Tutorial: Scanning Resolution

At Pixmonix, we can scan your slides and negatives at one of two resolutions - 2000PPI and 4000PPI. What does this mean?

Some background...

Digital images are composed of pixels. A pixel is one small point of color. When thousands of these very small points of color are put close together, an image results. The image on the screen you are looking at is composed of pixels. If your monitor is about average in size, there are about 800,000 pixels on the screen in front of you. Each pixel is able to show one of more than 16 million colors. When many pixels are placed close enough together and they can convey a wide enough range of colors our eye can no longer perceive the individual dots. Rather, we see a continuous picture.

Resolution

The resolution of a scanned image refers to the number of pixels in the resulting digital image for each square inch of the original material. The scanning resolution and the size of the original being scanned determine the dimensions of the resulting digital image, measured in pixels. Scanning resolution is commonly measured in pixels-per-inch, or PPI.

For example, if you have a 1 inch square original image (slightly smaller than a 35mm slide or negative) scanned at 4000 PPI, the result is a 4000 pixel by 4000 pixel image (or 16 megapixels). This is similar to the number of pixels in images shot with high-end digital single-lens reflex (SLR) cameras available today.

Not all scanners are created equal. Some scan at higher resolutions than others. However, this is not the only factor to consider when deciding which scanning service or which scanner to use. Scan quality is not the same across different scanners even if resolutions match. Steps taken during and after scanning can have a significant effect on the quality of the final digital images.

In general, you want to scan your slides or negatives at the highest resolution possible, as long as you understand a number of important issues up-front.

• Not all original images need the highest resolution. If your slide or negative film is fast (e.g., 400 ISO), of questionable quality (e.g., dirty or scratched), shot on less than top-notch equipment or with less than top-notch technique, you are unlikely to see significant improvement in scan quality by going to very high resolutions. What you can get from the higher resolution scan is all the detail in your film, down to the film grain. This resolution may allow you to create slightly larger prints, but it is no panacea.

• Digital image file sizes grow geometrically with resolution. Are you prepared to deal with 50MB TIFF image files? These beasts can take a good deal of time to load and process on average PCs. If you have a lot of images, you need lots of storage space and you have to be concerned with backing up and maintaining a large database of images; this can be a challenge. These issues clearly diminish in importance as computers get faster and faster, memories get larger and larger and disk drive capacities seem to grow without bound. Large file sizes should not automatically turn you toward lower resolution images, but you have to be prepared for what you are getting.

• Scan resolution is not the only quality factor. We use scanners that we provide consistent, excellent quality at their highest resolutions of 4000 PPI. However, scan resolution alone is not sufficient to assure a high quality digital image. We employ a variety of technologies in our scanners and processing that aid in producing high quality scans. We have rigorous procedures that assure that your scans are as good as they can be. We discuss these features of our services in detail on our website. We believe that the scanning methodology we employ ensures you the highest quality at a reasonable price.

The table below details the print sizes and storage requirements for various scanning resolutions.

For many of our customers, the most important figure in this table is the print size possibilities. At 2000 PPI, a scan from a slide can be printed reasonably large (up to approximately 6" x 9"). Going to 4000 PPI allows for larger print sizes (assuming that the source slide or negative is of high enough quality to realize a benefit from scanning at the higher resolution).

At what resolution should I scan my slides/negatives?

We are asked this question quite regularly. The answer is "it depends". For many people who don't plan on making large prints of their images or doing heaving editing or cropping of the digital images, scanning at 2000 PPI is sufficient. Additionally, much of the film that we see at Pixmonix was not shot on the best equipment or stored in ideal conditions. However, we do have many customers who have excellent photos shot on good film with good equipment and technique that benefit from higher resolution scans. On the other hand, scanning at the higher resolution doesn't cost much more and most computers can manage the larger files. The choice is yours. We are always happy to help you determine the right scanning resolution for your project. We are waiting to hear from you!

The examples below show the differences in the scans for a variety of scanning resolutions and processing options. We will add additional examples soon to show slide processing (including Kodachrome) and B&W negative scanning.

In each case, you may click on the small image to see a full-sized JPEG version of the images (saved with very little compression). This will allow you to download the example scanned images and evaluate the effect of the changes in scanning parameters yourself. For example, you may want to try printing one or more of the images. Note that the full-sized images are large - from approximately 2MB to 8MB and will take considerable time to download if you are using a dial-up connection to the Internet.

Example #1 Kodak Gold 400-6 negative film from 1999.
2000 PPI	4000 PPI
(Click each small image for a full sized JPEG image)
Raw scan - no ICE or color correction (2.5MB)	Raw scan - no ICE or color correction (8.1MB)
The raw scan shows substantial scratches and dust/dirt (very evident in the full-sized images). The scanned images also have a very noticeable blue cast, obvious grain noise (examine the full-sized images at 200% magnification) and low contrast.
With ICE (2.4MB)	With ICE (8.0MB)
The images scanned with ICE show the very powerful effect of this technology; the scratches and dust/dirt are gone with little or no loss of detail. The blue cast and grain noise remain.
Final Processed Image showing our full suite of corrections. (2.26MB)	Final Processed Image showing our full suite of corrections. (7.95MB)
These scans show the final results of the processing we perform on every image we scan. The dust and scratches are removed, the grain noise is greatly reduced, color casting is removed, contrast is improved. The images pop!