The effects that over compressing a JPEG file can have on your images
When it comes photo repairs you may scan in an image from a print, slide or negative and your scanning software does it all for you, it’s simple you just hit the go button and the job is done. Have you ever looked at the settings within the software to see how the software is saving your image? or if it gives you any options to alter the way the file is saved or scanned? Saving without compression or little compression is the best way to preserve detail in your image. A JPEG file is a “lossy” format, or a format where data is discarded in order to save space when saving. The higher the compression the more data is thrown away and the less detail there will be in your image.
There is no excuse these days, when hard disks are so very cheap to save your image with any other setting other than the best. It might be worth examining the software that came with your scanner or camera and check that you do have the option to change the way it saves and what format it saves your images in. Phrases to look for in the manual or software settings are “best setting” or the “lowest compression”, “lossless format” or “large file size”. You get the idea but do look for the top setting. Try to scan your image so that the longest side of the photo ends up at around 3000 pixels long, just scan the photo not the whole scanner bed.
To do this measure your photo in inches.
- 10 to 9 inches set the scanner to 300 Dpi
- 7 to 8 inches set the scanner to 400 Dpi
- 5 to 6 inches set the scanner to 500 to 600 Dpi
- 3 to 4 inches set the scanner to 1200 Dpi
- 1 to 2 inches set the scanner to 2400 Dpi
If you scanner manual lists its maximum optical scanning resolution then scanning above this will not really help with the final image. Any higher and the scanner will be adding pixels or data that were never there, this is called “interpolation”. We want to avoid any kind of interpolation if we want a good scan, we only want to restoring the original details not those that have been padded out by your scanner.
Saving your image.
To give you idea of what goes on when you save I have included an example below.
It is a photo of a M Street DC now let us save it at 3 different compression ratios.
As you can see the further we go with the compression the worse the data becomes and more the image suffers. The blocks you can see in the “0%” corner are where the JPEG algorithm splits up the photo into blocks in order to save it, the more data that is thrown away the more blocks there are visible. When this happens across a detailed section of the image the detail is lost, blocks meet and slurring of colours and details occur. This is called JPEG artifacts. It is these artifacts we do not want when our software takes over and saves our images for us.
On a further note, if you do save an image in this manner and then open it and make changes to it and re-save it with the same settings then the image will just get even worse.
In order for a restoration to be carried out to a high standard then highly compressed files must be avoided.
1. Scan with the appropriate DPI for the size of your image (see above)
2. Scan in colour, even if your photo is black and white.
3. Save with low compression, or biggest file size in JPEG or TIFF format.