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16 Bit Tiff's vs 8 Bit Tiff's

Kevin Trimmer , Mar 31, 2004; 09:37 a.m.

Recently i just purchased a Canon 10D, and have been overwhelming happy with both image quailty and camera design. I usually shoot everything in raw mode, and do an initial processing through the Canon software. However, when i then save the image as a 16 bit Tiff, i cannot make certain changes in photoshop, or drag the image into a different document without changing it to 8 bit, thus cutting the file size in half.

This may be a basic question, but i was wondering if there was an advantage to 16 bit over 8, and when using the image in another document, should i be concerned about the downgrade of file size that takes place when i convert it to 8 bits.

thanks alot

Responses

Beau Hooker , Mar 31, 2004; 10:34 a.m.

Hi Kevin, Versions of Photoshop prior to CS didn't support 16-bit images very well, but the new version (CS) does much better. I think most people would agree that, yes, there is an advantage to staying in 16-bit mode as much as possible or practical because there's much more information in a 16-bit file and things like tonal gradations should be much smoother. The downside to 16-bit is obviously the relatively huge filesizes you must deal with. They can really put a bite on your disk space, RAM and CPU. I tend to shoot RAW and use 16-bit only for images that are important to me - if it's just a few knock-off shots in the backyard, I set my 10D to large JPEG and don't fool with those big files. If you find working in RAW and using 16-bit images looks better, and most people do, you might consider springing for an upgrade to CS. Good luck!

Jean-Baptiste Queru , Mar 31, 2004; 11:01 a.m.

16-bit allows to avoid banding. If you start with an 8-bit picture and do a few operations on it, you'll quickly lose a few bits (When working from film I routinely do 2 or 3 histogram adjustments, a pair of color adjustments and a rotation, and probably lose 2 bits of data on the way).

I work with Photoshop CS and all the operations I want are supported in 16-bit.

So far I've only shot my 10D in JPEG, but the first thing I do when I work on a picture is to convert it to 16-bit. I still need to learn how to work from raw - but I've been happy with JPEG so far.

Andreas Carl , Mar 31, 2004; 11:52 a.m.

Yes, 16-bit is much better. Do all color adjustments, curves, levels, etc. in 16-bit!

QUESTION: Why do 16-bit files get larger with LZW "compression"???

Gordon Richardson , Mar 31, 2004; 12:27 p.m.

Andreas: 8-bit files also get larger with LZW - it's a known "bug"...

Ellis Vener , Mar 31, 2004; 12:36 p.m.

Upgrade to Photoshop CS. If you have a legitimate version of an earlier version of Ps the upgrade is $169.00

digital images are made up of three channels of picture data: Red, Green, & Blue. In 8 bit there are 256 "steps" of information per channel from absolute black to absolute white. In 12 bit (which is what your camera actually is producing, Photoshop just rounds it up to 16 bit) you get 4,096 "steps" per channel for the same range of information. In true 16 bit mode, you get 65,536 steps of color for each channel from absolute black to absolute white, so you can seethat as you use higher bit files each step is increasingly smaller. more and smaller steps means smoother color.

The reason this makes a difference, is that any editing step you make with a digital darkroom software program, cost you some steps of info for each edit. Start with more and smaller steps and the loss is increasingly minute with less consequences to the image.

Almost all output devices today are 24 bit (8 bits per R,G & B channel) the ideal is to be sendign the output device a "perfect 8 bit file".

Jeff White , Mar 31, 2004; 12:48 p.m.

Hi Kevin, I use Photoshop 6 and run into the same problems you do. Older versions of PS do not offer extensive manipulations in 16 bit mode because of the horsepower required. I may upgrade soon but until then.... 16 bit images are better from a quality standpoint because there is more information stored per pixel. When PS refers to "16 Bit color" it is talking about 16 bits per channel (there are three channels - R, G, and B). So you end up with an image that uses 48 total bits per pixel versus 24 for an "8 bit" image. That is why the files are so much larger. As someone in an earlier response noted, 16 bit mode will give you better color rendition because of the extra information stored. When you convert to 8 bit, you lose the additional number of colors that the extra information makes possible. HOWEVER, the difference is nonexistant for images to be viewed on the web and in most cases is negligible for those to be printed.

Jeff White
www.highlandgallery.com

Chris Combs , Mar 31, 2004; 02:04 p.m.

Re: LZW and larger TIFFs: As with any lossless compression algorithm, if the data is not inherently compressible, then the 'compressed' file will be larger... try ZIPping a ZIP. the second-generation ZIP will be larger than the first ZIP.

Jean-Baptiste Queru , Mar 31, 2004; 04:12 p.m.

Andreas : LZW isn't very efficient as a compression algorithm. A wide-window LZ77 (as used in zip or gzip) does better, to the point where zipping a gif often makes it smaller. LZ77 doesn't seem to pick any pattern in the Huffman compression used in JPEGs though.

Alistair Windsor , Apr 02, 2004; 04:33 p.m.

I still use PhotoShop 7 and it supports all the features I want on 16 bit files. For selections I create an 8 bit duplicate, save the mask as an alpha mask, and then load the selection into the 16 bit image. I also make extensive use of snapshots and the history brush. The only thing I really miss is the flexibility of adjustment layers.

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