How do I determine what the image file format and resolution should be when scanning?

Question

I was recently asked for help with digitizing a large set of photographs - 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 needed, 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 megabytes 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?

Answer 1

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!