This is a stupid question but I’ve never understood it so here goes. Why is the document size on bottom of the image window different from the actual file size? Why is the file size different if you look at the file in Bridge, or if you do a Get Info on it? Is there any way to keep track of how big the actual file is while you’re working on an image?
Because it’s the document size, not the file size.
As explained in the manual: it’s the size of the document in Photoshop’s memory. Saved in a file, there’s compression, and the fact that most files can’t use tricks like Photoshop does (sharing representation of duplicated layers, etc.).
The document size is a rough guide to the size of the file, but it’s not the same thing. And you can’t know the size of the file until you save it (and add the metadata, plus save some space from the compression, etc.)..
Sometimes you’ll be asked to send jpegs that are no larger than 500K, for instance. And you can’t really know how big a file is till you save it, then you may have to go back and change it. I guess you start to get a sense of this when you do it often enough- but I wonder if there’s any relationship, a percentage, that can give you some idea as you’re working on a file how big it is. And when you save a file as a jpeg, the size on the bottom of the dialogue box for jpeg quality is another number, not the file size.
In that case you use the file size estimate when saving, and change the quality to get the desired file size.
But the flattened document size is still a decent guide to how large the file will be (larger documents means larger files).
No, especially with JPEG because you can change the quality so much — you could have one picture that’s 100 Meg in memory and 2 Meg on disk, and another picture that’s 100 Meg in memory and 80 Meg on disk (depending on the picture contents and the quality setting you choose).
As Chris said, you won’t know until you save. The size savings you gain during compression will vary hugely from one image to another one, depending on things like the amount of detail contained within the image.
Yeah, that’s what I’ve thought and noticed, but just thought I’d ask if there was a secret to figuring it out beforehand. But still- that number that you see in the jpeg save as dialogue box, which indicates the file size and download speed, isn’t the same as the file size, either. So if I want to use as little compression as possible, and save as a maximum quality jpeg, the file size resulting from the save is a different number.
