Cropping and Print Resolution

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Cropping and Print Resolution

What is important here is how much cropping can be performed without seriously limiting print quality. This is not a problem that is new to digital photography, and was an issue with film photography. It is however much easier to work out for digital photography.

Ideally we would like the maximum resolution possible for our prints, but how much do we really need. The following definition is adopted from Norman Koren’s excellent web site.

Print PPI	Perceived print quality
300	Outstanding. As sharp as most printers can print; about as sharp as the eye can see at normal viewing distances.
200	Excellent. Close to 300 PPI for small prints, 8½x11 (or A4) and smaller. Outstanding quality in large prints, 11x17" (or A3) and larger, which tend to be viewed from greater distances.
150	OK for large prints. Adequate, but not optimum, for small prints.
100	Adequate, but not optimum, for large prints. Mediocre for small prints

So it seems there is little point in having more than 300 PPI of output resolution and 200 PPI should give very good results even with large prints.

The below tabulates the amount of sensor cropping in linear percent of full sensor image, that will preserve various levels of detail in various sized prints for the EOS 20D sensor. This is assuming that the image is well focused at that a good quality lens is used so that sensor resolution is the main limit of image sharpness.

Print PPI	Print Long Edge 6 inch (~A5)	Print Long Edge 10 inch (~A4)	Print Long Edge 16 inch (~A3)
300	52%	86%	~220 PPI @ 100%
200	35%	57%	92%
150	26%	43%	68%
100	17%	29%	46%

We can see that the 20D can only produce 220 PPI on A3 prints even for no crop and so very limited cropping will bring one to 200 PPI. If one is prepared to print a large print at 150 PPI then 68% cropping can be accepted.

If A4 prints are desired then much more aggressive cropping can be accepted, full quality being obtainable at 86% crop. Very good quality at 57%.

These are linear crop factors, so we can see that very acceptable A4 prints can be made with a lens that is almost half the focal length of one that would fill the frame.

This suggests that digital camera resolutions are now sufficient that a prime lens system based on focal length ratios of 1.4 to 2 are acceptable instead of zoom lenses, and some occasional cropping can be performed for images where it was not possible to move in closer without changing perspective.

Last Updated 05/06/2008

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