Size wrong when saved to folder

We have said it before, but nothing seems to confuse digital image users more than resolution, image size, size on the drive and size when open.  I've even seen international competition organisers get it wrong, so what chance do new users have or understanding it?

Way, way back, a camera club buddy of mine was having image quality issues and we couldn't understand what he was doing wrong.  The size on the drive was about right at 1.5MB.  When he sent the JPG file to me it was still full resolution (3072 x 2048 — digital image competition entries at that time were 1064 x 768 pixels. This was a long time ago) But he had saved it at minimum JPG quality which gave hime the tiny file size, but wrecked the quality)

Apples and pears 🙂

The size shown in Image size is the size in RAM, of a single layer using the basic calculation of pixel width x pixel height x 3  (for 8 bits/channel) or x 6 f(or 16bits per channel).

When saving on disk compression is used to decrease the saved file size size. The compression method varies by file type. In the case of TIFF or PSD it is usually lossless which means it can be decompressed when reopened and all original information recovered. In the case of jpeg compression can be more extreme, resulting in very small files, as it uses lossy compression. However, that does means some information is discarded and cannot be recovered. That is why jpeg should not be used when files are to be re-edited.

Dave

Edit : Oops sorry guys. It looks like we were all typing together - and saying the same thing.

Photoshop displays the uncompressed file size, based on pixel dimensions, bit depth, and number of channels.

If this is a format that uses compression, the file size on disk will be smaller.

10 MB vs. 1.5 MB suggests that you are saving a jpg.

You may find this article useful: File formats

Short answer: Compression.

What is the file format and save options?