There are ways to quantify a scanner's quality...


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Posted by diverd on June 14, 2005 at 22:36:28:

In Reply to: Multiple guess test posted by Elaine on June 14, 2005 at 13:58:23:

Wow, this is a very interesting problem. You may have some settings differences because your histograms are off the richter on the left hand side, but your DMax (Density Maximum/Total Ink Density) is higher on the right hand scan. But I'll play along.

What to look for in a scanner is it's abillity to gather information. The most obvious ways to quantify a scanner are to measure it's Density Range and it's Bit-Depth. But between the two, I would pick an 8-bit with an exceptional DMax over a 16-Bit with an adequate/average Bit-Depth. Of course you'd want some PPI (DPI, SPI) resolution too, but in this day-and-age of megapixel savy digital camera users, we'll assume that's a given.

In practice a scanner can have a Total Density Range of no more than 400%. In therory it can go higher but cannot be reproduced, (kind of like 16 bit color). A serious drum scanner tops out at about 370 to 390 (3.7 to 3.9) or so. A exceptional prosumer desktop model can be around 3.2 to 3.4. I'm not sure what the slide scanners can get up to, I've never used or researched one. The right hand scan has dMax approx. 10% greater than the scan on the right. So your first instinct would be to assume that that one on the right is from the best scanner. But in this case, this number only helps if the two images are scanned in a way to achieve the most identical results possible. An operator unsure of their settings may not be able to achieve the desired results to make a proper test. So on first inspection the one on the right wins for Density, maybe, depending on the operator. However the histogram shows a whallop of more information in the left hand scan. With this much more info, I'm suprised to see a 10 percent greater DMax in the right hand scan, hmm... settings, maybe? Maybe not.

What you would like in a scanner most, would be the highest Total Desnity Range and the highest Bit-Depth. Both of these numbers can be had by reading the spec's of the device in question. Examining the quality of the individual channels would be your secondary, but still important, consideration of the top DMax and Bit-Depth models. In our case this is a bit tougher to examine because our images have been JPG'd a bit and downsampled alot. But there is enough info to make some guesses.

Okay, lets inspect the channels. Of the two red channels, the one on the left shows a huge amount of contrast, it's almost pure black and white, the right hand shows lots of contrast but more detail than the left hand one -- but the red channel is the Contrast Channel, maybe more contrast is good, the left hand scan's got that. The green channel in the left hand scan shows lots of detail and contrast, the right hand scan also shows a good amount of detail and contrast but is mushy in detail and not as punchy or detailed in contrast as the one on the left. The left hand green channel is sharper. Well, the green channel is the Tonality Channel, maybe the clearer contrast and detail will give the left hand scan a richer tonality over the right hand scan. Now the blue channel, the Noise Channel. The left hand scan exibits less noise and greater detail over the right hand scan. Now this has just got to help the left hand scan over the right hand scan. So now I'll look at the composite images again. Now I can see what they all add up to.

Overall the detail on the right hand scan is mushier (yes, that is a technical term :) and dirtier, the dirtiness of the color is measured with the eyedropper tool. But the color on the right has a better black and white balance. I'm attributing this to the operator's settings, not the ability of the scanner to percieve and translate color. If I had to pick a scanner from these two images I'd be tempted to pick the left hand scanner.


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