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Robert D Feinman

How much resolution is really needed?



The availability of very high resolution scanners opens up the issue of just what settings should be used for normal sized prints.
The image we will be working with is a shot of a test chart taken so that the image on the film reads out in actual lines per mm. The small image shows the size of the segment from the original negative we will be working with.

Original Image
As Scanned film sample


The picture was shot using a tripod, electronic flash, and the focus set using the scale on the lens. The film was 400 speed color negative. This image show the results of scanning at 5400 dpi on the Minolta scanner.
Not surprisingly the best detail that can be resolved is slightly less than 50 lpmm.
I would say that the film is probably the largest factor in limiting the detail. This is, however, supposed to be a practical experiment, reflecting the type of images commonly made.
So 400 speed film is quite appropriate, being highly popular.
There seems to be no sign of color fringing so the lens and the scanner both seem to be well corrected for aberrations.
100 percent
As Scanned at 100%

The question we wish to answer is what are the best settings for producing prints at several different sizes. If we displayed the above image from the whole negative and printed it at 300 dpi the print would measure about 18 by 27 inches. This is probably much larger than we usually desire. Our segment will print out at about 5 by 7 inches. Here is the rough approximation.

Overview
Overview

Various suggestions have been given as to what the best approach to down sizing should be. We will try several.

This is scanned at 5400 using Vuescan and resized at output to 1350 dpi by setting size reduction to 4.

resized in vuescan
Resized at output
Here is the same scanned in Vuescan with a setting of 1350 dpi. The final file is exactly the same size as the prior image.
Scanned at 1350
Scanned at 1350 dpi
This is the difference between the prior two images. Notice that there is some difference between the edges on the larger bars, but very little in the finest ones.
differences
Differences between prior images

This one is scanned at 5400 dpi and then resized using bicubic resampling in Photoshop.

resampled_bicubic
  Downsized bicubic to 1350 dpi

This one starts with the same scan and is resized using the bicubic sharper resampling choice available in Photoshop CS.


resample bicubic sharper
Downsized bicubic sharper to 1350 dpi
Here is the difference between the two bicubic images. Again the larger features seem edge sharpened. But there is not much change to the finest details.
bicubic difference
Bicubic differences
Here is the difference between scanning at 1350 and downsizing using the bicubic resampling technique. The edge effects are more obvious.
bicubic vs 1350 scan
Bicubic vs scanning at 1350 dpi
Here are the variations seen side by side. This has been saved with minimum compression and each screen pixel represents an image pixel. Thus viewing online should not be too much of a compromise.
Notice that not much beyond 20 lpmm can be seen no matter which method is used.
To my eyes the bicubic sharper seems "sharper", but only in the larger bars. So is the issue detail resolution or "sharpness"?
A slightly different resizing algorithm in the scanner software, or using the Minolta supplied software might make the differences even smaller.
Other things that could be tried are various sharpening steps when editing. These, however, could be applied to any of the variations.
side by side
Side by side
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© 2004 Robert D Feinman