One word: Compression.
Image files are compressed when saved, expanded when open.
(I'll take that quasi-professional comment with a grain of salt. )
hey station_two, thanks so much. So which size is the REAL size? I've been sending files to Millers for 20 years and assumed the closed size was the real size. P.S., Did you just tease me, your less fortunate colleague? Ouch. I hate being dumb. How did I ever get to be a PPA Master, Craftsman and Certified!!
I was curious myself, so I did a little searching. I normally do not hand off files to others, so it is not a problem, but if this question comes up again I can explain it.
Here is the difference:
"For the sake of simplicity, let us just deal with the finished image after you have shot it, converted it, and done any cropping. This image, when opened in Photoshop, has an "open" or "working" file size, as seen in the Image Size Panel section above. However, when you come to save the finished image you will have to decide about file compression. Usually you will be save the image either as a TIFF file, which can have compression or none at all, or as a JPEG file, which will always have compression. If the image has been saved with compression then you notice that the save image has a much smaller file size than what it was when it was open. Think of this as "saved" or "compressed" file size.
The problem comes when you look at an image that has been stored on your hard disk and you need to decide what the "open" or "working" file size is as it is the "open" file size that counts when you have to send an image to a client."
You open the File and look at it in the Image Size box to see the open or working size.
Just kidding, Carol Ann. So, yes, teasing you.
The size you see after saving, in the Mac Finder or in Windows explorer is the size on disk, i.e. how much the file "weighs" on disk, how much space of the hard disk it occupies. It can be—to a very small degree —dependent of the size of the sectors on any particular disk, but that is really an insignificant difference. Bigger disks have bigger sectors, which matters to the aforementioned small degree because the small unused portion of at least the last sector in the file is also counted and shown as part of the saved image when it sits on a given disk.
The uncompressed size is the size the image itself occupies in RAM when you open it to display it and/or edit it.
Both are very much real sizes. Both are also utterly irrelevant to determine image dimensions (so many pixels wide by so many pixels high) and/or to give you a clue as to image quality.
Competition directors and managers who specify the minimum size for submission of an image in terms of megabytes should be taken behind the shed and whipped with a twig. Only pixels and megapixels make any sense, though they by themselves do not determine image quality either.
Sorry, my last post was sent a tiny bit prematurely, as I got called away to dinner.
To what I wrote above, I would simply add that the multiplication of the two components of the image dimensions (so many pixels wide by so many pixels high) is what gives you the size in Megapixels (MP), both of an image and of the resolution of a camera's sensor.
I notice MP is used as a digital camera spec, but not so much in digital files, I suppose because there Pixel width and height varies and is more informative.
I edited my post #5 for typos and for thoroughness, because, as I mentioned above, I sent it before being able to proofread it properly.
I notice MP is used as a digital camera spec…
Yes, camera sensors are often rated or described in terms of Megapixels to give the consumer an idea of much leeway you have when cropping one of their images and/or in enlarging the image for print.
Thank you Gener7 and Station_Two for two excellent answers. There will always be total confusion between MB and MP. But a good photographer should take pride in knowing the difference. Thank you both.
carol ann dwyer wrote:
…There will always be total confusion between MB and MP…
There has never been any such confusion in my mind.