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Friday, December 4, 2009

Image File Size and Resolution

Hi I just viewed the image size-resolution portion of your cs2 video and I can't figure out
why one of my images is 2900x2900 and the other is
about 2500x2700
the large one is 3.30 MB
and the other is 700 KB
why such a big difference in doc size?
I would like to learn more about this.


Hi Mike,

Hi Mike,
The two main things that control image file size are image size (size of image on page) and image resolution (number of pixels per inch). My guess is that your 3.30MB file is at 300 pixels per inch and your 700KB file is at 72 pixels per inch.
Also, everything else being equal, the image mode will also impact file size, RGB has 3 data points per pixel and CMYK has 4 data points, so at the same image size and resolution a CMYK image will have a larger file size than a RGB file.
Also different file types will have different file sizes, for example the Photoshop format (PSD) is uncompressed where as the JPEG file format is compressed, so at the same image size and resolution a PSD file will be much larger than a JPEG file. The main file types you will probably be encountering in Photoshop are PSD, BMP, TIF (all uncompressed) and GIF, JPG (JPEG), and PNG (all compressed - much smaller file sizes).
You can see the file type and image mode in the header of the image window, it will show you the file name, the zoom percentage and the image mode. You can see the resolution by going to the Image Menu and clicking on Image Size. The top of the box shows you the image size (usually in Pixels, number of pixels per inch) and the bottom of the box shows you the size on the page and the resolution. The higher the resolution, the more pixels per inch, and the larger the file size. Changing the resolution will not change the size on the page, but it will impact the file size (and quality of the image if you go to a lower resolution). My personal preference is to work on a image in PSD format at 300 pixels per inch, then convert the image to JPG or GIF compressed to a lower resolution for use on our disks or the internet. If I am creating an image for use in print I leave it at 300 pixels per inch.
George Peirson
How To Gurus
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