Resolution, Pixels, And DPI

Resolution can be a confusing subject so I hope to make it clearer for you.

Updates:
6th November ’07 – Scanner software (with screenshot). Red Bubble products. General tidying and clarifying. Table of Contents.
10th November ’07 – Added posters.

Contents:
Terms Used
DPI
Digital Camera Resolution
Aspect Ratios
Screen Resolution
Scanning & Resizing Images
…100% Reproduction
…100% Plus Reproduction – Enlarging
…Less Than 100% Reproduction – Reducing
Red Bubble Products

Terms Used
Resolution – the degree of sharpness of an image as measured by the number of dots per linear inch in a hard-copy printout or the number of pixels across and down on a display screen. Digital camera resolution is often referred to in mega pixels or MP. Mega means million, so 2mp = 2 million pixels.

Pixel – (Pix, short for pictures + el, short for element)
The basic unit of the composition of an image on a television screen, computer monitor, or similar display.

DPI – Dots Per Inch. This is a printing resolution term. Printers form dots on paper etc.

PPI – Pixels Per Inch. This is a screen resolution term. Screens display pixels.

Aspect Ratio – Usual form: 4:3, 3:3, 16:9, etc. E.g. a 4:3 image could be 4 × 3 inches, 8 × 6 inches or 12 × 9 inches, and would be in the 4:3 aspect ratio but different sizes.

DPI
Although DPI refers to printing resolution it has come to be interchangeable with PPI when used to describe the resolution of screen images.
The Pixels Per Inch refers to a linear inch not a square inch as you might think. So a 300PPI image is actually 300 × 300 pixels per square inch or 90,000 pixels.

Digital Camera Resolution
300PPI is photographic quality resolution, so a standard 6in x 4in photo at 300PPI is:
6 × 300 wide = 1800
4 × 300 high = 1200
1800 × 1200 pixels = 2,160,000 pixels or 2.16mp

If a 6in x 4in photo only needs 2 mega pixels why are camera sensor resolutions steadily increasing each year? Improvements in the technology and camera companies hoping their customers will think more mega pixels means better quality images. In fact the reverse is true. The more pixels you cram onto the same sized sensor chip the more ‘noise’ or electric charges effects images.
Most current compact camera sensor chips are around 1 square cm. So if you go from 4 or 5mp to 7 or 8mp on the same sized chip you are degrading the quality of images to some degree. This is especially noticeable in low light situations where the shutter is open for longer than usual. Most compact cameras become noisier as you get past the ISO 200 equivalent of film.

Aspect Ratios
Photographic prints generally have an aspect ratio of 3:2, e.g. 6 × 4 inches, whereas digital images are generally at the ratio of 4:3, e.g. 1600 × 1200 pixels, although some digital cameras have settings for the 3:2 ratio. This setting just ‘turns off’ the extra pixels to achieve the 3:2 ratio.
This means that you have to crop a 4:3 ratio digital image to the 3:2 ratio if you are intending to print at a photolab.

Screen Resolution
Computer monitors used to display at a resolution of 72PPI and often images intended for screens are at 72PPI, e.g. digital camera images are often at 72DPI when you open them in Photo Editing software. As monitor quality has improved, the resolution has increased to around 100DPI, but depending on the monitor size ie. 17", 19" etc., the display size chosen ie. 1024 × 768, 1280 × 1024 etc. and the quality of the monitor, the resolution varies.
Standard monitors (as opposed to widescreen) use a 4:3 aspect ratio, eg. 1024 × 768 pixels, or a roughly 4:3 ratio, eg. 1280 × 1024 pixels is a 4:3.2 ratio.

Scanning & Resizing Images
100% Reproduction
If you want to scan an image and reprint it at the same size then you must match the photographic resolution (300DPI), with the scanned resolution of 300PPI.
E.g. you have a 6 × 4 inch photograph that you want to scan into your computer, increase the brightness and then burn to CD or copy to thumbdrive to be printed at a photolab.
You scan the photo at 300PPI because this matches the photolab’s print resolution of 300DPI. The scanned image has not increased or decreased in resolution, therefore it will print at the same size as the original photo.

100% Plus Reproduction – Enlarging
If you want to scan an image and print it at a larger size, then you have to increase the scan resolution from 300DPI to something higher.
E.g. you have a 6 × 4 inch photograph (must be a film photo as these are printed at very high resolution and therefore enable enlargements without loss of quality) and you want to enlarge it to 8 × 12 inch and get it printed at a photolab.

First, let’s take a look at an example of scanning software:

Starting at the top and working downwards:

Colour Mode: Black and White, Grayscale, Colour
Black and White – reduces the image to two tones: black and white.
Grayscale – what most people call black and white, ie. an image that goes from black through shades of grey, to white.
Colour – colour.

Output Resolution: DPI (notice even scanner software refers to DPI…)
300DPI – if you want photographic quality

Selection: – for this example a 6 × 4 inch photograph (film) is being scanned
Width: 4
Height: 6
Units: inches
Note: the icon to the right of the Height & Width options is to lock/unlock the aspect ratio of the selection. When selecting the image to be scanned, ie. with the mouse, the selection maintains the aspect ration entered.

Print size: for this example an 8 × 12 enlargement is required. Conveniently the dimensions are at the same aspect ratio so no cropping is required.
Width: 8
Height: 12

The image is then scanned and the software scans the image at 200% or 600PPI.
Doubling the scanned resolution quadruples the size of the final print size:

Less Than 100% Reproduction – Reducing
If you want to scan an image for use on screens only, e.g. the WWW: then you wouldn’t need to scan at 300PPI for many images. Scanning at 100DPI would probably be enough. It depends on the size of the image to be scanned and the display size.
6 × 4 inch photos are 15cm x 10cm, so scanning at 100PPI would make a good sized screen image at 600 × 400 pixels. If you scanned the photo at 300PPI it would be 1800 × 1200 pixels making it larger than most people’s monitor display size.
The file size for a 6 × 4 inch photo at 300PPI would be approximately 500 kb to 2000 kb, whereas a 6 × 4 inch photo at 100PPI would be round 100 kb to 300 kb.

Red Bubble Products
How do I get my digital camera image to the right size for a Red Bubble card, print or T-shirt?

Firstly Red Bubble has minimum pixel dimensions:
Cards – 1300 × 900 pixels

Small print – 2400×1600 pixels
Medium print – 3240×2160 pixels
Large print – 3840×2560 pixels

Small poster – 2500×3500 pixels
Medium poster – 3500×5000 pixels
Large poster – 5000×7100 pixels

T-Shirts – 2400×3200 pixels

Secondly, Red Bubble uses the usual photographic aspect ratio of 3:2 (excluding posters and T-shirts), and digital cameras generally use the 4:3 ratio, unless there’s a setting for 3:2. Check if your camera has this setting if you would like to save having to crop your photos. The advantage of keeping the 4:3 ratio setting is you can choose where to crop if needed later.
If you have 4:3 ratio images there’s going to be a mismatch:

The 4:3 image must be cropped to 3:2 using image editing software; such as Gimp or Photoshop, that lets you choose an aspect ratio when using the selection tool.
After cropping you may have to resample the image to increase the pixels. Resampling is increasing an image’s pixels by using a mathematical algorithm to ‘guess’ where to put the new pixels and not degrade the image quality. There’s a limit to how much a given image can be resampled, and depends on the quality of the resampling algorithm used.
Photoshop’s Bicubic resampler is considered the best and can achieve 200% plus increases without noticeable degradation.

Cards: 1300 × 900 pixels
This equals 1.2mp, so as long as you set your camera to at least 2mp you will have enough pixels.

Small prints: 2400 × 1600
This equals 3.8mp, but due to the aspect ratio mismatch problem and the resulting cropping of the camera images, 4mp is not enough, so set your camera to 5mp if available, or 4mp as a second best option.

Medium prints: 3240×2160 pixels
This equals 7mp, set your camera to 8mp if available, or 7mp as a second best option.

Large prints: 3840×2560 pixels
This equals 10mp, set your camera to 11mp if available, or 10mp as a second best option.

Small poster: 2500×3500 pixels
This equals 9mp, set your camera to 10mp if available, or 9mp as a second best option.

Medium poster: 3500×5000 pixels
This equals 18mp which isn’t widely available as a camera resolution so some sort of interpolation (resampling) will be necessary. DSLRs have larger sensor chips which should allow greater enlargements than a compact camera’s small (typically 1cm square) sensor chip.

Large poster: 5000×7100 pixels
This equals 36mp. See above.

T-shirts: 2400×3200 pixels
This equals 7.7mp, set your camera to 8mp if available, or 7mp as a second best option.

If you only have a 4mp camera but want to upload images for larger than the cards and small prints don’t fret, as mentioned above, resampling with a good algorithm could get an acceptable image for the medium print size at least.

Other options:
Prints: photo collage made up of 4mp images
T-shirts/Prints: create art from scratch on computer at desired size
T-shirts/Prints: Using filters to transform photos into graphics allows for more resampling increase without as much degradation. Eg. posterize, half tone screen, and mosaic to name a few.

Journal Comments

  • peter
  • Lisadee Lisa Defazio
  • davoid
  • Patricia L. Ballard
  • mick8585
  • davoid
  • John Hurle
  • Andy2302
  • davoid
  • Andy2302