Why does Photoshop show my JPEG as 35 MB when the file is only about 4 MB?
Asked 8/25/2011
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I shot both JPEG and TIFF files on a Nikon D300. In Finder/Aperture, the JPEG is about 4 MB and the TIFF is about 35 MB, which seems normal. But when I open them in Photoshop CS3, Photoshop shows both at roughly 35 MB. The JPEG also seems slower than expected to open and rotate. The images are about 4288×2848 at 300 ppi. Why does Photoshop report the JPEG as so large, and what size is it actually using?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
18
The reason you got confused is that it's not the file size that is displayed in Photoshop.
Photoshop's status bar shows uncompressed size of image. With three 8-bit color channels, that's 3 bytes per pixel, resulting 34.9 MB for a 4288 x 2848 image from your camera. JPEG is a compressed format, so the actual file is smaller.
Showing compressed size would be complicated, as
- for accuracy, the data would have to be actually compressed after each edit, but that is certainly too time-consuming for just showing a number on status bar;
- you might choose another format or compression level when saving the image.
Uncompressed size is useful as you'll know how much memory is currently used to hold the image and you can calculate how much compression, cropping and/or resizing you'll have to apply to reach desired file size.
Originally by user4390. Source · Licensed CC BY-SA 4.0
user4390
15y ago
0
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Photoshop is not showing the saved file size of the JPEG. It is showing the image’s uncompressed size in memory.
A JPEG is a compressed file format, so it may only take about 4 MB on disk. But when Photoshop opens it, it expands the image into full pixel data for editing and display. For an 8-bit RGB image, that’s roughly:
width × height × 3 channels = memory size
For a 4288 × 2848 image, that works out to about 35 MB, which matches what you’re seeing.
That’s why a JPEG and a TIFF of the same pixel dimensions can appear similar in Photoshop even though the saved files on disk are very different sizes. TIFF may be uncompressed or losslessly compressed; JPEG is lossy compressed. Once opened, both become a bitmap in memory.
The 300 ppi setting does not determine file size here; pixel dimensions do. So nothing is wrong with your camera or cards—Photoshop is just reporting a different kind of size than Aperture or your file browser.
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