Camera Sensors

Reproduction resolutions

There are some points to bear in mind with sensors when considering resolution and reproduction sizes. Pixel size and sensor counts are not linked, except where the sensor size remains constant. It is possible to have a large pixel size and a high pixel count as demonstrated by Canon's 5D and Nikon's D3. Equally it is possible to have large pixel size and a low pixel count as per Sigma's SD14, or a high pixel count and a small pixel size as with many digicams and high count DSLR's.

Pixel size does not affect resolution which remains constant for the sensor count. A 6mp sensor say, produces the same resolution irrespective of the size of the sensor, whether it is a small digicam type, or a larger DSLR one. But the size of the sensor, and thus the size of the pixels, will determine the quality of that resolution in respect of how visible it will be in an image, how clearly and cleanly it will be reproduced.

Increases in sensor counts do not always bring the increase in resolution that might be expected. This is because the sensor count is not concerned with linear measurement but rather area coverage. To double a sensors resolution you must quadruple the pixel count. A 6mp sensor does not produce double the resolution of a 3mp one. A 12mp sensor is required to do that. Files sizes also quadruple as a result, meaning much longer processing times. 

To illustrate the differences in resolution that sensor counts give we have produced the following table. Although when this table was last revised in 2008 minimum sensor counts had increased quite considerably, with 5-6mp for digicams and 10mp for DSLR's, we have included lower counts from 3mp up for some comparison and perspective.

Sensor Resolution Table

It is generally accepted that 300 dpi/ppi is the basic requirement for a good print. But this is really the resolution needed to print an image in a magazine, and is why there is a continued call for ever higher pixel counts. 10mp or greater is needed to satisfy this demand. A 10mp resolution file is suitable for whole page (A4) reproduction, if the image quality is good enough. For a magazine double page (A3) spread then around twice this is needed, which is why high sensor count full frame DSLR's like Canon's 1Ds series 16.7mp & 21mp DSLR's, and Sony's forthcoming 24.8mp model are being produced. And why the expensive medium format digital cameras and camera backs are also being produced and developed that have sensor counts up to 39mp. These are for large high quality reproduction images of the kind found in large format books and magazines. Or hung on walls. 

Using an inkjet printer, good quality can be obtained with a lower figure, and anywhere between 150-200dpi has been proved to be more than acceptable. Which means that these high sensor count digital cameras aren't really needed by or aimed at the ordinary camera user, be they enthusiastic amateur or semi-pro. 

All of this is relative to the printer that is used and the size an image is to be reproduced at. If all you want is to print out your images at say 6"x4", then a 3mp camera is quite sufficient, and a 6mp or larger sensor will not give a substantially better print at this size. If however you want to make an A4 size print, then the 6mp image will look better than the 3mp. Again, if you use an ordinary inkjet printer then the difference will not be as great as it will be if you use a dedicated Photo inkjet printer.

As you can see from looking at the chart it is now widely accepted that for all general purposes a 6mp camera will be a good all rounder, since its images can be printed at full A4 and look quite alright. So in many cases a camera with a large resolution sensor may cost more in terms of image storage space needed and yet not produce images that are significantly more useful to the 'average user'. 

However if you are intending to publish your images in a magazine that is printed using conventional printing methods, known as 4 colour printing, then you do need as high a pixel count as possible. This is because of the different way an image is printed, requiring far higher resolution to produce acceptable final image quality. 300dpi is the standard resolution required, and sets the maximum size an image can be reproduced at.

Please visit  Printing Photos for more information. 

Another point to bear in mind when considering sensor's is that as pixel counts increase, so do image file sizes, so you need larger memory cards, and more storage space and a much more powerful computer to process the images at a reasonable speed.

Here is an illustration showing 6mp and 10mp counts placed on a 12mp count in relation to each other. The relationship remains constant, as a ratio, whatever printed/displayed resolution is used.  The difference between the resolution of a 6mp and 12mp sensor whilst significant is nevertheless perhaps not as big as you might expect. You only have to crop the 12mp image a bit to end up with the equivalent of a 6mp capture. And the difference between 10mp and 12mp is frankly not worth worrying about, it's minor in resolution difference terms, about 10%, as the previous table shows.

Printing Comparisons

To illustrate all of this here are some shots taken with three Digital Cameras. Two digicams of 1mp and 3mp resolution, and a D-SLR with 6mp resolution. All the shots were taken using as far as is possible comparable focal lengths. A crop was then taken from a small portion of each one and resized to the same size to illustrate the difference in quality and detail that exists between them. On the lower left is the 3mp shot of the image used showing the area cropped. On the lower right is a composite which shows the size ratio each will be at the same resolution.

1mp, 3mp, and 6mp image comparison