photos will be submitted by field staff, (inspectors, engineers, surveyors, architects) and will be taken using various smart phones and digital cameras.

Since DPI is not an attribute of the photo and many cameras and smart phones default to 72 dpi in the meta data, what is a better way to describe the attributes someone should look for to know if an image is suitable to submit to the archive?

10

Image resolution/pixel dimensions are the attributes you should probably be looking for.

To find out what numbers you should specify in your requirements, you'll need to decide on the maximum size that you'll want to print an image from the archive, then from that you can derive the minimum resolution that images in the archive should have.

For example, if you decide that the largest you'll want to print an image is the full width of an A4 page:

A4 page width in inches= ~8.27 inches

8.27 inches x 300ppi = 2481 pixels

From these numbers you could set a requirement that images submitted to the archive must have a minimum number of say 2500 pixels across.

However, there are a number of further issues that complicate matters.

Firstly (and most importantly IMO) the number of pixels that an image is made up of isn't necessarily a measure of image quality. As an example, my mobile phone and my old Canon 20D dSLR camera, both produce pictures of near identical dimensions, but the quality of those pixels is worlds apart.

Secondly, I imagine most of your images will be submitted as jpeg's so there is a question of the compression setting used. If the camera in use was set to apply heavy compression to the image before saving that'll reduce the quality of the image, whilst maintaining it's outer pixel dimensions.

This means that although you can set requirements for minimum dimensions, it's possible for images to technically meet those rules, and yet actually be of a quality lower than you need.

Thirdly, calculating the minimum image dimensions as above may be overly optimistic. Suppose you need to crop or rotate the images to fit a certain layout, that may necessitate a larger starting file to still get a good print.

Conclusion

Personally, if I were building an image archive for marketing use, I'd want to source images of as high a quality as possible, so the general guidance I would issue would be to make sure the camera was set to it's highest resolutions and quality settings.

Resolution wise I'd issue a guideline that images should be at least 6 Mega Pixels or have their longest side be at least 2500 pixels. (These are the numbers I'd go with, naturally they are dependant on your specific printing requirements.)

6

Imposing a minimum DPI value is meaningless as images do not have any intrinsic physical dimension.

A better approach is to work out the maximum size you want to guarantee to be able to print, work out the image size in pixels necessary to achieve 300 PPI at this size, and then impose a minimum on the image dimensions in pixels.

1

For people who have a decent image editor I would say -- the picture should be at least 300 ppi when sized down without resampling to the dimensions it will be printed.

As the people you are communicating with may not have those tools or know the print sizes, I'd ask your marketing people the maximum printed size they are aiming for and then find someone with Photoshop (or equivalent) who can create an X by Y inch document at 300 ppi and see what the pixel size is. Then you can tell folks that images submitted should be at least these many pixels in size.

An 8x10 inch image at 300 ppi would come out to 2400 x 3000 pixels, for example.

If the pixel size is hilarious and would mean most people can't submit anything, you might have to negotiate a more reasonable print size or dpi/ppi with marketing. 300 ppi is really only needed for materials that will be held by the viewer (magazines, etc), for posters and billboards the standards can be lower.

  • " create an X by Y inch document at 300 ppi and see what the pixel size is." Do you mean the dimensions in pixels? Or is 'pixel size' an attribute of the digital photo? – Vidro3 Dec 9 '14 at 15:10
  • Sorry, yes "dimensions in pixels" would be a better way to word that. – David Rouse Dec 10 '14 at 13:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.