How Much Resolution is Enough?

Humans are incredibly curious creatures and we like to ask the big questions such as “How many licks does it take to get to the Tootsie Roll center of a Tootsie Pop?” The answer, of course, is not “one, two, three *crunch*…three!” but “Do not give nocturnal predatory birds your Tootsie Pop because a lot of them are jerks.”

We also like our quantifiable data such as how many miles there are to the sun or what’s the longest palindromic word? (93 million miles and saippuakivikauppias, a 19-letter Finnish word for a lye dealer.)

Another question that humans love to ask, at least to me, is “Okay, wise guy, if scanner resolution doesn’t equal quality, how much is enough?” It’s a fair question because scanners, especially photo models, tout all kinds of really high resolution specifications. What are those 4800 and 6400 dpi ratings for?

The answer is enlargement, to embiggen, if you will.

The standard for a 1:1 image size from a scanner is generally 300 dpi. This is based on how well a typical human eye can resolve images made up of pixels (dots) at typical viewing distances of about 12”. In old printing technologies that used dot patterns lower than 150 dpi, like Golden Age comic books, the dots were very visible at normal viewing distances and it looked as if Superman’s face was covered with Kyrptonian super measles.

Given this information, if you are scanning prints or documents and want them to be reproduced at the same size, 300 dpi will give you enough pixels. Given that baseline, you can do some basic math to determine how much resolution you need for making enlargements. If you wanted to create an 8” x 10” print from a 4” x 6” original, divide the two short sides (8 ÷ 4 = 2), then multiply that by the 300 dpi base (2 x 300 = 600). The 600 dpi scan will give you enough pixels to resize up to 8” x 12” and with a little cropping (you didn’t want Uncle Schmendrick in that photo anyway) you have your 8” x 10”.

The big resolution numbers come into play when you’re scanning something very small, like a 35mm slide. If you wanted to create a 16” x 20” print from a slide (1.29” x 0.8” roughly), you can run through the formula of dividing the shorts sides (16 ÷ 0.8 = 20) and then multiplying that by 300 (20 x 300 = 6000) meaning you’d need one of our 6400 dpi models to get that big print from your slide.

Just like almost everything you learned in math class, now that you understand what the resolution numbers mean, you can let a machine do all of the actual work. In the Professional mode of your Epson scanner driver, you can leave the output resolution at 300 dpi and use the pull down menu to choose an output resolution. The driver scales the resolution accordingly and even provides a crop frame so you can see how best to crop out Uncle Schmendrick.

Epson Scan dialog box showing resolution and output size.

Epson Scan dialog box showing resolution and output size.


A thumbnail 35mm slide with the scaled crop frame.

A thumbnail 35mm slide with the scaled crop frame.

So in conclusion, that’s how your scanner’s resolution works. Don’t give owls candy unless you want your heart broken and please send me the contact for a good saippuakivikauppias, I’m almost out of lye.

Image: ©

Resolution ≠ Quality


In my senior year of high school, I took an elective semester course in archaeology. On day one of class, the instructor wanted everyone to understand that real archaeology has no resemblance to the adventures of Henry Jones, Jr. (aka Indiana). Those of us in the classroom with bullwhips and fedoras in our backpacks were disappointed to learn that real archaeology was about slowly digging around shards of pottery with very small brushes and not about melting bad guys with ancient artifacts.

As the crew of MythBusters has demonstrated for over a decade, a lot of what is depicted in film and television is about spectacle over reality. You can sit back and enjoy the cinematic ride, but it can be hard to turn off that part of your brain that really understands how certain things work. My poor wife must cringe whenever something appears on screen that might put me into a scanner-nerd fugue state where I am compelled to explain, yet again, how “it doesn’t work that way.”

Take for instance the popular spy/action movie trope of the heroes enhancing the footage from a security camera to get a crystal clear image of the bad guy. If you’ve seen real security footage on the evening news, even the enlarged version is usually badly pixelated and motion blurred, but it seems as if every security camera in a movie is at least HD quality and can be zoomed in to see the miniature map to the bad guy’s lair hidden in the design of his cuff links. (“Hah, your fashion sense has done you in again, Baron Nefarious!”)

In working with scanners all these years, the myth of “resolution = quality” is one I’ve encountered a lot. There is a correlation with printers and the dots per inch on paper to image quality, but in scanners, the dots per inch of resolution has a lot more to do with image enlargement than quality. In an imaging system, the quality of the image is ultimately decided by the optical sensors. A scanner with a good sensor and high resolution can give you a great enlargement from small media, like a 35mm slide, but only if the source is also great.

Once at a trade show, a gentleman wanted me to scan his 35mm slide at highest resolution and was irritated that the final image wasn’t as tack sharp as the other slides I was scanning. He was not aware that his slide was simply out of focus and the scanner was enlarging exactly what it was scanning – a fuzzy, out of focus image.

As the great humorist Tom Lehrer quipped on his album That Was the Year That Was, “Life is like a sewer, what you get out of it depends on what you put into it.”

The same can be said about scanning – no amount of resolution can fix a bad image.

Thoughts? Questions? Tom Lehrer Quotes? Drop me a line!