how to order scans & understanding 'resolution'.
Ordering scans can be confusing because Scan Resolution is often misunderstood. Those not knowledgeable about scanning often ask for High Resolution scans.
The terms "high" and "low" resolution have no meaning without knowing how a scan will be used. Resolution has to relate to something before it can be judged as high, low, or just right.
Simply asking for a 'high' resolution scan is like asking an architect to design a 'large' home without knowing the size of the property on which it will be built.
Every use of a scan requires a certain amount of resolution (or data) for optimum results. Uses might include photographic printing, posting to the web, emailing, projecting, inkjet printing, commercial offset printing, etc.
Too little data results in quality that is less than optimum. Too much data does nothing to improve quality; data in excess of what's required for a particular type of output is tossed out.
There is only one type of resolution, neither high or low; it's the "correct" resolution for a particular use.
It is the same way with reproductions made from traditional film. Each film size provides a certain amount of 'data'. The larger the film, the greater the enlargement that can be made. 35mm film is suitable for enlargements up to a certain size, but to go larger -and maintain quality- more data is needed, in the form of larger film.
When you know how a scan will be used, it's easy to determine the correct size for the job. However, since scans are often used for a variety of purposes, it can be difficult to predict the best scan size in advance. When unsure, it's best to base scan size on the largest conceivable output for a particular image. Files that are larger than needed can be successfully downsized or "re-purposed", but it doesn't work the other way; small files that are upsized lose quality.
We often refer to uncompressed file size (like 22MB) rather than resolution (like 2400x3600 pixels) when discussing scans (compressed file formats like .jpg must be opened and uncompressed in order to determine resolution). File size indicates how much data a file contains regardless of how it's expressed as resolution. Here's why...
Resolution is expressed as a certain number of pixels in a given space, say a linear inch. Imagine 25 dots marked along one inch of an empty balloon. This equals a resolution of 25 dots per inch.
Now add air. As the balloon increases in size (an enlargement), the dots stretch further apart, lowering the resolution. If we increase the balloon five times in size, the resolution is 1/5th of what we started with, or 5 dots per inch. But the amount of data, 25 dots, is unchanged.
The same is true of file size. Regardless of how we express its resolution (balloon full or balloon empty), the amount of data it contains is fixed. Some people refer to pixel dimensions rather than file size when discussing image resolution, such as 1200x1800 pixels. This information is valid and accurate, just not as readily available as file size (which is available from the desktop without opening the file).
A 6MB file might be expressed as 4x6" at 300ppi, or 8x12" at 150ppi, or 2x3" at 600ppi (depending on how 'full' the balloon is). No matter how you express it, it contains 6MB of data. Resolution is a moving target, file size isn't.
We drum scan film at 4000 to 8000 pixels per inch. Some might say that's pretty 'high' resolution. But that amount of data could be 'high' for one use, and 'low' for another. Without relating to some form of output, it's neither.
If a 35mm slide is scanned at 4000ppi, and the resulting file is enlarged 200%, the resolution drops 1/2 to 2000ppi. Enlarge it 400% and its resolution is 1/4 or 1000ppi. Enlarge it 20 times and the resolution is 1/20 or 200ppi. Therefore, a 4000ppi slide scan might be high resolution for one enlargement and low for another.
Since the amount of data in a given file is the same no matter how you calculate and express its resolution, when you place a scan order, you will be asked how large a file is needed. If you don't know, we'll calculate that for you by asking how the scan will be used.
If we make a 12"x18" photographic print, which we image at 300ppi (ppi=pixels per inch), the optimum file size is calculated using the following formula. Substitute your own numbers for width/length/ppi to calculate the ideal scan size* for your use.
(LxWx300x300)/349,525=File Size in Megabytes
When people ask for 'high resolution' scans, what they are really asking for is the best possible quality for the use they have in mind. But bigger is not better when it comes to scans... "Correct" is not only best, it's the most economical, and that's what we'll give you when you order.... 'high resolution'.... scans.
(*) In 8-bit/RGB (the topic of a future article)