What should I look for in optical/hardware resolution when buying a scanner?

Question

I'm debating between these two scanners:

1. Canon CanoScan LiDE 110 Flatbed Colour Scanner 2400DPI 48/24BIT USB2.0
2. Epson Perfection V33 Flatbed Scanner USB2.0 9600X4800DPI

I've got some old family photos I want to scan and archive. I'd like high-quality scans.

I'm a bit confused about the differences between "Optical" and "Hardware" resolutions, and why they are giving X*Y DPIs; normally DPIs are given as a flat numbers. Does that mean they have a higher horizontal resolution than vertical resolution?

The interpolated/software DPI is meaningless if I'm not mistaken. I can blow it up in Photoshop if I really want to.

What should I buy based on these technical criteria?

Answer 1

The word scan-head simply means the horizontal bar that goes up-and-down along the scanner (for flatbed scanners). It is static for auto-feed scanners.

Optical resolution

This is often given as a single number say 600dpi and is related to the amount of detail the scan-head can capture horizontally -loosely speaking.

Technically, it is the number of sensors placed per inch (horizontally) on the scan-head. In this case, there are 600 sensors horizontally per inch on the scan-head.

Side note: If the maximum paper size of a scanner is legal, then there are (600 x 8.5 = 2550) sensors along the scan-head. some scanners using a single-pass might have 3 rows of sensors on the scan-head (this doesn't improve quality of scan, it only increases the speed).

Hardware resolution

This is often given as two numbers say 600 x 1200 dpi; the first is often the same as the optical resolution and the last is related to the quality of detail that can be captured vertically - again loosely speaking.

Technically, the last number is the number of vertical step movements of the scan-head per inch. In this case, this means the scan-head moves 1/1200 inches (vertically) per sampling - allowing for more detail to be captured vertically than one with a 1/600 step.

Maximum, Interpolated, etc resolutions

These are just software/hybrid means of increasing the resolution. They are not important because they simply add (gradient) pixels horizontally/vertically to the original scan (to give a smooth larger image); they don't add any further detail to the original scan

Verdict

Pick a scanner where the optical or hardware resolution is as high as possible.

I know that the optics: lens, type of light source and sensor technology etc play a role in scan quality, but these are out of this question's context. Oh, and don't forget to up-vote if you found this useful

Regards.

Answer 2

ScannerGalaxy has a guide for picking scanners.

The follwoing was taken directly from ScannerGalaxy.com.

Choosing the Right Scanner

When choosing a scanner, there are several key attributes that you should take into account. 1. Color Depth: The number of bits captured per pixel, which is related to the number of possible colors. Higher color depth equals better image quality. Typical color depths are 24, 32, 36, 42 and 48-bit. If you plan on doing document scans, then you may not even need color. However, for those that wish to scan photographs or projects requiring great detail, then look for a scanner with a color depth closer to 48-bit.

2. Optical Resolution: A measure of how well a scanner can capture an image. It is the actual number of pixels that the scanner provides when scanning an image. The higher the optical resolution, the higher the quality of the image captured. Measured in dots per inch (dpi). Not to be confused with interpolated resolution. 300 dpi will do well for most office applications. However, look for at least 600 dpi when photos are involved.

3. Interpolated resolution: The ability of the scanner software to "fill-in" spaces between scanned dots. This is really only a factor if you plan on enlarging images. The higher the interpolated resolution, the smoother your enlarged images will look. 9600 x 9600 dpi is a typical interpolated resolution offered today.

4. Flatbed vs. Sheetfed: Flatbed scanners offer a flat, glass surface. The image to be scanned is placed on the glass surface and the lid is shut. This is typically used for environments where very limited scanning takes place or for "irregular" objects (i.e. a book or hard cased object). Sheetfed scanners are useful for environments that require frequent volume scanning. They allow you to scan large projects automatically. Simply place the sheets that need to be scanned in the automatic document feeder. The sheetfed scanner will automatically process the sheets and scan them automatically. Scanners offer various sizes of automatic document feeders. Larger capacity feeders allow for larger jobs to be completed with fewer interventions from the user. Some scanners offer both flatbed and sheetfed capabilities. This allows for more flexibility in a single unit.

5. Simplex or Duplex: Simplex scanners have the ability to scan one side of a document. This is useful when a majority of your items that need to be scanned are single-sided. Duplex scanners allow for the scanning of both sides of a document in a single pass. A majority of duplex scanners have a feeder with a dual sided lens that reads both sides of a document as it passes through. Duplex scanners can be set to scan as simplex or duplex, depending on the job the user wishes to scan.

6. Scan Speed: Scan speed is typically rated in pages per minute (ppm) for simplex scanning and images per minute (ipm) for duplex scanning. Pages per minute (ppm) measures the amount of pages scanned in a given minute. Images per minute (ipm) measures the amount of actual images (front and back) scanned in a minute. The higher the scan speed, the more scans a user can do in a set amount of time. Look for higher scan speeds if you plan on doing frequent or high-volume scanning. Scan speed decreases as you increase the scan resolution.