Digital Basics: Pixels

 

Contents:

 

Introduction

Megapixels and Image Quality: Not all pixels are created equal

Size of each pixel

Depth of field

Size and quality of the optical lens

RAW or JPEG

How many pixels do I need for a print?

Table 1: Desired print size and quality

Table 2: Possible print sizes by camera megapixel number

Graphic: Print sizes in inches at 300dpi

 

 

Introduction

A pixel is a single point in a graphic image. The technique is similar to the way Pointillist artists used to build up an image from separate dots of pure colour in the late 19th Century.

However, unlike pointillism, pixels are of identical size and neatly arranged in rows and columns. Pixels are also so close together that they appear connected. Generally speaking, the more pixels are used to represent an image, the closer the result can resemble the original – though this general rule does not always hold true.

Each pixel stores (colour) information in form of a number variable. How much colour information a pixel can record or display depends on its colour depth, which is measured in bits per pixel (bpp). Some camera sensors can only record 2 different values (monochrome); others can differentiate between 65,536 (highcolour) possibilities. 		Painting by George Seurat La Parade
Georges Seurat – Detail from “La Parade” (1889) showing pointillism

 

Megapixels and Image Quality: not all pixels are created equal

A common misconception is that a camera’s Megapixel number is a fair and reliable measure of comparison between cameras. The sad truth is that literally nothing could be more misleading: even if 2 cameras have the same number of megapixels, it does not mean that they will deliver equal image quality for a number of reasons:

 

Firstly, the physical size of each pixel differs between camera types. Assuming hypothetically that all camera types of the graphic on the right have the same number of megapixels, say 10MP, a camera phone would record 10MP in the blue area - whilst the full frame DSLR records 10MP in the yellow area. Larger individual pixels record a smoother and more detailed image with less noise. They can also contain more light sensitive particles (photons), which results in a higher dynamic range (light and dark parts of an image). Digital Camera Sensor Sizes
Scaled comparison of common camera sensor sizes

 

Shallow depth of field

Secondly, smaller image sensors (such as used in mobile phones) have limited control over the depth of field of an image. You may have noticed before that compact cameras are almost incapable of achieving background blur (or selective focus) unless on a macro setting, whilst this is relatively easily achieved with a DSLR.

Therefore, a 10MP camera phone is unlikely to produce a portrait of a person that is as pleasing and focusing the viewer’s attention as much as one taken with the 10MP DSLR.

Thirdly, the size and quality of the optical lens have a phenomenal impact on the quality of an image. When comparing the physical mass of a camera phone’s lens with the size of even the smallest DSLR lens, the difference in image quality and sharpness between the two mediums is easily explained.

Shallow depth of field is pleasing for portraits

 

Fourth, it makes a big difference whether an image is recorded in RAW or JPG format. RAW images are being recorded uncompressed (lossless) and with an average colour depth of 14 – 16 bit. By contrast, the JPG format uses ‘lossy’ compression (i.e. loses data) and is only of 8 bit colour depth. RAW image files come especially into their own when image editing is required though because they offer a vast array of image corrections – even exposure corrections of up to 2 f/stops. However, RAW files do have very large file sizes because they are uncompressed and offer the greater colour depth – and for most purposes, the JPG format is perfectly adequate.

 

 

How many pixels do I need for a print?

A megapixel is simply a unit of 1 million pixels. If you want to make a photographic print of a certain resolution and detail then there is a maximum print size you can achieve for any given number of megapixel.

It is often argued that the ideal photographic resolution is 300 dots per inch (dpi). However you may find that a resolution of 200 dpi is perfectly acceptable, especially if the photograph is large and will therefore be viewed from a distance. Many magazine articles and posters are printed at a resolution of less than 300 dpi.

However, the following tables and graphs may prove helpful. The first one provides guidance on approximately how many Megapixels are required to make a certain size print in either medium or high quality:

 

Table 1: Desired Print Size and Quality

Table 1

 

This second table starts with your camera’s (or photo’s) megapixel size and indicates what maximum print sizes can be achieved in medium (200dpi) and high quality (300dpi):

Table 2: Possible Print Sizes by Megapixel numbers

Table 2

 

Graphic representation of the above tables:

Maximum print sizes at 300dpi for givel MP size