“I need a high res file”
High and low resolution are terms oft used to describe what you need from an image. But what does does “high res” actually mean, and how is it measured?
Resolution is best specified in pixel dimensions. An example of a high resolution file would be 3200 x 5000 pixels (px). More pixels equals higher resolution. File size is not a reliable indicator of image resolution. A JPEG file straight out of a digital camera might be:
 4MB and 2400px x 3600px
Now compare that to a JPEG file from Adobe Photoshop that may have a file size of:
 2MB and 4200px x 6300px
That’s almost double the resolution yet half the file size! How’s that possible? Well Adobe has a much more efficient JPEG compression engine than camera manufacturers can offer in camera. That compression can reduce file size to a point that is visually identical.
“You need a 300dpi image for print”
Firstly, DPI (dots per inch) comes from prepress and refers to the dots printed on a page and are specific to each printing device, ink and paper. Since the advent of digital we now refer to Pixels Per Inch (PPI) which is easily compared. Last century 300dpi was a typical number of dots required to make a photograph appear as continuous tones.
In terms of resolution, digital surpassed film over a decade ago. Today’s high end digital cameras can print to that same 300dpi sharpness at 75ppi because there is no film grain to worry about or generational loss in scanning.
PPI is not an indicator of resolution, it indicates the scale of reproduction. Divide the number of pixels by the scale required and that gives you the size of the image. Let’s look at the same image scaled for print and web use (on a screen):
 2100px x 3600px @ 300ppi = 7″ x 12″
 2100px x 3600px @ 72ppi = 29″ x 50″
Just because the latter is at 72ppi doesn’t mean it’s low res.
“The photo looks soft”
Back in the film days people rarely saw a photograph at 100%. You would need a microscope. Today it’s all too easy to zoom into 100% and make value judgements on image quality. Those who judge images solely on this basis have been called pixel peepers.
To assess an image to simulate sharpness as it will look printed instead view it at 25%. True image (and camera) sharpness is tested by printing an image, not viewing it on a computer monitor.
The megapixel race is a lazy way of selling the latest camera model. At some point there will be diminishing returns with higher megapixels (MP) unable to compete with image noise. Higher megapixels does give you more options for cropping and printing extremely large sizes. How large do you need to print? Over the years I’ve created thousands of images used in 6ft banners with 5MP cameras, and billboard ads using 12MP professional cameras.
When I deliver image files to clients they come in different sizes. The highest resolution is intended for repurposing and unsharpened. Final sharpening is made at the last stage of production after resizing, colour adjustments and conversion. Sharpening multiple times increases image artefacts and is best left to the end.
So when discussing resolution look at the pixels count. For more information on best practice in digital image delivery visit dpBestflow and the Universal Photographic Digital Imaging Guidelines for Image Receivers at www.updig.org.