On occasion I read in messages here references to scanners' actual resolution. I'm wondering how you make these tests? I own an Epson 4990 and I've been curious about its actual max resolution, so I decided to see if I could figure out a way to test it myself.
To test the scanner, I thought of the 1951 USAF test chart, and after hours of searching the 'net for a suitable example, I finally located a pdf of it. Actually, the pdf is not an actual copy of the 1951 USAF chart, but it's pretty close. I found it here:
http://photo.net/learn/optics/USAF1951.pdf
After wasting a bunch more time trying to get the .prn translated to a file my image processing software could read without prompting me for dpi parameters, I finally just gave up and printed it out with my Epson Workforce 600. I also printed it out on my HP Laser Jet 1200. Both have 1200 dpi output, and as it turns out, the Epson does a slightly better job. But it wasn't nearly good enough. The printer output fell apart long before the scanner's resolution did. So now I know at least that my 4990's claimed 4800 dpi is way better than 1200 dpi.
It occurred to me then that I wouldn't necessarily have to be able to read the smallest area of the chart that was supposed to be there, but that I could make an accurate estimate based on the smallest feature I could see in the scanned page, and measure that against one of the groups in the chart.
Then a little light went off in my head, and I realized I didn't need the chart at all. The image processing software I use -- Paint Shop Pro X2 -- has an image scale that can be toggled on, which gives a readout in pixels. I'm sure PS has this same ability. So, who needs any sort of test chart, I ask? So, I loaded a 4800 dpi scan of a fine-grained slide into PSPX2 and had a look.
But then things become very subjective to interpretation, I'm finding. I'm looking at this scan, which is of a Kodachrome 64 slide, and with only a bit of contrast and color adjustment done in the way of post processing. In the scan, the 4990 is resolving grain particles, or textures in the pigment, whatever you want to call them, that are 3 to 5 pixels in size. So, resolving an object of 3 pixels in a 4800 dpi scan would tend to make the argument that the effective resolving power is about 1600 dpi. But if this is grain, then, it would seem that the scanner can do even a bit better. And it's pretty clear that it does much better, which is obvious by looking at 100% scans of the 1200 dpi USAF test target, which is only 400 dpi away from the test target scans. As it turns out, at much higher levels of magnification, there are other objects in the image in which a continuum of coloration is present, and the coloration changes from pixel to pixel in an even, non-random manner. This strongly argues for a full 4800 dpi to me. What I seem to be encountering is a few things: resolution limits in the film caused by grain, resolution limits of the image caused by the sharpness of the camera lens, sharpness limits caused by the function of the photographic gear (taking into mind camera shake and the like), and image characteristics that are not as dependent upon resolution limits as they are upon image density and color rendering accuracy, and finally somewhere in all this, resolution limits of the scanner itself. But perhaps even more important from a practical standpoint than resolution limits is resolution detail. It just seems to me my 4990 should be able to render sharper images than it does.
I also have scans of this same slide done by an HP S20 (2400 dpi dedicated film scanner) and an Opteka slide duplicator used with my 10.1mp DSLR (effective 2400 dpi resolution). Both of these images compare favorably to the 4990 scans until one begins to enlarge them. The two lower resolution images quickly begin to pixelate before image detail is exhausted. With the 4990 scan, it's just the reverse. Image detail tends to become exhausted before pixelation occurs.
These are at very high magnifications, though. Way above 100%. The reason why I have scans of this same image done by all three devices is for a comparison I did. At 100% image size, a slight nod can be given to the Opteka/DSLR combo in terms of sharpness, while the HP S20 and the Epson 4990 are practically dead even when looking at the S20's scan at 100% and the Epson's at 50% (because it's 4800 dpi vs. the S20's 2400 dpi). You can read it here:
Dead Link Removed
So, in a way, there's not much difference to be offered between 2400 dpi scanned output and the 4990's 4800 dpi output. But to me that's only a part of the picture. Sure there isn't much difference in many instances. But when one wishes to enlarge their image to a larger size than the others can support, then a usable 4800 dpi shines. And the 4990's is usable, IMO. I do wish it was a bit sharper though.
So, while I still can't point at a hard number with much confidence, and say, "Well, my scanner's effective resolution is yadee yadee dpi," I can at least say, hey, it's actually better than I expected.
To test the scanner, I thought of the 1951 USAF test chart, and after hours of searching the 'net for a suitable example, I finally located a pdf of it. Actually, the pdf is not an actual copy of the 1951 USAF chart, but it's pretty close. I found it here:
http://photo.net/learn/optics/USAF1951.pdf
After wasting a bunch more time trying to get the .prn translated to a file my image processing software could read without prompting me for dpi parameters, I finally just gave up and printed it out with my Epson Workforce 600. I also printed it out on my HP Laser Jet 1200. Both have 1200 dpi output, and as it turns out, the Epson does a slightly better job. But it wasn't nearly good enough. The printer output fell apart long before the scanner's resolution did. So now I know at least that my 4990's claimed 4800 dpi is way better than 1200 dpi.
It occurred to me then that I wouldn't necessarily have to be able to read the smallest area of the chart that was supposed to be there, but that I could make an accurate estimate based on the smallest feature I could see in the scanned page, and measure that against one of the groups in the chart.
Then a little light went off in my head, and I realized I didn't need the chart at all. The image processing software I use -- Paint Shop Pro X2 -- has an image scale that can be toggled on, which gives a readout in pixels. I'm sure PS has this same ability. So, who needs any sort of test chart, I ask? So, I loaded a 4800 dpi scan of a fine-grained slide into PSPX2 and had a look.
But then things become very subjective to interpretation, I'm finding. I'm looking at this scan, which is of a Kodachrome 64 slide, and with only a bit of contrast and color adjustment done in the way of post processing. In the scan, the 4990 is resolving grain particles, or textures in the pigment, whatever you want to call them, that are 3 to 5 pixels in size. So, resolving an object of 3 pixels in a 4800 dpi scan would tend to make the argument that the effective resolving power is about 1600 dpi. But if this is grain, then, it would seem that the scanner can do even a bit better. And it's pretty clear that it does much better, which is obvious by looking at 100% scans of the 1200 dpi USAF test target, which is only 400 dpi away from the test target scans. As it turns out, at much higher levels of magnification, there are other objects in the image in which a continuum of coloration is present, and the coloration changes from pixel to pixel in an even, non-random manner. This strongly argues for a full 4800 dpi to me. What I seem to be encountering is a few things: resolution limits in the film caused by grain, resolution limits of the image caused by the sharpness of the camera lens, sharpness limits caused by the function of the photographic gear (taking into mind camera shake and the like), and image characteristics that are not as dependent upon resolution limits as they are upon image density and color rendering accuracy, and finally somewhere in all this, resolution limits of the scanner itself. But perhaps even more important from a practical standpoint than resolution limits is resolution detail. It just seems to me my 4990 should be able to render sharper images than it does.
I also have scans of this same slide done by an HP S20 (2400 dpi dedicated film scanner) and an Opteka slide duplicator used with my 10.1mp DSLR (effective 2400 dpi resolution). Both of these images compare favorably to the 4990 scans until one begins to enlarge them. The two lower resolution images quickly begin to pixelate before image detail is exhausted. With the 4990 scan, it's just the reverse. Image detail tends to become exhausted before pixelation occurs.
These are at very high magnifications, though. Way above 100%. The reason why I have scans of this same image done by all three devices is for a comparison I did. At 100% image size, a slight nod can be given to the Opteka/DSLR combo in terms of sharpness, while the HP S20 and the Epson 4990 are practically dead even when looking at the S20's scan at 100% and the Epson's at 50% (because it's 4800 dpi vs. the S20's 2400 dpi). You can read it here:
Dead Link Removed
So, in a way, there's not much difference to be offered between 2400 dpi scanned output and the 4990's 4800 dpi output. But to me that's only a part of the picture. Sure there isn't much difference in many instances. But when one wishes to enlarge their image to a larger size than the others can support, then a usable 4800 dpi shines. And the 4990's is usable, IMO. I do wish it was a bit sharper though.
So, while I still can't point at a hard number with much confidence, and say, "Well, my scanner's effective resolution is yadee yadee dpi," I can at least say, hey, it's actually better than I expected.
