James Photography Scanner Bake-Off 2004 Results: What do the Numbers Mean? MTF is "Modulation Transfer Function". It's synonymous to SFR, or "Spatial Frequency Response". These are measurable qualities that directly relate to image sharpness. So, the higher the number, the sharper the image. That simple. It is measured in this test with software that looks at how quickly the image transitions from one colour to another (white to black). Also referred to as edge sharpness. The actual number is "line pairs-per-millimetre", and is calculated by multiplying the MTF50 cycles-per-pixel by the DPI of the scan, and dividing by 25.4. (Thanks Don). This provides an absolute equal footing from which to compare scanners. CA is short for chromatic aberration, which measures the scanner's lens quality in terms of how it keeps the colours together and not spread them out like a prism. The lower the number, the better. There's a direct relationship between the DPI you choose to scan at, and this number. For instance, a 2400 DPI scan produces a CA number of 2.19 in one of the results. The CA pretty well doubles to 4.36 for the 4800 DPI scan. The CCD is faithfully capturing the lens' output. Clipping contributes to the apparent sharpness of the image, but it skews the MTF50 rating as well - so it's cheating! Clipping is caused by overexposure or underexposure, creating too many pixel levels at (or very close to) 0 or 255. It falsely adds to the measured image quality-- sharpens edges and reduces noise. So although it has swayed these results somewhat, look at the clipping number in the MTF figure if one image appears sharper than another, but with a lower MTF. Anything over 5% is not good... As a matter of fact, I've asked anyone with a value over 5% to resubmit, so stay tuned... The signal-to-noise ratio gets better as the MTF worsens. This is caused by the softening (or blurring) of the image, which makes the s/n look better as a result. Kind of like turning the treble down on a stereo, which reduces hiss... which effectively hides the noise. This analogy is even mathematically correct according to the analysing software's author. 10-90% Rise is an important metric in that it measures how many pixels it takes to go from dark to light. A sharp edge will do it in 2 or 3 pixels; a fuzzy edge in 4 or more. Oversharpening is measured and noted in the MTF chart. If you see a slight rise in the graph before it falls, you'll see a slightly higher oversharpening number. This affects MTF as well, so please interpret the results with this in mind. For more details on what all the other numbers mean, go to the software author's web page: www.imatest.com. Lots of good info there. Standard Deviation and other analysis Thanks to Guy Tal for compiling some interesting stats based on the results entered up to June 7. Download the Microsoft Excel spreadsheet here. Typical Disclaimer Stuff There are no alliances or special considerations when it comes to brands, as this scanner bake-off is not an endorsement of any product or manufacturer. Any and all participation is voluntary. All results are impartial and fair, so if anyone takes offence, or even wants to litigate, they can bite me.