The resolution of a digital image is defined as the number of pixels it contains. A 5 megapixel image is typically 2,560 pixels wide and 1,920 pixels high and has a resolution of 4,915,200 pixels, rounded off to 5 million pixels. It is recommended to shoot at a resolution which corresponds with the camera's effective pixel count. As explained in the pixels topic, shooting at higher (interpolated) resolutions (if available as an option) creates only marginal benefits but takes up more card space. Shooting at lower resolutions only makes sense if you are running out of card space and/or image quality is not important.
We measure resolution using the widely accepted PIMA/ISO 12233 camera resolution test chart. This chart is excellent, not only for measuring pure horizontal and vertical resolution, but also to test the performance of the sensor with frequencies at various angles. It also offers a good reference point for comparison of resolution between cameras. The chart is available for every camera which comes through our test labs, both in the camera reviews and our extensive products database.
|Resolution test chart from the Nikon Coolpix 8700 review. The areas indicated in red are shown as crops below.|
|Crop A. The black and white lines can be distinguished from one another until position "16", so the Horizontal LPH is 1,600, as explained below.||Crop B. The black and white lines can be distinguished from one another until position "15", so the Vertical LPH is 1,500.|
Horizontal LPH refers to the number of (vertical) lines measured along the horizontal (x) axis or width of the image. Crop A shows a test pattern consisting of 9 black lines with 8 black white lines in between. From the crop you can see that below label "16" the 17 lines start to merge and become hard to distinguish from one another. The crop shows that at label "16", the 17 lines cover a horizontal distance of 26 pixels. Since this sample image is 2,448 pixels high, the horizontal number of lines per pixel height is therefore 2,448/26*17 or 1,600 LPH. So in general a value of "16" on the resolution chart equates to 1,600 lines per picture height (LPH).
Likewise, the Vertical LPH refers to the number of (horizontal) lines measured along the vertical (y) axis or height of the image. Crop B shows that in this example the vertical LPH is around 1,500 LPH.
Because the resolution is "normalized" to the picture height, the results of cameras with different aspect ratios can be compared easily.
Since we normalize on picture height, the absolute number of (horizontal) lines the camera is able to resolve along the vertical axis (image height) is equal to the vertical LPH. The absolute number of (vertical) lines the camera is able to resolve along the horizontal axis (image width) is equal to the horizontal LPH multiplied by the aspect ratio. In this example, this would work out to be 1,600 x 1.333 = 2,133 since the camera has an aspect ratio of 4:3.
You will immediately notice that 2,133 x 1,500 or 3,200,000 is much lower than the image resolution of 8,000,000 (3,264 x 2,448). This is because the data collected by a sensors with a color filter array have to be interpolated and because many cameras have an anti-alias filter. In Foveon sensors, the image resolution is close to the sensor resolution, as shown in the Sigma SD10 review on this site. Limitations of the optics required to create an incredibly sharp image on the small sensor area can also affect the image resolution.
Our reviews also state the 5° Diagonal Lines LPH, measured in crop C in this example. Since the chart only goes up to 1,000 LPH for this camera, the review states 1,000+ as LPH.
|Crop C. The black and white 5° diagonal lines can be distinguished from one another until position "10", the maximum of the chart, so the 5° Diagonal Lines LPH is 1,000+ for this camera.|
The above explanations refer to "Absolute LPH" which is an LPH with clearly defined detail. Our reviews also state the "Extinct LPH". This is the LPH at which the lines become solid gray. The detail at that level is beyond the camera's definition. Between the Absolute and Extinct LPHs only some detail can be captured.
|Crop D. Around label "18", the black and white lines merge into solid gray, so the Vertical Extinct LPH is 1,800 LPH.|
|This article is written by Vincent Bockaert,|
author of The 123 of digital imaging Interactive Learning Suite
Click here to visit 123di.com