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Thread: Scanner DPI Options

  1. #1
    Bart B's Avatar
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    Scanner DPI Options

    I'm searching for a decent scanner for my 4x5 B&W negatives. Something that will do a good job on negatives with images resolved in the 30 to 60 lines per millimeter range my lenses produce.

    The largest prints I'll make from a 4x5 negative are 20 x 30 inches. From 120 film 2.25 x 2.75 inch negatives, 16 x 20 inch prints are about as large as I'll go. So I'm thinking that a scanner that does 1600 or 3200 dpi should do the job. Such scanners are out there for 70 to 140 bucks used. I think 1600 dpi should suffice so that's what I'm considering.

    Question is, is 1600 dpi enough or should I opt for a 3200 dpi one?

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    Re: Scanner DPI Options

    Quote Originally Posted by Bart B View Post
    Question is, is 1600 dpi enough or should I opt for a 3200 dpi one?
    The question is, does the scanner really deliver the DPI figure in the specs...

    Michael

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    Bart B's Avatar
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    Re: Scanner DPI Options

    Quote Originally Posted by Michael E View Post
    The question is, does the scanner really deliver the DPI figure in the specs...
    Michael, that's a good question. I'd guess flatbed negative scanners deliver close to specs. If not, then a higher spec'd resolution would best ensure you got at least what your minimum is.

    If anybody with an Epson 1600, 1680 or 3200 used to scan B&W negatives that could offer setup vs. actual dpi numbers, that would be of great help.

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    Re: Scanner DPI Options

    It can be rather difficult to determine exactly what a scanner delivers from the advertised SPI figure.

    Firstly consider that the SPI figure says nothing about the contrast at that SPI.

    Next you will not know if that SPI is specified for capturing the full density range of your image. You can always increase the contrast to boost the apparent SPI figure at the cost of sacrificing valuable intermediate densities and/or densities at both ends.

    Lastly the size of the aperture used when scanning is seldom specified. The aperture as referenced to the film surface can resolve detail in the image of about twice that aperture diameter. So an aperture of 25 um (.001 inch) will adequately resolve detail about twice that diameter with no bets on the contrast within the 50 um. That's just due to Nyquist limitations.

    Your requirement of 30 l/mm (15 lp/mm) at the low end corresponds ideally to about 770 SPI and at the high end 60 l/mm (30 lp/mm) to about 1500 SPI. so you're in the ball park at 1600 SPI. But if you want some real detail from the film you'd better go for 3200SPI which would, I think, from most experiences here, provide you with a visually pleasing 20 X 30 inch print that would stand up under close inspection.

    An example of the fallibility of scanner SPI specs can be found by actually measuring the performance of the scanner using a resolution target. I've been doing this with an Epson V750 just so I could understand what I was getting from the scan vs what was on the film. The results don't particularly match the scanner claims, although the claims are of course quite vague and imprecise.

    I have a ton of data, but here are two test runs of interest. Focus point 3.37 mm above the platen.
    4X5 Film Holder and using Film (with Film Holder). 2400 SPI with no Adjustments or Sharpening.
    Scan of 20 um lines and spaces (25 lp/mm; 1270 SPI). Contrast determined by the difference divided by the sum method X 100, with readings obtained by measuring the K values of dark lines and white spaces at 1600% magnification in PS. Post histogram adjustment of K values from 1% to 99% for full density range in PS.

    Positive Film setting with RGB scan = 28% contrast at 25 lp/mm (1270 SPI actual)
    B&W Negative Film setting scan = 54% contrast at 25 lp/mm (1270 SPI actual)

    BTW, consider what happens when the scan is done at 45 lp/mm (2300 SPI) still using the 2400 SPI machine setting:

    Positive Film setting with RGB scan = 4% contrast at 45 lp/mm (2300 SPI actual)

    At 52 lp/mm (2700 SPI) the contrast falls to zero, there is no discrimination between lines and spaces. All is greyed out. But also note that this is for a full density range condition, 1 to 256.

    Nate Potter, Austin TX.

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    Re: Scanner DPI Options

    I couldn't have said it better. I really couldn't. I hardly understand any of it. :-)

    This is a good example: According to Nathan's tests, the Epson V750 delivers 2400 spi. According to my german dealer, it delivers ca. 2200 ppi. Epson claims 6400 dpi. Apart from the confusing nomenclature (I think Nathan got it right with his use of spi), it shows that even a very good scanner doesn't necessarily conform to the numbers on the box. It's probably worse with el cheapo models.

    Besides that, an Epson V700 or V750 would probably be a very good choice for your application.

    Michael

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    Bart B's Avatar
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    Re: Scanner DPI Options

    Nate, thanks for your excellent post.

    I meant 30 line pairs per millimeter in my original post; I know it takes 60 lines, half white and half black alternately positioned, to see 30 of 'em across about 1/25th of an inch. Apologies for not being clear.

    Found this web site:

    http://www.largeformatphotography.info/scan-comparison/

    ....that, to me, shows pretty good comparisons of scanner results. Compared the Epson 3200 to their V750 and there is a difference. Not much, but it can be seen. Biggest difference is in the shadow areas and that's where the V750 is much better. I don't know if that much difference can be seen with B&W negatives.

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    Jim Jones's Avatar
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    Re: Scanner DPI Options

    Quote Originally Posted by Bart B View Post
    . . . Compared the Epson 3200 to their V750 and there is a difference. Not much, but it can be seen. Biggest difference is in the shadow areas and that's where the V750 is much better. I don't know if that much difference can be seen with B&W negatives.
    4x5 negatives scanned on an Epson 3250 made 16x20 prints that suited me. However, your standards (and those of others) appear to be higher. Certainly the loss of contrast in dense areas was a limitation.

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    Re: Scanner DPI Options

    I've plotted the contrast vs spacial frequency (and SPI) for 12 data points using a step function resolution mask.
    This for an Epson V750 scanner on the B&W negative setting.

    While this is a kind of academic exercise it gives a bit of insight into the scanner capability for the indicated settings. This is what you get by scanning at 2400 SPI. Notice that for 30 lp/mm (1500 SPI) the contrast is about 50%. If you can tolerate 10% contrast you get a considerably higher 43 lp/mm (2200 SPI) capability. As suggested by some on the forum here 2200 is about the upper limit of the V750 for B&W. Color scans reduce this number significantly; maybe even halve it.

    Bart, maybe this makes it a bit easier to see how this works; it's really fairly complicated especially when you fold in some requirements for maintaining the full density range from the film master.

    Nate Potter, Austin TX.


    EPSONcont-web-1 by hypolimnas, on Flickr

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    Bart B's Avatar
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    Re: Scanner DPI Options

    Quote Originally Posted by Nathan Potter View Post
    Bart, maybe this makes it a bit easier to see how this works; it's really fairly complicated especially when you fold in some requirements for maintaining the full density range from the film master.
    Nate, thanks for the link to the curves. I understand better now that I've seen the data.

    'Twould be nice to see the results of scanning a really good B&W negative at different scan lines per inch; 800, 1600, 2400, 3200 and so on. I'm gonna search our nororious web to see if I can find some.

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    Re: Scanner DPI Options

    BartB, I'm a workin on 6400 SPI @ B&W setting on the V750 but that's a lot of data with a lot of pixel by pixel density measurement.

    Even though the resolution mask is only two densities, clear and opaque I need to be careful that the full density range 0 to 256 is within the histogram in order to insure that the % contrast is computed about correctly. Even that poses problems due to the higher spacial frequencies being smaller than the scanner aperture.

    Nate Potter, Austin TX.

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