Expert: LizaL - 5/17/2006

Question
Hi, I would like do know what is the difference between file size,image size and document size? I am photoshoping my images to send to Alamy stock. The minimum size required is 48MB. I use Adobe Photoshop CS. I use a nikon D2X, shoot in raw file and open the document with photoshop CS and I also have nikon capture image which I haven't use yet, for I am still learning the basic photoshop. When I open the image the side bar with all the information about it shows reads, for instance: file size: 19.3M; on the bottom left of the image we read document size: 69.9M and if I go to the toolbar IMAGE then image size, I read 69.9M. I am a bit confused for I do not know which information the stock images will read. I would appreciate your help. Thanks a lot. Consuelo

Answer
Hi Consuelo,

This is kind of a confusing thing to figure out. But actually all three things -- file, image, and document size -- are related. Here's how it works:

1. When the term 'file size' is used, what's meant is the *digital* size of the image file, and that's what we measure in kilobytes or megabytes (or even gigabytes, sometimes!). The file size of an image is proportional to the pixel dimensions of the image (which is the resolution). So if we say the picture is '48MB', we're referring strictly to the digital size of the image (the number measured in -bytes.)

Images with more pixels produce more detail -- and again, this refers to the resolution of the image.

2. Document size is how big the image would be if you printed it.

3. Image size refers to the resolution of the image. Resolution can seem confusing. But basically, all resolution is is the number of pixels in an image, measured both horizontally and vertically.

The more pixels in an image, the higher the resolution -- and vice-versa.


I looked up Alamy stock photography and it looks to me like they're referring to digital size when they list their submission requirements.



Here's a little tip about knowing what your file size is. Open an image and look at the bottom bar, on the left. You'll probably see two numbers there, separated by a slash. In this area you can see all sorts of information about your image, at a glance. Here's what it means:

1. The number on the left represents the *printing* size of the image -- this is approximately the size of the saved, flattened file in Adobe Photoshop format. The number on the right indicates the file's approximate size including layers and channels.

2. If you click and hold on that area, you'll see a list of choices revealed. These are different views you can select and change as you wish, that tell you different things about your document.

Here's what that little bar looks like, expanded:

http://little-works.com/all_experts/file_info.jpg


Hope this helps,

Lisa