View Full Version : Image compression

01-18-2007, 04:23 AM
I've noticed that when I take a jpeg and save it in Adobe Photoshop using the tool "save for web" which saves it as a gif file, it seems to be half the size of the original jpeg, without a loss of image quality.

Is this because gif files are naturally better compressed, or more likely I wil guess because the particular Photoshop tool just knows how to make files destined for web viewing small as far as image file size?

01-22-2007, 08:33 PM
Not sure if this is what you are looking for::confused:

JPEG mode is currently the mode used by the vast majority of photographers as their capture mode of choice. Why? I’m glad you asked. The two primary benefits JPEG mode offers are the small file size it produces and its compatibility. Because of the small file size, your camera is able to write from its memory buffer to the on-board memory card and from its memory card to your computer much faster than with other formats. For those action photographers out there, this means that you can capture more images more quickly without missing that critical shot. Additionally, the JPEG format is compatible, right out of the camera, with every – yes, I said every – image editing, image management or image printing software out there. As a result, you do not need special software to process the files to make them harmonious with your software, as you do with some file formats. Once downloaded from the camera to your computer, the smaller file size makes editing the image progress much more quickly.

You are probably asking yourself, “What’s wrong with JPEG?” For starters, JPEG is a lossy compression format which throws away pixels during the compression process. This brings us to the second disadvantage of JPEG, which is that the more you manipulate/edit and save your JPEG files, the more information you lose. After saving a JPEG file three or four times, even at low compression rates, you begin to notice visible degradation of your image. This means that you need to plan so that you can edit your image in one or two sessions, thus avoiding the loss of critical data and image quality. Finally, some say, and this point is debated by the photo gurus, because of camera compression, JPEG images may not be as sharp as TIFF or RAW files right out of the camera. I will leave this to your keen eyes to experiment and see.

The final contender in the battle of the supreme file format is the RAW file format. Unlike its two competitors, RAW files are not processed by the camera’s internal software. Instead, they are stored on the camera’s media in an unaltered, unadulterated form. This is great but is probably not enough to make RAW a viable contender. What does put RAW in the running is that RAW files have much great bit depth than its rivals. TIFF and JPEG capture images with only 8 bits of color per pixel. This translates as 256 shades of color per pixel, whereas the RAW file captures 12 bits of color per pixel or 4096 shades of color per pixel. Clearly, RAW has the advantage. With so much more information to work with, RAW files can be exploited by the greater processing power of a computer. High bit depth files therefore, are great for images that will need extensive tonal manipulation in Photoshop, or where you just want incredible fidelity to ensure smoother color transitions in the final photograph.

All right! All right! I know I said RAW capture was in 16-bit color earlier. The reason for this is that your camera actually captures 12 bits of color per pixel, but applications like Photoshop, which, because of the way computers work, process information much more efficiently in multiples of 8 (computers store numbers in 8-bit units,) uses the 16-bit figure to represent bit depth. Confusing? Maybe. Confounding? Quite possibly. But since when has anything digital been easy.

What is the down side of RAW capture? The file size of this format is large. Although not nearly as large as TIFF, it is still 5 times larger than JPEG. In addition, no standard for RAW files exists in the industry. And this leads to the most troubling part of RAW capture. In order to use the RAW file you must first process it using special software so that it is recognizable by applications like Photoshop. This software doesn’t mess with the pixel information, it just converts the file to a format that Photoshop can interpret. Since each manufacturer has its own flavor of RAW, each manufacturer has its own software for the conversion. Some of these programs work very well and some leave a great deal to be desired. As the RAW format becomes ubiquitous, more software companies are making programs that more effectively process RAW files, including Adobe, which makes CAMERA-RAW, a plug-in for Photoshop, which can convert RAW files from most of the major digital camera manufacturers. As of the latest version of Photoshop, Photoshop CS, the Camera RAW plug-in has been incorporated into the program and is no longer a separate program that needs to be added to Photoshop.

Finally, to answer the ultimate question “which format should you use?” I am going to tell you it depends. Here are my recommendations. If you want to capture a lot of images and the images are going to be used for the Web or are going to be printed on an inkjet printer (even at larger sizes) at home, JPEG would be a great bet. If you intend your images for use in the publishing industry and/or your subjects are perfectly lit so that there is minimal Photoshop work afterwards, then TIFF is the file format you want. If you are a control freak (which most of us photographers are when it comes to our images) and want ultimate control over the data in your image without manipulation by your camera and with a balance between file size and speed, then RAW is “it.” Ultimately, there is no right or wrong answer to the question of which file format to use. The answer is best based on your personal preferences, needs, experience and the ultimate use you intend for the image. Take the sage advice of great photographers throughout the ages — Test, Test and Test again. The only way to find out what you like and what works best for you in all the varied shooting situations you encounter is to experiment. Ultimately, it is through experimentation that you will be able to know what works and this alone will be the best guide to choosing the right file format. So when someone asks you if you shoot RAW, simple answer, “perhaps.”

01-23-2007, 12:24 AM
Yeah, what he said!

Great post SeaFox.

01-23-2007, 05:03 AM
Hey Cornel, thanks for the great explanation!

PS. Let's get out for some RB diving before we go to Truk.

01-28-2007, 04:19 AM
Excellent post Seafox. Personally I shoot with a Cannon 20D configured to produce both RAW and JPEG. It reduces the number of shots I can put on a card (not too worried I shoot with a 4G CF going to the new 8G) and slows my fps rate down but I love processing my work using Photoshop Camera Raw.

I find for the best manipulation of the RAW format the Cannon software is easier to manipulate but Photshop allows you greater control of the finshed product.

Dive safe :)


01-28-2007, 03:18 PM
You're quite right Dave.
Using the manufacturer's SW is indeed easier and faster and is useful for the small "quick and dirty" fixes.
PS is more complex and time consuming, but allows for much more control and flexibility over the finshed products.
Personally I am reluctant to use very large capacity cards, ie 4GB.
That's too much work lost should the unexpected happen, ie: flood!
Keep in mind that it's not if it's when!
I find that for me, 1 or 2 GB cards do well for a 2 hrs CCR dive.
I use Extreme lll cards for their speed and use in different environments ie" hot, warm and cold, both on deck and UW.



01-29-2007, 06:05 AM
You are correct about the CF card this is my 2nd 4G. I usually travel with the 4G as my primary, 2G backup and a 256K emergency. I have found you can never have enough for those occasions when something goes wrong.

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10-24-2009, 05:53 AM
Image compression is the application of data compression on digital images.. In effect, the objective is to reduce redundancy of the image data in order to be able to store or transmit data in an efficient form.

07-25-2011, 10:59 AM
Good info Seafox ... I often wondered about all these different formats but it would be easier for me now.