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Resolution - Myths and Legends

People can sometimes confuse image resolution with print resolution and are unsure about the differences. So lets define resolution.

  • Image files usually have their resolution expressed as pixels per inch - e.g. 300 ppi .
  • Print resolution is usually expressed as dots per inch, that is so many dots per inch printed on paper or other print media - e.g. 300dpi.

These terms are frequently misapplied, most commonly when image files are referred to as having a resolution expressed as dpi. Although incorrect, in practice this really doesn’t matter, as the error is so common that we all understand what is meant.

Pixels - Image Resolution

Pixels are the picture elements making up the image file, which may be compared to the grain structure of photographic film. The reason for this comparison is straightforward - if you enlarge a photographic film image too far, the grain structure starts to become visible, just as if you enlarge a digital image too far, individual pixels start to become visible.

The resolution of a digital image file is measured in pixels per inch (ppi) or less often as per cm (ppcm). The image has height and width dimensions made up of a certain number of pixels. These make up the pixel map i.e. the amount of data in the file. In fact a digital image file does not have a size or resolution that is absolute, it just has a certain number of pixels spread over a certain area. The resolution will change, of course, if the image is printed larger or smaller as the finite number of pixels is spread across a larger or smaller area.

Digital image files stored on your computer take up hard drive space and the memory size of a digital image file on your computer is measured in megabytes (Mb) and very small images, less than 1 Mb, would have a size expressed in kilobytes (Kb).

A frequently asked question is: "How large can this image file be printed?" What should really be asked is: "How large can this image file be printed before the individual pixels become unacceptably visible?" The answer is subjective, as it depends on the viewing distance of the print. For example: a billboard image looks fine when view from the other side of a road but not when viewed close up, as the print resolution is quite coarse. A fine art image printed at much higher resolution may be viewed satisfactorily from quite close up. Thus in practical terms, resolution may be considered relative.

Dots - Printer Resolution

Most good quality desktop printers from the major manufacturers, such as Canon, Epson and HP, may have a print resolution of 1440 dpi. Whereas they are capable of printing that many dots per inch, it should be remembered that most quality colour printers use at least six inks (CMYK + LC, LM). This means that the resolution of 1440 is divided by 6 = 240dpi.

This should be considered the minimum print resolution for a good quality fine art print. If your image is printed with a resolution below this, expect to see deterioration in print quality and the lower the print resolution the lower the quality will be.

Enlarging or Re-Sizing Image Files

However, in practical terms, an image file can usually be enlarged by 200% or sometimes 300% and still produce an acceptable quality print. Note that a lot depends upon the individual image as to how much enlargement may be achieved whilst still providing an acceptable result. Remember, there is no rule of thumb applicable; it just depends on the image.

Image files may be resized in several ways, using imaging software such as Photoshop or a Photoshop plug in, such as Genuine Fractals. However, we find that in terms of image appearance, better results are frequently achieved by letting our Onyx Printer RIP (Raster Image Processor) do the job of resizing. The output results from a modern RIP using complex interpolation algorithms is far superior to Photoshop's cruder image interpolation.

It should be remembered that any image editing in Photoshop is destructive and should be kept to a minimum i.e. once any edit has been made and saved, it cannot be undone.

Compression Artifact

A compression artifact is a noticeable distortion of an image or loss of image quality due to lossy compression techniques. Compression is a way to reduce file size by sacrificing image quality. Compression artifacts occur in image file formats such as JPG. The visible signs of excessive JPG compression are artifacts, which include areas of similar colour that become "blocky" and sharp contrasting edges in the image will have "vague smudges" surrounding the sharp edges.

Original Image heavy JPG compression
Original Image
Loss of edge clarity and fuzziness
due to heavy JPG compression

Sharpness, Sharpening, Resolution and Acutance

The sharpness of an image is measured as resolving power in lines per millimetre - lpmm. Some good quality lenses might resolve (at optimum aperture) greater than 100 lpmm and some fine grain B/W film, if processed with a high acutance developer, might achieve a resolution of greater than 150 lpmm.

Photographic papers might achieve resolution of 75-100 lpmm. B/W photographic emulsions always appear to render sharper results as colour emulsions have three layers (tri pack) one on top of another. The human eye however can only resolve a maximum of around 10 lpmm. So, if a printed image (by photographic, ink jet or other CMYK process) can achieve a resolution of at least 10 lpmm, then it should look sharp. In practice a resolution of 25-30 lpmm is required due to the effect of acutance with low contrast images.

Acutance is all about the contrast between the edges of pixels of differing density. Digital capture, whether by scanning or digital photography has the effect of softening image acutance. To negate this unwanted side effect, sharpening or unsharp mask is applied to a digital image file to increase the edge contrast between pixels of different density and thus increase the apparent overall sharpness of the image. It follows that a high contrast image will frequently require less "sharpening" than a low contrast image. It is worth remembering that once unsharp mask has been applied in Photoshop, for example, and the image file subsequently saved, that action cannot be undone. Over sharpening is a common error in image post processing, as the requirements for different out put applications vary. So beware, once size does not fit all!

The apparent sharpness of an image, whether generated by a digital camera or a scan from a film/print original, depends on a variety of factors. These include, amongst others, the quality of the lens (camera or scanner), whether the subject was moving or the camera was moving (camera shake) or both! Also, the native resolving power of the digital chip or film used and the accuracy of the alignment of the film plane or digital chip. It also follows that an image captured by a cheap consumer camera is unlikely to be capable of enlargement to the same degree when compared to the same image captured by a high-end professional camera. Thus the answer to the often asked question "How large can this file be printed?" will depend on many factors, not just the file size or resolution.

We are sometimes asked about printer resolution such as "why only 1440 dpi, why not 2880 dpi - surely that is twice as good?" Well, a printer with a resolution of 1440 dpi printing an image file with a native resolution (not resized by interpolation) of between 300-400 ppi (or dpi if you prefer) should be achieving a print resolution of about 8 lpmm. As this is approaching the limit of the resolving power of the human eye, it can be seen that a printer resolution of 2880 dpi is not going to make a discernable difference. Merely put twice the amount of ink down and thus increase the print cost.

 

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