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I'm a novice, and don't have a detailed understanding of the technical terms, but I've scanned hundreds of 35mm and medium format negatives to get the best quality RAW files at certain presets which were set up for me by others, using Nikon CoolScan and Epson V500, V550 flatbeds.

Some negatives were accidentally scanned with the film upside down in the scanner, giving me a flipped (mirrored) image.

Are these photos missing any quality (channels, color, detail etc) because of being scanned upside down, and they must be rescanned for the best possible image? Or can I just flip these photos in Photoshop and they will have all the quality they would have had, had I scanned them right side up?

Additionally, I have the same situation with scans done on a flatbed as drafts, as opposed to full RAW scans.

  • Scanning upside down does not give you a mirrored image. It give you one rotated by 180 deg, not flipped. – Olin Lathrop Jul 22 '17 at 20:12
  • Consider that the negatives are actually exposed upside down and flipped in the camera, yet we are still somehow able to reconstruct the original scene right side up and not flipped. – Olin Lathrop Jul 22 '17 at 20:13
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It depends on what you were scanning and at what resolution, but in general you're missing out on a small amount of sharpness by scanning them upside down. However, if the scanned images look fine, it probably isn't worth the trouble to scan them again.

The main issue with scanning them upside down (film base down) is that you might put the emulsion layers out of reach of focus for the scanner. The stock trays that come with Epson scanners put the film at the correct distance so that they can be scanned in focus, whether right side up or upside-down. But if you were scanning on third-party trays or thicker mediums like glass plates, you might have issues with the scanner maintaining focus during a scan, as the registration distance for film scanning is within a millimeter range a few millimeters above the scanning bed. It's more likely you would have bad scans caused by film curling in the trays than film orientation.

For the scans done on the flatbed, the same issue exists, but the maximum dpi direct from the flatbed is considerably lower (reputed to be about 600 dpi) than it is for medium scanned in the tray. You'll be less likely to notice a loss of sharpness here. If they're drafts, say, like contact sheets, you probably don't need to rescan these either. In addition, you might run into newton's rings in your scans by scanning directly against the bed.

Anecdotally, myself, I've intentionally flipped some negatives/film when scanning if the emulsion side has a lot of issues (severe scratches, fine embedded dust, rinse marks) or the film doesn't lay flat enough the correct side up in the tray. Some issues are just too difficult to fix and it usually proved faster to just get some reasonable scan done first to get a closer look at the image before committing to fixing the film for a final scan.

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If you are using any kind of Flatbed scanner (like the Epson), don't worry about it. The optical quality of such devices is quite poor, so no one will be able to tell the difference. On a super high quality scanner at a very high DPI count, it is true you might see the image getting softer because it is focusing on the base side and not on the emulsion. But for most scanners, like the ones you mention, the difference would be so negligible that I would not loose any sleep over it.

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If they are in focus, flipping them in Photoshop will not cause any problems. Of course, bear in mind that every consecutive save of a JPEG file does reduce image quality to some degree, even at the highest quality setting. For the absolutely, numerically, least amount of degradation, you'd save the flipped JPEGs (assuming the scans are JPEGs) as TIFF files, or some other non-compressed format.

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    Worth noting that you can flip JPEG files losslessly. – mattdm Jul 21 '17 at 20:02
  • ^^ I did not know that! Definitely worth noting. – digijim Jul 21 '17 at 20:12
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Depending on the scanner features you use, yes.

Some scanners have advanced features ('ROC', 'ICE', etc.) where they focus separately on the different color layers, scan each with UV or IR light for remainders of decayed coloring materials, and calculate the original colors from it. Obvioulsy, with the colors in the wrong sequence and position, this will give incorrect results.

For a normal, one-plane focus scan with visible light, the scanner's autofocus should be able to focus either way, and there would be no difference.

Note - as others mentioned - that saving in JPG might be a much bigger issue. Is is typically adjustable, but the default is 20% loss per save; so if you don't adjust it to the minimum of 1% loss, you lose 20% with each save - including the original save of the scan result. The best way to avoid this is to use a loss-less format; PNG is good, but not supported by most scanners; TIFF is also good (but very large).

I use TIFF, and then batch-convert the huge TIFF files to PNG (PNG is compressed, but loss-less, so the files are much smaller).

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As an addition to the answer of @meklarian: it depends what side the emulsion is on. At my work, we are scanning thousands negatives: color negatives on film, B&W on film, B&W on glass plate etc. For most of the materials, the 'upside down' is the emulsion side. So you actually get more quality scanning the other side and performing a flip afterwards. We always scan the emulsion side, so flipping is part of our general workflow.

I guess that the difference is bigger with glass plates with a thickness of a few millimeters than with film.

To determine which side is the emulsion side:

  • emulsion side is more 'mat' (matte, dull) and depending on the support, you can see the structure of the image (sometimes you can really see the lines within the picture as 3D lines on the film)
  • non-emulsion side is shiny, glossy and there is no texture, only the smoothness of plastic (and additional scratches)

Note that this really depends from film type to film type. Sometimes it's really clear, sometimes nearly invisible.

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Having come from a darkroom background. Lightsource at the top, filmbase diffusing the light below. Emulsion on the underside is directly focussed by the lens without having to travel through the filmbase. Image is reversed, but easy to flip. Emulsion on top is advised by scanner manufacturers for simplicity. (No need to post process/flip).

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