Photoshop resizing is giving wrong file size

[aprilf41538545]

I'm resizing some images for upload to a web page for an art competion. I open the image, adjust using levels, merge down to the background layer, resize using image size, check the file size is 4.83 mb. But when I go to upload, my Mac is telling me that the file size is 1.1mb, NOT 4.83 mb. I've never seen this before, what's going on??

[Tom Winkelmann]

You confuse file size with image size...

[aprilf41538545]

I thought they were the same, please explain?

[Tom Winkelmann]

https://cornicello.com/itfigures/image-size-vs-file-size-vs-pixels

https://www.essentialdigitalcamera.com/2015/11/what-is-difference-between-image-size.html

https://www.gestalterkram.de/2015/02/berechnung-des-speicherbedarfs-von-bildernvideos/

[davescm]

Hi,

Image size is the number of pixels.

Image size in RAM is the number of pixels multiplied by 3 for an 8 bit/channel image or by 6 for a 16 bit/channel image.

File size on disk is the the file size after any compression used in the save process has been applied and any meta data added in.

To export a jpeg at a target file size, use File - Save for Web and adjust the compression to reach the target file size shown under the Image preview.

Dave

[aprilf41538545]

thanks!

[D Fosse]

The piece in the puzzle you're missing is jpeg compression. File size on disk/over the wire depends on the data compression applied when saving.

Image size shows you the decompressed size of the data, as it sits in RAM. Layers are not counted.

[Trevor.Dennis]

Can you give us a link to the competition rules?  It is unlikely that they require an exact file size.  It's more likely to be a maximum file size, and maximum pixel size.  The file size will depend on how 'busy' the image is, because JPG compression will be more effective on a image with few colour transitions, than on a noisy complicated image.  The trick is to use the highest JPG quality setting you can get away with, while staying below the maximum file size.

Also make sure your image has the required colour setting which is usually sRGB.  If you submit an Adobe RGB image, for instance, then the colours are likely to be muted and dull when displayed to the judges.  I've tripped up with this one myself moaning about the projector calibration when it was actually my mistake. 

[aprilf41538545]

https://www.callforentry.org/uploading-images-audio-and-video-files/

• File Type: JPEG or JPG only.
• File Dimensions: 1,200 pixels or greater on the longest side.
• File Size: Under 5 MB.

I've set compression to 12 (maximum), which does compress a file size of 7.92 mb down to 1.3 mb, but what's driving me nuts is trying to figure out what file size to start with to get best quality. I have set the color space to SRGB.