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I have an EOS 60D that has 18Mpixel. An expert said I should use at least 24MB pictures for stock photography. My 18Mpixel Jpeg at 72 DPI are between 5MB and 14MB. What is the purpose of a 24MB file? Just to resize the picture on print size? Thanks

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An expert said I should use at least 24MB pictures for stock.

You sure the "expert" meant 24 MB, not MP? I don't know much about the higher MP cameras, but I can't imagine anything but maybe the 36 MP Nikon D800 having a 24MB file size. If you take a look at Nikon's official website, you'll see file-sizes ranging from 17 to 29 MB. If it's true that you are required to have a 24MB file, you'll have to invest in a better camera!

I took a look at the shutterstock.com submission guidelines, and to quote them:

Images must be at least 2.5MP (2.5 Megapixels/2.5 Million Pixels). To calculate the number of pixels in a photo - multiply the width by the height. For example - a photo that's 1700x1300 is 2.21 million pixels. The minimum size for new photographers is now 4.0MP.

iStockphoto:

iStock accepts files 1600 x 1200 pixels or larger.

Naturally they want more than that if possible, they all say "give us the largest file you've got"

File size doesn't necessarily equal quality, the photo submission guidelines will tell you what they're looking for, and resolution is not at the top of the list. Proper focus and exposure is much more important.

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  • \$\begingroup\$ @AJHenderson it's not hard to make a 24+ MB raw file, but the question specifically asks about JPEG. Plus, stock photo websites do not typically accept RAW files anyway. \$\endgroup\$
    – SaltyNuts
    Commented Oct 31, 2014 at 20:19
  • \$\begingroup\$ @SaltyNuts - the question mentions JPEG, but said the expert said 24MB images (which doesn't necessarily mean JPEG). There is more than 24MB of raw data in cameras as low as 14MPix. \$\endgroup\$
    – AJ Henderson ♦
    Commented Oct 31, 2014 at 20:28
  • \$\begingroup\$ Yes, I would expect a 22MP camera to have RAW files that size. However, we're talking about JPEG files. \$\endgroup\$
    – ezmopho
    Commented Oct 31, 2014 at 21:47
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I recall, 20+ years ago, people in the graphic industry referred to file size as a proxy for resolution. Given that files were never compressed, and the aspext ratio was normal and pixels were 3 bytes, it made sense. Although it bothered and amused me that they often had no concept of resolution and couldn't handle linear pixel measures and sometimes couldn't grasp that print size was not the same thing; file size was used extensively for such purposes until "megapixel" was popularized when digital cameras became practical in their work. Lossless compression in TIFF files became usful earlier, but wasn't used due to lack of trust and because it messed up their meaningful file sizes!

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  • \$\begingroup\$ I did IT support for a printer around that time and there was another very good reason: TIFF is (or at least was then) a complete mess that resulted in implementations which were frequently incompatible - even between versions of the same package. That led to a lack of trust as files that weren't produced in house or at least by the same software (and version) had a tendency to be unreadable by anything else once compressed... \$\endgroup\$
    – James Snell
    Commented Oct 31, 2014 at 10:27
  • \$\begingroup\$ Ah yes, the "non-standard standard". I don't know when that got better. \$\endgroup\$
    – JDługosz
    Commented Nov 1, 2014 at 1:59
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My gut feeling tells me that the expert probably doesn't mean 24mb. Probably megapixels.

From the number of pixels alone, you would probably never be able to output a 24mb JPEG with that resolution. JPEGs are compressed 8bit per channel images. So if there's no compression at all, your images would be a maximum of 18x3=54mb. You (typically) except a 5 to 10x compression ratio (a very rough estimate, and the compression ratio is image dependent) for JPEG at maximum quality. Which is what you are getting.

Computing backwards, that would mean that your image would have to be 40mega pixels to begin with to have such a file size.

Even RAWs may not have such large file sizes as well as they are compressed with a lossless algorithm.

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