Print Sizes For Different Digital Camera Resolutions

This table gives a very rough guide to reasonable quality print sizes for various camera resolutions.

 

Resolution Megapixels	Example Camera	Image Size (Pixels)	Photo quality (300 DPI)	Poster quality (150 DPI)	Billboard quality (40 DPI)
21.1	Canon 5D - high	5616 x 3744	18.7" x  12.5"	37.4" x 25"	11'8" x  7'9"
12.1	Canon 450D - High	4272 x 2848	14.2" x 9.5"	28.5" x 19"	9' x 6'
10	Nikon S60 - High	3648 x 2736	12.2" x 9.1"	24.3" x 18.2"	7'7" x 5'8"
8.2	Fuji J10 - High	3264 x 2448	10.9" x 8.1"	21.8" x 16.3"	6'9" x 5'1"
6.3	Canon 450D - Med	3088 x 2056	10.3" x 6.9"	20.6" x 13.7"	6'5" x 4'3"
3.4	Canon 450D - Low	2256 x 1504	7.5" x 5"	15" x 10"	4'8" x 3'1"

 

This table shows the camera resolution (megapixels) needed to fill a given size of paper.

 

Paper	Size (mm)	Size (inch)	Photo quality: 300 DPI		Poster Quality: 150 DPI
			Pixels	Megapixels	Pixels	Megapixels
A0	841 × 1189	33.1 × 46.8	9933 x 14043	139.5	4966 x 7021	34.9
A1	594 × 841	23.4 × 33.1	7015 x 9933	69.7	3507 x 4966	17.4
A2	420 × 594	16.5 × 23.4	4960 x 7015	34.8	2480 x 3507	8.7
A3	297 × 420	11.7 × 16.5	3507 x 4960	17.4	1754 x 2480	4.4
A4	210 × 297	8.3 × 11.7	2480 x 3507	8.7	1240 x 1754	2.2

 

Camera manufacturers specify the level of detail that their cameras take in terms of Megapixels. The more megapixels, the more detail. More detail means that you can print your pictures at a larger size without quality suffering.

 

A Megapixel equates to 1 million pixels. In digital camera terms, a pixel is a picture element, or a single 'dot' of colour that makes up the image.

 

Enlarging the image (left) shows the rectangular blocks of colour (pixels) that make it up.

 

The megapixel count for a camera is calculated from the size of the array of pixels in each image. So, an image of 1000 pixels high by 1000 pixels wide would be 1 megapixel (1,000 x 1,000). The Canon 450D produces images up to 4,272 x 2,848 pixels which equates to 12166656 pixels, or 12.1 megapixels. Note that this is an inverse square relationship, so doubling the megapixel count does not double the pixel length or height of the image.

Print sizes are often specified in DPI (Dots Per Inch) or PPI (Pixels per inch). The latter term is more accurate, but DPI is also used interchangably with PPI in many applications to the horror of print professionals. In printing terms, a dot is also a single block of colour that will make up an image on paper.

To work out the print size of an image, you can take the actual pixel count of the image and divide it by the DPI count. So printing a 1 megapixel (1,000 x 1,000 pixels) image at 150 DPI will produce a print that is 1000 / 150 = 6.67 inches to each side. Printing a top resolution picture from the 450D will produce a print that is 4,272 / 150 = 28.48 inches long by 2,848 / 150 = 18.98 inches high.

 

To work out the DPI required to print at a given size of paper, simply divide the number of pixels along one side of the image by the size of the page in inches. So, for the Canon, a 12 Megapixel image has a long side of 4272 pixels. Printing this on A4 (with a long side of 11.7 inches) will require a DPI of 4272 / 11.7, or 365 DPI. Note that the proportions of the page may be different from that of the photograph, so you will either end up cropping two edges of the picture, or leaving unprinted strips along two edges. 

As the DPI count specifies the amount of detail is packed into each inch of the printed image, the ideal figure will depend upon the distance that the print is being viewed at and the quality of the paper being printed on. A normal photograph on glossy paper that is viewed at close distances would typically have a DPI count of 300 or higher. A street-side advertising hoarding is typically printed at only 40 DPI. For wall-hung prints, 150 DPI is a reasonable compromise between detail and size. Usually rougher paper is more forgiving of lower DPI - so printing on canvas at 150 DPI can look better than printing on glossy paper at 150 DPI. 

 

 

What about file sizes? If you have a 12 megabyte (Mb) file, doesn't that mean it'll print at a bigger size than a 5Mb file? In short, the answer is usually no. Due to the way images are stored on disk, the number of pixels does not directly translate to file size. Typically an image with more detail will take up more space than an image with less detail even though they have the same number of pixels in the image. Therefore the pixel count, rather than the file size has the strongest affect on print size. The only exception to this is when the image has been heavily compressed to save disk space, in which case the compressed image may have 'noise' that will look poor when printed.

Digital cameras can save their images in two different ways - either 'raw' or compressed. Raw images record the exact colour of every pixel in the original image. Some savings can be made by noticing that adjacent pixels are the same colour, but otherwise each and every pixel must be recorded and the file sizes are therefore pretty large. For instance, the 12.1 megapixel images of the 450D take up around 15 Megabytes.

 

Compressed images take advantage of similarities between adjacent pixels to store a rough estimate rather than an exact colour. There can be some fairly mind-bending maths involved, but the result is that you can get a fairly accurate representation of an image without storing each and every pixel precisely. This means that you get smaller file sizes. The exact size will depend on how much detail the original picture contains, and how accurate the stored version is required to be. The same image can be stored with higher compression (lower accuracy) and take up much less space on disk. The penalty is that higher compression introduces 'noise', where the clever maths tries to guess at the exact colours of each pixel and gets it a little bit wrong. Noise typically appears as speckles, particularly around sharp edges in the image and areas of high contrast.

 

Most digital cameras save compressed images using the JPEG (.JPG) file format. The compression level in the camera is usually set at a good compromise between image quality and file size. However, photo professionals will save their images in a raw format (which differs between camera manufacturers) so that no detail of the original image is lost.