The Best Way to Compress a JPG in Photoshop
Compressing image files in Photoshop saves disk space and allows for easier file transfers. The primary compression option in Photoshop is JPG, also known as JPEG. JPG is popular because it allows high levels of compression and varying quality. It is important to know when JPG compression is appropriate and how much image quality must be maintained for a given graphic design project.
Understanding Lossless Vs. Lossy Compression
There are two kinds of compression: Lossless and Lossy. Lossless compressions, such as LZW and Zip, write all the original data in shorthand. It is like condensing a grocery list with the name of an item written down for each number of that item needed (e.g. "banana, banana, banana") by writing each item once with a number next to it (e.g. "3 bananas"). No data or quality is lost with Lossless compression, but it takes longer to perform.Alternatively, Lossy compression, like JPG, makes estimations about the image data by saving average values. Sticking with the grocery list analogy, a list of items (e.g. "banana, banana, banana") can become a generalized statement about those items (e.g. "yellowish fruits"). Some data is lost with Lossy compression, but it is faster. Further, Lossy compressions greatly decrease file size in comparison to Lossless compressions and provide an image quality that is good for emailing and some printing purposes.
JPG Compressing in Photoshop
To compress an open image to a JPG file in Photoshop CS3, click on the File menu and choose "Save As." JPG is already chosen in the format dropdown box. Type a name for the compressed JPG file and press Okay. A new dialog box will appear titled JPEG Options.
This dialog box allows for the adjustment of JPG quality, which is directly related to size. If the image is for general use, choose the 10 quality setting to be safe.A higher-quality compression requires a larger compressed file size, while retaining lower quality requires a lesser file size. Aside from the level of compression, the final compressed image quality will depend on the image's resolution and content. If an image is being saved for on-screen viewing or thumbnails, choose a smaller quality of 6. If an image is going to be printed or published, a larger compression of 10 is a safe way to preserve quality while still significantly decreasing the file size. For example, a 12x8 inch image at 300 ppi (pixels per inch) compressed at the 6 quality setting, can be transferred in approximately seven seconds over high-speed Internet. The same size image at 300 ppi compressed at the 10 quality setting can take approximately 19 seconds.
Tips for Maintaining JPG quality
JPG compression has difficulty averaging data related to text because of text's hard edges, so avoid compressing text heavy images. JPG does well with natural, gritty images because it mimics those kinds of rough edges. Finally, make sure you have done all of your editing before you compress to JPG. Color, tone and sharpening alterations directly on a JPG compressed image will exaggerate quality degradation.