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
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.
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.
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!
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.