The DPI versus MP Confusion

For a couple of years now, the Smart Photography Magazine has welcomed entries from the general public for featuring in it’s Center-spread and Photo of the Month features. The requirement for the images are:

  1. Landscape format.
  2. 17″ image printable at 300 dpi.

As a digital photographer, you may wonder how to give a 17″ image at 300 dpi considering that your entry-level DSLR camera shoots images in mega pixels.

What is DPI?

dpi illustration

Illustration of dpi / ppi

Simplistically speaking, DPI is the density of the ink dots that are printed to form the image. i.e. the number of printed dots packed into one inch. In the printing technician’s world, there are other parameters such as dot-pitch, bleed, dithering, LPI etc. but for general purpose the ‘dpi’ requirement is sufficient.

It is generally perceived that if an image is printed at 300 dots per inch, and the image is seen in normal reading mode (about 1 foot away from the eye), the human eye cannot make out the individual dots. The dots seem to merge and the resultant image seems continuous rather than made up of individual dots.

Magazines are typically printed at 300 dpi. News papers are printed at approximately 150 dpi and very high quality images are printed at 600 dpi.

What about Electronic DPI?

DPI is applicable to the electronic screens too. On a computer monitor, the image is made up of coloured pixels instead of coloured ink dots. Hence, simplistically speaking, dpi of the printed world can be substituted for ppi (pixels per inch) in the electronic world.

CGA graphics were displayed at 23 dpi.

CGA graphics were displayed just 23 dpi.

Earliest ‘graphic quality’ displays from Apple featured monitors with display resolution of just 72 ppi. For a long time, Microsoft Windows assumed that computer monitors operated at 96 ppi. The appearance of Apple iPad with Retina display pushed electronic displays towards 300 ppi resolution. A few devices on the market even claim 550 ppi resolution!

On older CRT monitors and LCD panels, you can make out the individual dots if you look closely enough. It is easier to do this if the monitor is displaying images with varied coloured areas instead of just solid colours. On the newer LCD panels, it is increasingly getting harder to make out the individual pixels and the images seem continuous rather than made up of individual pixels.

What is Mega Pixel?

Your digital camera shoots images and stores the information as pixels. For example, it may shoot an image and store data for 4000 pixels horizontally and 3000 pixels vertically. Hence the total number of pixels that are stored for the image are 4000 x 3000 = 1,20,00,000 pixels (1 crore 20 lakh pixels or about 12 million pixels). Instead of uttering this long string of numbers, it is convenient to reduced the number by a factor. The factor chosen was Mega Pixel or 1 Million Pixels. Hence, the 12 Million Pixel image can be simply described as 12 Mega Pixel image or 12 MP.

0.3 MP images on a 1.44 MB Floppy

0.3 MP images on a 1.44 MB Floppy

If I remember correctly, the first digital camera I ever used was in 1999. It was a Sony Mavica featuring 640 x 480 pixel images or approx. 0.3 MP. My most recent camera purchase, a Nikon D5500 features images that are 6000 x 4000 pixels across or 24 MP. A considerable jump. Back then, viewing a 640 x 480 pixel image on a 14″ colour monitor capable of 800 x 600 pixel display resolution resulted in a large viewing experience but no ability to zoom in to see details. Today, watching a 24 MP image on a 2 MP (Full HD) LED display allows me considerable leeway in zooming into the image and more details becomes visible as I keep zooming in till 1:1 ratio.

Medium Format cameras from Phase One now shoot 100 MP images. Even phone cameras featuring Nokia’s PureView technology claim to shoot 40 MP images. MILC such Olympus OMD EM-5 shoot 40 MP images using sensor-shift technology.

Even if you have an entry-level DSLR with a basic 18 – 55mm lens, you can shoot super-sharp 200 MP photos using Panorama stitching!

What is the DPI – MP connect?

Printer DPI test image

Printer DPI test image

The image out of your camera is fixed in it’s resolution. For example, your camera may deliver 12 MP images at maximum. This is a hard-limit and cannot be changed.

However, when you print it, you can opt to print it at a DPI of your choice.

For example, if you print a 4000 x 3000 pixel image (12 MP) at 300 dpi, the resultant image will be:

  • Horizontally: 4000 pixels / 300 dots per inch = 13.3 inches
  • Vertically: 3000 pixels / 300 dots per inch = 10 inches
  • i.e., you will get a sharp 13″ x 10″ print.

If you were to increase print resolution to 600 dpi, you will only get a 6.5″ x 5″ print which will be perceptibly sharper than the 300 dpi print but probably an overkill. The maths is:
Horizontally: 4000/600 = 6.66″, Vertically: 3000/600 = 5″.

Effect of dpi on continuous tone

Effect of dpi on continuous tone

Similarly, if you were to decrease the print resolution to 150 dpi, you will get a 26″ x 20″ print. Sure, when seen up close, you will be able to make out the individual printed dots. But such large poster sized images are meant to be seen for a distance of a couple of feet. At such distances, again your eye simply cannot resolve the individual pixel.

The conclusion is that:

  1. The MP capability of your camera is fixed.
  2. The DPI you select for printing, is simply dependent on how large a print you want or how sharp a print you want.

Activity:

  • Download the printer DPI test file and print on your printer at different dpi to check how fine your machine can print. Note that at higher DPI, the printed image dimensions should get smaller. At 300 dpi, the image should print at 9.41″ x 7.67″.
  • Try and photograph a high-quality print of the image to see how much detail can your camera resolve.

What about photography magazine requirements?

Assuming that the magazine wants a image that is 17″ wide horizontally at 300 dpi, the image needs to have 5100 pixels horizontally. Since digital cameras shoot images in 4:3 ratio, the vertical dimension will be 3825 pixels (5100 x 3/4). Hence the image has to be 19.5 MP (5100 x 3825 = 1,95,07,500).

In the year 2016, even entry-level DSLRs are offering a resolution of 18 MP and most amateur grade DSLRs are offering resolutions such as 24MP, 36MP etc. Shooting and submitting an image at 19.5 MP does not seem so hard. However, please consider the following factors:

  • Professional grade DSLRs featuring full-frame sensors do not offer high MP images. They focus on colour accuracy, dynamic range, pixel-well etc. For example, the Nikon D4S costing $6400, only offers a 16 MP image. However it’s image is better than 40 MP image from a Nokia camera by several orders of magnitude. So are we to assume that a top-notch world renowned photographer’s image from his Nikon D4S camera is ineligible for printing in the magazine?
  • It is a highly recommended technique that photographers should not crop in camera. i.e., when shooting, do not frame the image too tightly. Leave a little breathing space. Allow some extra parts of the scenery to be included. Cropping is ideally done in post-production. When you are examining the images on your computer, you may apply the cropping in a much more flexible manner. If you have cropped the scene very tightly while shooting, you have discarded information that you cannot grow back on your computer. Considering this, an image that is 24 MP in size, will probably become 12 – 14 MP once the photographer crops the image in post-processing. Although, the resultant photo is certainly better in it’s framing, has it just become ineligible for printing in the magazine?
  • The chase for 300 dpi is generally for sharpness. The magazine wants images that are super-sharp. The objects in the image should present knife-like edges. Every single detail should pop-out. This is quite contrary to what professional photographers preach. They say that photographers should focus on composition, depth of field, moment-anticipation etc. If fact, blurriness or softness are friends that can be employed creatively to create award winning images! So is the magazine incorrect in chasing sharpness over merit?
  • The magazines frequently diss images shot by Compact System Cameras (CSC or Pocket Cameras) that employ a tiny 1/2.3″ sensor. In fact Smart Photography went on to recommend CSC cameras that offered RAW output whereas noted photography experts stated that RAW images from CSC is crap because of the extremely high noise. They clearly state that the electronic image processor of the CSC cameras is designed to reduce the noise in the images from the sensor using proprietory technologies and we should let the camera do it instead of futzing around in Photoshop on our computer. In the last two months, I have read numerous articles in photography magazines which emphasized on why RAW format is an overkill for most photographers and they are simply wasting disk-space by stuffing them with RAW images that they will never get around to editing. So is the magazine’s awareness of technology a big suspect?
  • Till a few years back, photographers regarded the ability to shoot on 35 mm film highly and praised it over DSLR cameras. Considering that 35 mm film is actually 36 mm wide (horizontally), scanning such a film at 300 dpi yields a image that is just 425 pixels wide! It is a far cry from the 5100 pixels demanded by the magazine. To get 5100 pixels wide image from a 35 mm film, it would have to be scanned at 3600 dpi using specialized film-scanner. This is clearly out of scope for even many professional photographers. Considering that we are used to projecting 35 mm slides onto 12 feet walls, but will fail to scan it at 3600 dpi, will the magazine be right in rejecting a 600 dpi scan of the slide?

The net conclusion I can derive is that:

  • The magazines are unrealistic in technical and aesthetic terms about acceptable submissions for printing in their magazine.
  • While they should ask for images with a minimum MP resolution, demanding it in dpi is unrealistic. They can very well print it at a dpi that is optimum for their magazine’s requirements.

End note

Shot on iPhone 6 and printed on super-sized billboards

Shot on iPhone 6 and printed on super-sized billboards

If you have seen huge billboards featuring images shot using iPhone 6, you should know that sensor size, mega-pixel are fast becoming irrelevant. Imaging and printing technology have improved by leaps and bounds and the focus now is on creativity rather than technical superiority in numbers.

At times, my Lenovo K3 Note mobile phone takes images that are more satisfying because of it’s spontaneity and acceptable image quality rather than my Nikon D5500 DSLR which is terribly fiddly when shooting images as a tourist. Professional wedding photographers are using a mix of iPhones and Full-frame DSLR cameras. People are shooting more images with mobile phones than traditional cameras.

No one is challenging the superiority of large sensors, higher mega-pixels, fast frame-rates. The important thing to consider is that a 6 year-old kid has now been published by National Geographic for shooting with a Fujifilm Instant Camera. The key is creativity and aesthetics. If he can do it, so can you. To hell with magazines with archaic requirements from photographers.

The DPI versus MP Confusion
The DPI versus MP Confusion
How to translate mega-pixels of an image into a print with specific dpi requirements.
Rajib Ghosh

Rajib Ghosh

Based in Hyderabad, India.

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