Finding a scanner's "sweet spot"

[G. Couch]

Most consumer grade scanners have what is known as a "sweet spot"- an area where the scans will be at their sharpest, and with the least amount of noise. For most scanners this a narrow area running down the middle of the bed, but it can vary greatly from scanner to scanner. Higher end scanners will generally have much larger sweet spots.

A simple way to find your scanner's sweet spot can be found here

Basically, you place a sheet of paper in your scanner and scan the entire bed at 72-100 dpi. In Photoshop, use the Equalize command to exaggerate the differences in the scan and the resulting image will give you a good idea of where your scanner's sweet spot is!

Here is the result for my scanner, an Epson 2450...pretty much the entire upper half of the bed is the sweet spot.

[Ed_L]

Aha! A very useable post. Thanks Greg.

Ed

[DJ Dubovsky]

Very handy info Greg.
DJ

[DannyRaphael]

Clean your scanner!

Greg...

I just tried this out and was dismayed at how "unsweet" my scanner is... Then I had a glimmer of hope.

I actually cleaned the glass and tried again. To my relief (and edification), now I've got a sweet spot too! This will change how I scan from now on. Excellent tip.

Danny

[jeaniesa]

Thanks Greg. I'll have to add this to my list of "installation steps" when I get my new scanner!
Jeanie

[Ed_L]

Just FYI, I scanned a piece of cheap inkjet paper. The scan looked absolutely terrible after using equalize on it. Another scan was done with matte heavyweight paper. This scan was much better. But I did notice that if I took the paper out of the scanner, then inserted it for a re-scan, it looked different each time I did that. Is it possible that very small dust particles could be moved around each time I did that? The glass was cleaned before scanning the first time.

Ed

[Steve Taylor]

I'm curious as to why there is a need to put a piece of paper in there at all.

[G. Couch]

Ed - Do not worry too much about dust. You are basically looking for large areas of dark and light. I get slightly different results with each scan but the lower part of the bed is always darkest as is the extreme left hand side. Try a few different scans and make note of the areas that always remain lighter...this will correspond to the sweet spot.


Steve - I think the only reason for the paper is because of the glossy surface of most scanner lids. I tried mine with just the lid down with no paper, and the reflected light caused a great deal of distortion.

[PixelMover]

Ok..I scanned the paper (full scanner bed size) @ 78dpi, tried every filter, gimmick, tweak and mode I could find in PS (including equalize, of course), but all I came up with was a completely evenly colored image, ranging from pale grey to bright green, but no dark/light differences.. Did I miss something?

[G. Couch]

Maybe you have a very high end scanner?

[jeaniesa]

Hmmm - given that you just posted a question about Linux drivers for a Scitex, I think that perhaps you DO have a high-end scanner!
Jeanie

[Doug Nelson]

Since the flatbed scanner sensors that read the top of a page are the exact same sensors that read the bottom, I wonder why scanners have a 'sweet spot'?

[Blacknight]

lack of equal quality throughout in the glass??

[G. Couch]

Originally posted by Doug Nelson
Since the flatbed scanner sensors that read the top of a page are the exact same sensors that read the bottom, I wonder why scanners have a 'sweet spot'?

Good question! I have heard several people talk about scanners having a "sweet spot", but no one ever really explained why. None of the seb sites I found mentioned much more than "scanner inconsistencies". I guess lower end CCDs tend to degrade a bit toward the edges? I'll see if I can track down some more info.

[BigAl]

Since the flatbed scanner sensors that read the top of a page are the exact same sensors that read the bottom, I wonder why scanners have a 'sweet spot'?

Light intensity decays as a 1/r^2 function (where r is the distance between the source and the detector), so it is easy to see that any irregularities or uneveness in the track that the source/detectors follow can have a marked effect on the amount of light reaching the detectors.

This can be clearly seen in the dark backgrounds of some of the pics in the "Scanning Objects" thread.