Image quality explained

A benefit of digital imaging is that "quality" can be more easily expressed in quantifiable terms. The aim of this article is to try an show why image quality is such an issue for me.

Here is a (sadly) typical reference photograph

Imagine that you send me a photograph like this, and want me to make an A4 size drawing or painting (about 10 times the size of the original photo).

This image is a section of a print measuring 1.139 inches square, and has been scanned at 72 dpi (dots per inch).

The scanned image is physically 82 pixels square, and composed of 724 pixels in total (82 x 82 = 724). In other words, 724 pieces of information.

Dots per inch

This is how the right eye looks on my PC monitor when enlarged 10 times. You can see that the picture is made up of squares of colour (pixels), and it is impossible to see any details at this level of magnification. You wouldn't be very impressed with your portrait if I drew or painted what I can see here!

Here is the same image digitally enlarged, so it is physically 10 times bigger (820 pixels square). However, it still only contains the original 724 pieces of information. The increase in the physical size of the image has been achieved through "interpolation" (to insert or introduce between other elements or parts ). For example, if one pixel is black, and its neighbour is white, interpolation software will assume that a new pixel between the two will be mid-grey. The interpolated image is merely smoother, but it does not reveal or create details.

If the scan resolution is increased, the photo better withstands enlargement. In this image, the original scan was made at 300 dpi, and the right eye is seen at 10 times magnification. However, the level of detail is still pretty poor, and not adequate to work from. I could theoretically take the scan resolution much higher, but it would be highly unlikely to reveal any further detail. Most prints do no hold any more detail than can be captured at 300 dpi.

When working from a digital image, as opposed to a print, the exact same problem exists: the level of detail has been fixed by the camera at the time the shot was taken.

Contrast and saturation

Sometimes, additional details are recorded by the photograph, but are not immediately visible in the print or digital image. Careful adjustment of colours and contrast can reveal more information. In this unadjusted photo, the hair colour is solid black, with no apparent detail.

In this adjusted photo we can see more detail in the dark areas. Other adjustments are sometimes possible in the mid and high tones.

Colour casts

Certain types of light can cause film and digital cameras to have a colour cast. Sometimes these are obvious, but other times they are not. For example, in my demonstration photograph, I know the subject, and her true colouring. The green/blue colour cast is fairly obvious, and probably contributed to by the background colour.

In conclusion

Some times I get lucky: I can work with a poor photograph and squeeze-out enough detail to allow a reasonable drawing to be made. I never dismiss a photo without attempting to make it work.