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Thread: Aliasing and scanning resolutions

  1. #1

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    Aliasing and scanning resolutions

    I wonder if some of the conflicts about scanning resolutions and print quality arise from confusion about alaising and scanning. From what I remember about sampling theory, you have to sample at twice the frequency of the data you want to capture. If I want a 16 x 20 printed at 240 DPI, then I need a data file that is 3840 x 4800. This would be scanning a 4x5 at 768 DPI. Doubling this for alaising, I get 1536 DPI. If I want to put 300 DPI data down on the print, this goes to 1920 DPI. Thus in our thread about comparing 4x5 and 8x10, the 100 meg scan of the 4x5 was at too low a DPI to collect enough data to even get 240 DPI down on the print.

    This would be consistent with the observation that good consumer scanners can produce 16x20 prints from negatives that are indistinguishable from drum scans, because a good consumer scanner can probably produce a good 2000/2200 DPI scan from a negative by scanning at 4800 DPI and downsampling. (DMAX and noise become a big problem for chromes.)

    Above 16x20, the advantage of drum scans would become more obvious as the print increased in size. Having an optimum negative might push the best printable size with the consumer scanner up to 20 x 24. Of course, this assumes fine detail in the picture. Clouds over a pond might look the same in any size print. I think that is also why we see people saying that their 8 MP DSLR makes great 16x20 prints - it will if you have no fine detail and properly process the data to smooth out the pixalation.

    Comments from some of our math oriented scanner mavens?

  2. #2

    Aliasing and scanning resolutions

    Ed,

    I believe that aliasing is handled internally by the scanner so a "dot" in the output file will already be the result of whatever interpolation the scanner is doing (depending on the scanner technology) from multiple samples.
    You should also distinguish between resolution and dynamic range. You can't compare scanner performance based on resolution alone as the sibjective "quality" of the image you get is a combination of detail and tone, not just one or the other.

    Guy
    Scenic Wild Photography

  3. #3
    Abuser of God's Sunlight
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    Aliasing and scanning resolutions

    some general ideas ...

    How many samples you need to accurately reproduce a line pair worth of detail is the subject of some discussion. The points of view I've read suggest that two samples is optimistic, and that four is overly conservative. A lot of people consider 3 to be a good estimate. So to record 10 lp/mm of resolution, you would want at least 30 pixels /mm sampling rate.

    Avoiding aliasing is a more complex topic. One of the reasons desktop scanners tend to have double the sampling frequency of their actual optical resolution is to provide a simple answer to aliasing problems. Drum scanners provide more sophisticated controls, but making them work right takes a lot more skill.

  4. #4

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    Aliasing and scanning resolutions

    Nyquist theory is such that at LEAST twice the sampling frequency is needed to resolve a given resolution in line pairs. Convention places this figure at approx 2.3

    Thus, for a 1200 dpi scanning rez, you should scan at least at 2700 dpi or above on a cheap flatbed. Higher quality scanners reduce this figure as it is already built into the scanning system software. On a flatbed like the Epson 4990, scan at least 2X your desired rez.

  5. #5

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    Aliasing and scanning resolutions

    ... and then what? Use resize to get back down to your working resolution? Sorry if this is obvious to others, but the concept of scanning at a higher resolution is new to me.

    -Ben

  6. #6
    Resident Heretic
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    Aliasing and scanning resolutions

    Rather than comment on the mathematics of sampling theory, I'd rather point out an often discounted advantage of drum scanning - that of wet mounting. Not just wet mounting to a flat surface, but wet mounting to the precision curved surface of a drum.

    Mounting on a curved surface holds the film rigidly in correct focus position. Then the scanner fluid fills in the surface imperfections of the film such as small micro scratches, and coats and floats at least some of the dust. The resulting scans are visibly improved, even at small enlargement factors.

    Even if you make just small enlargements like your example 16x20 print from 4x5 film (only a 4x enlargement), you'll be able to tell the drum scanner print from a flat bed print. The effect isn't negligible in my book, nor will the two scans be indistinguishable.

    Is the improvement worth it on such a small enlargement? Only you can say.

    Bruce Watson

  7. #7

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    Aliasing and scanning resolutions

    Ben,

    you would then downsample to your desired resolution in Photoshop. Some people prefer to downrez using "bicubic" sampling, others with "bicubic sharper." It depends on the amount of pre-reduction USM you've applied to your raw scan. Film tends to need higher USM amounts to achieve the same accutance as a digital source. You can find this through experimation. As a starting point, prior to downsampling, apply a USM of about 300, .9, 2 for a 4x5 scan. See how this looks and work from there.

    If you need anything further, email me offline & I can run through a routine with you.

    Best of luck.

  8. #8

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    Aliasing and scanning resolutions

    Dave, Just to make sure I understand what you are saying.

    For example, if I scan a 4x5 negative in order to make a 4x5, 300 dpi inkjet print, then I should scan at at least 600 dpi, then downsample in Photoshop to 300 dpi before printing?

  9. #9

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    Aliasing and scanning resolutions

    The real problem is that nobody ever defines what they mean by "resolution".

  10. #10

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    Aliasing and scanning resolutions

    That's it Ron. Best to scan a little higher than 600, but that is the idea. Now that said, the higher quality scanner, the less important the 2X+ threshold becomes, but considering memory and HD space is cheap, it's good to scan at a higher rez. I always scan my MF work at 3200 DPI on my Imacon and store that as the master scan. I scan 4x5 on a collegues scanner (Imacon as well) at 3200DPI. I can always downrez, but it's best to avoid upsampling if at all possible. ( a small amount of upresampling, +15% or so wouldn't be noticable). This allows you to keep a master, unsharpened file. That way in the future, you don't need to rescan, and you can sharpen depending on your output size.

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