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I understand the title to my question is very vague, but bare with me, I think I can make it a bit more precise in the question body.

Recently my mother was asking for help with digitizing a large set of photographs her partner has - tens of thousands of the things, that need to be digitized so they can be sent to book publishers and possibly other types of clients. So I needed to work out how much space she'd need, which raised the question of how high quality the scans would need to be.

That's just an example situation, my question is a bit broader than that: in general, in which industries and situations is quality the most important? One thing I was curious about was what file formats are typically used. Personally I see no visible difference between a 100% quality JPEG and lossless file, but how do professionals feel about that? Is there every a situation where that minute difference becomes important? If so, is it just being overly fastidious, or does it genuinely matter?

Recently, I also heard about photography sessions running into the hundreds of gigabytes. That just blows my mind, wouldn't the images need to be hundreds of megs each? What would ever make them that large?

I hope my question isn't too broad. To break it down into more concrete chunks:

  1. When does the lossless/high quality lossy distinction matter?
  2. Can you provide a rough comparison of what's considered an acceptable DPI in various industries/situations, or in your particular industry?
  3. What file formats are typically used? Just JPEG for lossy and PNG for lossless? Or are fancy domain-specific formats preferred? If so, why?
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The JPEG format is always lossy. –  Mechanical snail Dec 13 '12 at 3:53
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Equating 'image quality' to 'industry' isn't really all that relevant. The image quality is really connected to means of how it will be displayed, the size it will be displayed at, and other project-centric variables. There really aren't blanket industry-wide connections to image quality. –  DA01 Dec 13 '12 at 7:36
    
@Jack M: are you only asking about photographic images? –  e100 Dec 13 '12 at 10:42

1 Answer 1

up vote 4 down vote accepted

This really is too broad. But broad recommendations can be given for a broad problem ...

Resolution considerations

In general terms

  • 300ppi for print
  • 72 for web
  • More every day for mobile and tablet (the highest right now is 433ppi, I believe).

The catch is that your resolution is output size. If you can guess the final crop and dimensions and intended use of every one of those many thousands of photos then you're a psychic.

Calculating required source resolution

With the above info in mind, you can take a shot at the largest reasonable use (8x10" @ 300ppi?) and the most dramatic crop (50%?) and calculate your scanning resolution from there. With those numbers, a 4x6" source image should be scanned at 1200ppi.

The formula is:
((target size ÷ source size) ÷ crop factor) * final resolution = source resolution

So the scenario I described would be: ((8/4)/.5)*300=1200

Notice the result if you use the vertical dimension: ((10/6)/.5)*300=1000

Because of the difference in aspect ratio between source and target, you can get yourself into trouble if you calculate from the closer dimensions.

Of course, you also have to take into account

  • Production quality of the destination media
  • Expectations of your audience
  • Quality of the originals

File types

I prefer a lossless format like tiff with compression to be sure nothing is lost.

Jpeg is intended for outputting to final size for low bandwidth applications. Jpeg files should not be resaved because you're giving up more info: They should be re-exported.

I really don't know happens to quality with a 100% jpg because I don't see the point. Just use compressed tiff and you'll save size.

Photo shoots and file sizes

The reason a professional shoot results in such large files is RAW format. A professional size sensor capturing in RAW format produces mammoth files. But it also holds on to a whole lot of good data! Even in the enthusiast point and shoots, the RAW files are pretty large.

Fortunately, SD cards have gotten cheaper, faster, and really large!

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On the point of 100% JPEGs: I find that a 100% JPEG beats out a PNG in terms of size by a factor of about 3. Saving a 23.5 meg PNG as a 100% JPEG produced an 8.3 meg file when I tested it just now. TIFF was 30 meg! (though I've never used TIFF before so I might not be using optimal compression settings) Good answer, thanks. –  Jack M Dec 13 '12 at 1:24
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Well, png is lossless, jpg is lossy. If the size is smaller then I'd guess you're losing information. Test it against a tiff with zip or LZW compression and see what you get. The fact of the matter is storage is cheap! Why give up quality when the extra capacity can be bought for pennies. –  plainclothes Dec 13 '12 at 1:48
    
TIFF with ZIP and LSW compression produce 27.4 and 30.0 meg files, respectively. By the way, could you clarify what you mean about "the final crop", and when you mention a crop of 50%? You mean 50% of the image just... cropped out? If so, what does that have to do with resolution? –  Jack M Dec 13 '12 at 1:52
    
The lossless formats are comparable so you can see what the jpg format is doing: throwing info away. It's also worth noting that the PNG won't hold color profile info. –  plainclothes Dec 13 '12 at 2:40
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Note that 72/300 are display pixel density resolutions and, while useful on some level, don't really correlate to 'image quality and file size' in terms of storage in and of themselves. –  DA01 Dec 13 '12 at 7:38

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