Sensor resolution is probably one of the most talked about subjects that concern digital cameras, yet is also possibly one of the most mis-understood. Despite this it is without doubt the key factor that most use when choosing a camera. Although we try to give full details on it in our pages on  Sensors  and  Photo Printing  using tables, figures and illustrations to deal with the various factors that arise, we often feel there is the need for a simpler explanation, especially for those just about to enter digital camera ownership, who just require for the time being what could be called an overview. A basic interpretation of sensor resolution and the implications that flow from the use of different resolutions. And thus what type of digital camera they need in terms of the sensor resolution provided.

Sensor count

Sensor resolution is measured in terms of area pixel count, the number of pixels located on it. Pixels are the individual components on a sensor that collect the light information the camera lens produces, and from which an image is constructed. They are normally situated in a square pattern grid formation, so in rows vertically and horizontally. It is the number in a row, and the number of rows, that determine not only what is known as the aspect ratio of the sensor, the width to the height, 3x2, 4x3,16x9 etc, but also the total sensor resolution, the number of pixels that cover the sensor area. This is usually expressed as the pixel number across multiplied by the depth.

So a sensor with 3000 x 2000 pixels is of 3x2 format ratio, and has 6 million pixels. This figure is usually written as 6mp. Although it means 6 million pixels there has for some years been the alternative custom to referring to this as a 6 mega-pixel resolution. We have no idea how this arose, but it is quite common usage now, the expression mega, although quoted as signifying a term in most dictionaries; jumbo, large, mammoth, replacing million. 

(Recently we were informed by a visitor to our pages that 'mega' is also used as a math term, and refers to the power of 10 - million,10^6 - following the SI - Standards Institute - standard for prefixes, and might have arisen out of the usage of 'megabyte' to refer to memory capacity - 1 million bytes, 8 x 10^6 bits).

Indicating a sensors resolution this way is the accepted practice, 6mp, 9mp, 12mp etc. Although early digital camera resolutions were quite low, often less than 1mp, this is not the case today. Minimum sensor resolutions for digicams today are around 8mp rising to 14mp. For DSLR's the minimum is 10mp and rises to 24mp.

When you see a camera's sensor resolution stated this way you have to accept that this is just a rounded off figure. It may be slightly less or slightly more. If the discrepancy exceeds 100.000 pixels it is usual to state this. So a sensor may be classed as 6.3mp, or 10.2mp. Similarly it is unusual to find sensor linear figures in round numbers, as with the example given above which was done purely for ease of understanding. A 10mp sensor is for example 3872 x 2592 (10,036,224mp), and a 12.2mp, 4288 x 2848 (12,212,224mp). These are both at 3x2 ratio.

Sensor format

There are just two aspect ratio formats most sensors are made to and with which the camera user needs to be concerned. The 4x3 ratio most digicams use, and the 3x2 ratio of most DSLR's. The aspect ratio of a sensor is not usually mentioned, little attention being paid to it. It will be listed in the details of a camera's specifications somewhere, but is rarely considered of prime importance, although it does have a direct bearing when images are displayed or printed, either the horizontal or vertical edges needing trimming to fit the space used or the size required, and known in photography circles as 'cropping', this being accepted as part of the process.

Today with the rise of the common A4 size ink-jet printer it's somewhat surprising to realize that neither sensor aspect format matches that of A4, and that cropping of both is needed when the maximum fit is desired on a whole A4 sheet. This issue actually arises with all sensor formats and at many reproduction sizes. Very few paper sizes match sensor format ratios whether they are metric or inch based. 

Sensor Size

A sensors actual physical size, and thus the size of the individual pixels on it, has no bearing on it's resolution. A 12mp sensor has 12 million pixels on it whether it is a tiny digicam sensor, or a larger DSLR type. However it often has a direct effect on overall image quality, just as other factors such as image processing, image exposure and lens quality do. These are separate issues, best ignored in the context of this page, and ones that we consider and comment on elsewhere in pages on this website, including those we have already mentioned.

Printing resolutions

The size an image may be printed at is related mainly to the sensor resolution of the camera that produced it, in other words the size of the image file in pixels. However this is calculated using not the area measurement of it's resolution, but rather the linear measurement of the number of pixels it has either horizontally or vertically. This is often where confusion arises.

To find for example the printing resolution for a 6mp image at 6"x 4" it is necessary to divide the sensor pixel width, 3000, by that of the print, 6. A figure of 500dpi results. For a 10"x 8" it is between 300/250dpi depending on how you crop the image, (it doesn't fit the format ratio). A 12"x 8" would still be 250dpi. The same goes for A4 size, between 240/255dpi. So at it's most basic, as you increase printed size, resolution drops because of course the same number of pixels are being spread over an ever wider area.

It is generally stated that the minimum resolution for good printing quality is a figure of 300dpi. Many use this as the benchmark. This is a standard that is set for the printing of images in good quality magazines by the printing industry. Using this benchmark, images from a 6mp sensor are fine for prints up to around 10"x 6½", but are not suitable for printing at A4 (11¾"x 8¼"), and a sensor resolution of around the 10mp mark is needed (13"x 8¼").

These figures are quite correct - for magazine printing.  However, unless you regularly have your images printed in magazines, and not many do, and they are produced full page A4 size, (which few are), then this benchmark should not really be used because it's higher than that needed for photographic prints and ink-jet printing.

As a rule a figure of around 180dpi upwards is sufficient. It is not an exact science, and depends on the quality of the image itself, because a poor image is a poor image whatever it's resolution may be. Using this benchmark you will discover that 6mp resolution can be produced at A3 size. And 3mp resolution is all that is needed for 6"x 4"'s.

So today even the minimum sensor resolutions available are more than enough for the average camera user, and in some cases exceed what is needed, as most use A4 size printers. Only those who regularly produce large prints, A3/A2 upwards, really need consider higher ones.