What settings are needed to achieve maximum resolution from an Epson V850?

[williaty]

So, speaking of resolution and the V850, I'm getting some results I'm not sure how to explain. Most of the online reviews estimate the true optical resolution as somewhere between 2000 and 2400 dpi. While I was hunting for best focus, I played around with some settings and turned up results that don't perfectly line up with that. Specifically, I get significantly better real resolution scanning at 6400dpi (software setting) and downsampling to 3200dpi in PS (Bicubic downsampling) than I do scanning directly at 3200dpi (software setting). The test image I've been using has an air conditioner in one window and the cooling fins on the back of it make great test bars. They're are much better resolved when scanning at the higher setting. Film grain is similarly better resolved at the higher setting.

Is this simply a case of having to ask the software for 6400dpi to get the most possible optical resolution even if a proper test chart would show the true optical resolution is down in the 2000-2400dpi range?

[RHITMrB]

It may also be a product of the fact that bicubic resizing (Photoshop's default) also sharpens an image.

[williaty]

I know Bicubic Sharper does, but I didn't think Bicubic did. That being said, the differences don't look like increases in actuance (from sharpening) in that actual separation is improving, not just contrast between them.

[Oren Grad]

I've moved this to a thread of its own, since it's a different topic from the "step up" discussion.

[Peter De Smidt]

Note that a flatbed scanner, unless it stitched ala some Creos/Eversmarts, will have fixed resolution in the direction of the sensor. For instance my scanner has 8000 light sensitive areas in one direction. Those 8000 remain the same when scanning 1 inch and when scanning 8 inches. For my scanner, that means that if I do one pass scanning of an 8x10 negative, that resolution is limited to 1000 (i.e.8000/8) in one direction. This also applies to an Epson. It may achieve 2-2400 dpi over, say, 1", but it'll be lower for bigger scans. The only way with these scanners to increase the resolution for big film is to scan in strips.

[williaty]

Note that a flatbed scanner, unless it stitched ala some Creos/Eversmarts, will have fixed resolution in the direction of the sensor. For instance my scanner has 8000 light sensitive areas in one direction. Those 8000 remain the same when scanning 1 inch and when scanning 8 inches. For my scanner, that means that if I do one pass scanning of an 8x10 negative, that resolution is limited to 1000 (i.e.8000/8) in one direction. This also applies to an Epson. It may achieve 2-2400 dpi over, say, 1", but it'll be lower for bigger scans. The only way with these scanners to increase the resolution for big film is to scan in strips.

Can you provide a reference to back this up? That's contrary to all the other explanations I've ever heard. Everything else says it has a fixed linear resolution along the length of the sensor and your resultant scan has a width of however many pixels your film covers. IOW, if the sensor is a 6400dpi linear, then you'd get 6400px across a 1" wide target, 12800px across a 2" wide target, etc.

[Peter De Smidt]

How would it do that without making more than one pass? If the sensor has 6400 light sensing elements, then that's the maximum amount of points it can read from at one time. The stepping motor/linear drive system might be able to make finer steps in the other direction, but that's it. For a long time, the best sensor used in pro flatbed scanners was made by Kodak. It was a 3-line CCD array (red, green, and blue) with 8000 elements/line. If the entire sensor is being used to give maximum resolution over, say, an inch, i.e. all light sensing elements are being used over that inch, how could the scanner give additional info over the next inch? There'd either have to be another sensor, which there's not, or more than one pass would have to be made.

There are different kinds of flatbed scanners. One has a fixed resolution over the whole bed. This is fairly low, as the sensor reads the whole bed each time. Since the sensor has a given amount of light sensitive elements, this leads to a low obtainable resolution, since if you don't use the whole bed, you wasted sensor sites on blank space. Next up, there are some scanners that can switch lenses, such as the Agfa T2500. It gave one max resolution setting for the whole bed, but it could switch lenses and read a higher resolution strip down the middle of the bed. Getting more complicated there's scanners like my Cezanne, which are xy zoom scanners. This means that the sensor can be moved front to back on the scanner, and that the lens is a zoom lens, which can use all of the sensors elements for any size, with 12" wide or 1 " wide, no matter where the film is placed on the scanning bed. No elements are wasted. Obviously, this is a more complicated way to build a scanner than the earlier mentioned ways, and these scanner were very expensive. Finally, Creo/Scitex/Kodak made some scanners that were xy zoom scanners, but they could also scan more than one strip and combine them. They had a patent on that, and so other scanner manufactures didn't follow suit.

The resolution for the V850 is 4800 x 9600 dpi, according to http://www.epson.com/cgi-bin/Store/j...sku=B11B224201 . That implies that the sensor has 4800 sensing elements per line. To do 4800 over 8 inches would require a sensor with 38,400 sensing elements-per-line. I have not heard of such a line sensor being produced.

[williaty]

How would it do that without making more than one pass?

If it were a linear array with 6400 photosites per inch over the entire width of the scannable area, it would behave like this. Epson says the max width the high-res camera can scan is 5.9". A linear array 37,760 photosites long (5.9" times 6400dpi) would sample the whole width of the scannable area at once and then be stepped down the scan one row at a time. Interestingly, Epson spec sheet says the maximum resolution for a single-shot scan is 37,760px across the short axis, implying that the sensor physically has that many photosites. However, it also mentions "Alternative 6 Lines Color Epson MatrixCCD", so it may actually have 6 lines of 37,760 photosites each, for a total photosite count of 226,560 photosites. A sensor with 37,760 photosites, or even a 6-line one with 226,560, is tiny by today's standards and wouldn't be any big trick to produce with high yield efficiency. This would produce behavior consistent with everything I've ever read about scanners and in conflict with what you said.

Again, can you site specific authoritative sources that show that scanners work the way you say they do? I would be very surprised to find out it's true and it would certainly change they way I understand them, which is why it's important to me to find out if you're right.

If the sensor has 6400 light sensing elements, then that's the maximum amount of points it can read from at one time.

Why would the sensor have only 6400 photosites?

[RHITMrB]

What makes you assume the sensor is exactly 1" long or scans exactly 1" at a time?

[williaty]

What makes you assume the sensor is exactly 1" long or scans exactly 1" at a time?

Yeah that's exactly my question to him as well. Why does he think the sensor is 1" long? Everything I've ever read about scanners says that the camera samples the full width of the scannable area at the stated resolution in one shot, even if there's a lens involved that "condenses" the image so that the physical sensor occupies a smaller space at a higher pixel pitch. What Peter is suggesting would require the scanner to have a zoom lens that could change how much width it "sees" as you change the scan area in software and that would cost a LOT more than just a simple CCD that was the size it needed to be to do the full width in one shot.