Posted by John Tobias on September 27, 2009 at 10:13 AM
Greetings,
I am new to the NPN, but have been involved in photograhy for 50+years. But, the computer and I are not friends! My question is when I resize an image in Photoshop and then save it in a folder the size changes, why? This is causing me problems in posting photos.
thanks for your help.
John Tobias
Posted by David Bostedo on September 27, 2009 at 1:01 PM
John - I'm not sure what you're asking because I'm not sure if you're referring to image size or file size when you say "the size changes". If you're talking about file size, then that makes perfect sense, because the file size is determined partly by the number of pixels that comprise the image. So if you re-size your image, and re-save it, the file size will be different than before.
Posted by John Tobias on September 27, 2009 at 3:25 PM
David, thank you for your reply. I am referring to I think the image size when I resize the photo to beable to meet the size requirement of posting, no larger than 200kb. I just posted a photo in the fauna gallery. I resized it in Photoshop to just under 200kb, but in the folder I saved it in the size is much less. That is what I am asking about. While I have been using Photoshop for sometime, it is only since I joined NPN I have needed to post photos at a certain size.
thank you for your help.
John
Posted by Jim DeWitt on September 27, 2009 at 4:14 PM
Some terminology may help. The logical size of a file is the size in bytes of a stored file. The physical size is the amount of disk space it takes up, and very often a different number than the logical size.
It sounds like you know how to adjust the logical size of a file by using the "Save for Web" feature in PS and then using the slider to adjust JPEG compression to get to the target logical size. Note you are not "resizing the photo" by adjusting JPEG compression; you are tweaking file compression. Your original file is unchanged. All you are doing is adjusting the logical size of a copy by adjusting compression.
"Resizing a photo" means something else. More correctly, several somethings.
The reason why the logical file size (what you see using the "Save for Web" menu in PS) and the physical file size is geeky. Data is stored on drives in "blocks," and the size of a block varies depending upon operating system, the format of the drive and the size of a drive. On my MacPro, with a 1 terabyte hard drive, with old style HFS formatting, my block size is 64 kilobytes. It takes four blocks, or 4 x 64kb, to store a 200kb file. So the physical size of a 200kb file is 256kb. With modern Unix formatting, partially used blocks can be "scavenged," so the operating system might show a 200kb file stored in 160kb of physical space. The bottom line is that you shouldn't expect a strong correlation between logical size and physical size of files.
It's more complicated still when you consider file formats. Applications like PS use proprietary file structures which do some file compression. Unhappily, the world wide web only recognizes about three kinds of picture file formats; of those three, JPEG generally works best. But JPEG, apart from compression, isn't a very efficient file format. So it is not uncommon for a PS file to actually be smaller than the same file uncompressed or slightly compressed under JPEG.
Bottom line: don't sweat variations in file sizes on your computer. Compress just enough to meet NPN's file size requirements.
Use "Save for Web & Devices."
Set pixel size (if not already done).
Convert to to sRGB (check box).
Use the optimize menu (looks like a stack of parallel horizontal lines in the upper right of the pop-up window) to choose the desired file size.
Save the file, but don't overwrite your original. Save if with a new name, perhaps just adding WEB to the existing name, or save it in a new folder for web images.
Discussion
4 repliesI am new to the NPN, but have been involved in photograhy for 50+years. But, the computer and I are not friends! My question is when I resize an image in Photoshop and then save it in a folder the size changes, why? This is causing me problems in posting photos.
thanks for your help.
John Tobias
thank you for your help.
John
It sounds like you know how to adjust the logical size of a file by using the "Save for Web" feature in PS and then using the slider to adjust JPEG compression to get to the target logical size. Note you are not "resizing the photo" by adjusting JPEG compression; you are tweaking file compression. Your original file is unchanged. All you are doing is adjusting the logical size of a copy by adjusting compression.
"Resizing a photo" means something else. More correctly, several somethings.
The reason why the logical file size (what you see using the "Save for Web" menu in PS) and the physical file size is geeky. Data is stored on drives in "blocks," and the size of a block varies depending upon operating system, the format of the drive and the size of a drive. On my MacPro, with a 1 terabyte hard drive, with old style HFS formatting, my block size is 64 kilobytes. It takes four blocks, or 4 x 64kb, to store a 200kb file. So the physical size of a 200kb file is 256kb. With modern Unix formatting, partially used blocks can be "scavenged," so the operating system might show a 200kb file stored in 160kb of physical space. The bottom line is that you shouldn't expect a strong correlation between logical size and physical size of files.
It's more complicated still when you consider file formats. Applications like PS use proprietary file structures which do some file compression. Unhappily, the world wide web only recognizes about three kinds of picture file formats; of those three, JPEG generally works best. But JPEG, apart from compression, isn't a very efficient file format. So it is not uncommon for a PS file to actually be smaller than the same file uncompressed or slightly compressed under JPEG.
Bottom line: don't sweat variations in file sizes on your computer. Compress just enough to meet NPN's file size requirements.
In Photoshop CS4:
Use "Save for Web & Devices."
Set pixel size (if not already done).
Convert to to sRGB (check box).
Use the optimize menu (looks like a stack of parallel horizontal lines in the upper right of the pop-up window) to choose the desired file size.
Save the file, but don't overwrite your original. Save if with a new name, perhaps just adding WEB to the existing name, or save it in a new folder for web images.
Hope this helps.
Pete D.