July/August 2005 HelpLine

JPEG Reloaded

    * Saved By JPEG
    * Timing Of File Saving
    * It’s All In The Name

I received a lot of responses to a HelpLine question regarding JPEG and its effect on image files that ran in the May issue of PCPhoto. That much interest shows that readers are concerned about image quality. Since this is an important issue, I thought I’d revisit the topic. So, with apologies to the Wachowski brothers, here’s “JPEG Reloaded.”      
Saved By JPEG

Q)  I’ve heard from many sources that a JPEG file that’s opened and resaved multiple times will lose quality. But I’m starting to wonder how much this is really a factor. I did a quick test and showed the results to others in the office, and no one could tell the difference after 10 saves. Your thoughts on this?

Tim Jenkins
Via e-mail

A)  I appreciate and welcome your skepticism. It shows how much you care about image quality. There are many occasions where I’m skeptical, too. It helps us understand the technology and how it can be used to make great images.

I’ve run a simple test on a large file. The image printed here was photographed using a Canon EOS 20D shooting in the RAW mode. I took the image into Photoshop CS2 via Camera Raw, doing little to optimize the shot. I saved the file as a .psd file, the native file format for Photoshop.

Next, I saved a copy of the file as a JPEG, using maximum quality settings, and called the file c01.jpg. (Keep in mind, however, that in many image editors, maximum isn’t the default setting.)


Here’s the important part. I closed the original file in Photoshop and opened c01.jpg. This has now reconstructed the compressed file.

Then, in order to simulate some editing, I cropped the file by a pixel on the top and left. After that, I did a Save As, again selecting JPEG for the copy, using maximum quality settings, and called it c02.jpg. I closed c01.jpg, opened c02.jpg (reconstructing the file again) and continued this process (Open, Crop, Save As, Close) until I got to c10.jpg.

Finally, I opened the original, together with the c10.jpg image and took a look at them to see what kind of compression artifacts I’d get. You can see them, although you have to enlarge the image a bit to do so. Around the child’s head, you can see obvious compression artifacts. You also can see them in the skin tones next to face details. While they might not show up in certain uses like small Web images, they will appear when printed.

For the record, I did this same test without adjusting the image at all—no cropping—just Open, Save As and Close 10 times. The artifacts still were there, but they were just a little less obvious.

So if you want to keep as much honest data in your images by keeping them artifact-free for as long as possible, only compress when you have to and stay away from using JPEG as an archival or storage format. Why wouldn’t you want to use JPEG for archives? First of all, when you save the image for the last time, you’re compressing it again, so what you see on the screen isn’t what you saved. Also, can you honestly say that you’ll never reprocess that image? Might you use it in a montage with another image, or would you edit differently for a different type of output? If you can avoid compression, you’ll have more imaging options in the future.

Timing Of File Saving

Q)  In the May 2005 PCPhoto, you suggest the following: “[I resave] the edited images in the native format of the image-editing application that I’m using. When I’m satisfied with my adjustments, I also save a copy of the file in TIFF format.” I believe this leads you to a wrong sequence of events.

Tom McGrath
Via e-mail

A)  This brings up a couple of misconceptions about saving image files. The first is how saving or resaving a file in a different format works. Let’s take an example of a simple file shot with a digital camera and recorded in a high-resolution JPEG format. I’ll call the file image.jpg.

First, I’ll open the file in Photoshop. Regardless of the name of the file, it’s now sitting in Photoshop in Photoshop’s native file format. Something similar happens in any image-processing program. It can only edit in its native format.

I make some adjustments. Next, I decide to save the file, so I’ll save it first in the native image format. At this point, I have to tell Photoshop to save it in something other than its original file type, even though it’s in the native file format while being worked on. I go to File > Save As and select .psd and save the file as a copy. Now, I end up with a file on my hard drive called image.psd.

I also want to save this file as a TIFF file so that I can put it on a CD and send it in for publication. With the image still open in my image editor, I select Save As and choose TIFF as the file format. The new TIFF file is created on my hard drive, but the actual file open in Photoshop is still in its native file format because, again, an image-processing program can only make adjustments to its own file format.

Next, to send the image via e-mail to friends, I select Save As and this time choose JPEG. I could continue doing this, saving GIFs, PNGs, TARGAs, PICTs, BMPs and all sorts of other file formats, but realize that I’m still working from the native file format (which is the same as the image.psd file). When you use Save As, a new file is created, but the open image isn’t affected.

Ultimately, it makes no difference in what order you save files from an open file in your image processor. It also doesn’t matter if the files are compressed or uncompressed because as long as the file remains open, it’s still at work in the native file format. It’s only when the file is closed that you could have problems, but even then, it would be with compressed files, such as JPEG, not uncompressed files such as the native file format or TIFF. If you closed a file to a JPEG format, then reopened it and saved it again as a TIFF, you’d lose quality because of the compression changes.



It’s All In The Name

Q)  You’ve said that you batch-rename all your photographs prior to archiving them. Would you clarify what batch-renaming does without altering the original JPEG files? What program might you recommend for batch-renaming?

Paul Castenholz
Colorado Springs, Colorado

A)  First, it helps to understand how file names are changed, as batch-renaming is simply renaming many files at once. The long way would be to open the file and resave it with a new name. After reading the previous answer, you know that this would not be the best procedure, particularly for compressed file formats.

Another approach is to rename the file at the operating system level. With Windows, right-click on the file, choose Rename, and the system highlights the file name so you can type a new one. If you prefer, you can click once on the file name, wait for a second, then click again on the file name and type the new one. (You have to wait between clicks or the system will recognize this as a double-click, and the file will open.)

On a Mac, click once on the file name (not the icon) and it will highlight so you can rename. You also can select the file and use Command+I to bring up the info window with the file name highlighted so you can rename.

Notice that when you rename using either o
perating system, you’re not opening the file; you’re just changing the name. This prevents the file recompression we’re trying to avoid.

To automate the task of renaming, check to see if your image-editing software has any batch or automatic functions. Many do and typically are located under the file menu (in Adobe Photoshop, check in the Automate menu). In addition, most browser programs, such as ACDSee from ACD Systems or iView MediaPro, offer extensive batch-renaming capabilities.

If you have any questions, please send them to HelpLine, PCPhoto Magazine, 12121 Wilshire Blvd., Ste. 1200, Los Angeles, CA 90025 or helpline@pcphotomag.com.


Leave a Comment