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Thread started 29 Jan 2003 (Wednesday) 19:37
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Best format to save a digital image without lossing quality

 
JR92
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Jan 29, 2003 19:37 |  #1

What is the best way to save a digital image and not lose quality over time. I know it is not JPEG, but what else, TIFF? EPS? Photoshop Doc.?




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robertwgross
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Jan 29, 2003 20:22 |  #2

JR92 wrote:
What is the best way to save a digital image and not lose quality over time. I know it is not JPEG, but what else, TIFF? EPS? Photoshop Doc.?

I think you will get your best results with TIF. There are at least two variations of TIF. One is "straight TIF" and one is "LZW TIF". The latter does a lossless compression on the file, so it takes a little longer to save and it packs into a tighter file size. Some images will "pack tightly" in LZW TIF, and some will come out about the same as straight TIF.

We could have a debate about 8-bit-per-color TIF (24-bits) or 16-bit-per-color TIF (48-bits). Obviously the latter takes up twice as much file space, and it could be debated as to what that really buys you.

The bottom line is: It depends.

It depends on what you are trying to shoot and what you want to do with the image afterward. If you tell us that you are simply posting a small image to the web, then that means something. If you tell us that you are printing at 24x36 inches, then that means something else.

---Bob Gross---




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JR92
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Jan 29, 2003 21:14 |  #3

Thanks for your advise Bob.
I am printing many of my images and I don't want to lose quality. Some of my wildlife shots I am having made into 20x30s. I will have my web-site up in a few weeks and probably just use JPEGs for my thumbnails.
Whta do you advise for best resolution? right now I have just been using 72. I don't know any better.
Thanks again, JR92




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Roger_Cavanagh
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Jan 30, 2003 07:42 |  #4

If you are using Photoshop (can't speak for other editors), there are now plug-ins available for formats such as PNG and JPEG 2000 that will provide lossless compression with good results. The downside is that not all applications support these formats and the formats don't support stuff like layers and EXIF. I have been wondering about J2k, but haven't made this switch yet because I don't get thumbnails displayed in IE or ThumbsPlus. Cerious are promising better J2k support in their next version, so I will reconsider then.

As for resolution and JPGs. If you are preparing the JPGs solely for screen/web use, then DPI is irrelevant. What matters is the pixels dimensions. DPI will only affect the physical print size of an image.

Regards,


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Dans_D60
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Jan 30, 2003 08:25 |  #5

Roger_Cavanagh wrote:


As for resolution and JPGs. If you are preparing the JPGs solely for screen/web use, then DPI is irrelevant. What matters is the pixels dimensions. DPI will only affect the physical print size of an image.

Regards,

I agree that DPI depth for JPG images on the web has little effect unless you drop below 72 DPI. Display screens cannot reproduce images larger than 72-96 DPI depending on the pixel layout. So anything larger just gets lost anyway. But, larger DPI images means larger file sizes. So as a rule of thumb all my images for web reproduction are set to 72 DPI while my print resolutions are set at 240 – 300 depending on the printer selection.

Dan
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Roger_Cavanagh
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Jan 30, 2003 13:39 |  #6

Dans_D60 wrote:
I agree that DPI depth for JPG images on the web has little effect unless you drop below 72 DPI. Display screens cannot reproduce images larger than 72-96 DPI depending on the pixel layout. So anything larger just gets lost anyway. But, larger DPI images means larger file sizes. So as a rule of thumb all my images for web reproduction are set to 72 DPI while my print resolutions are set at 240 – 300 depending on the printer selection.

Dan
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Dan,

Nope, DPI will not change the file size just by itself. If you are using Photoshop and change with DPI with the resample box checked then the dimensions of the image in pixels will change so that the physical print size of the image remains the same at the new DPI. But you can change the DPI with resample off and the number of pixels does not change. And, broadly speaking, the file size will be related to the number of pixels (ignoring JPG compression, layers, EXIF and other stuff).

The size of the image on screen will not change NO MATTER WHAT DPI - a pixel is a pixel is a pixel. If an image is 400 x 400 pixels that is how much real estate it will take up on your screen whether DPI is 72, 240, 300 or whatever. I'd post some example, but Photoshop is tied up doing a batch conversion right now.

Regards,


=============
Roger Cavanagh
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Dans_D60
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Joined Apr 2002
Temecula Ca
Jan 30, 2003 14:44 |  #7

Roger_Cavanagh wrote:

Dan,

Nope, DPI will not change the file size just by itself. If you are using Photoshop and change with DPI with the resample box checked then the dimensions of the image in pixels will change so that the physical print size of the image remains the same at the new DPI. But you can change the DPI with resample off and the number of pixels does not change. And, broadly speaking, the file size will be related to the number of pixels (ignoring JPG compression, layers, EXIF and other stuff).

The size of the image on screen will not change NO MATTER WHAT DPI - a pixel is a pixel is a pixel. If an image is 400 x 400 pixels that is how much real estate it will take up on your screen whether DPI is 72, 240, 300 or whatever. I'd post some example, but Photoshop is tied up doing a batch conversion right now.

Regards,

I guess the use of ‘size’ with pixel dimension, document size, and file size should have been more clearly explained. By maintaining the same document size (8X10) and reducing the DPI (300 to 72) would reduce the pixel dimensions (3200X2100 to 780X520) thereby reducing the file size. I totally agree a 400 X 400 image is what it is - and basic mathematics is constant (at least I hope so!). Sorry about the confusion!
Dan
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chewiebakka
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Feb 06, 2003 03:47 |  #8

so lzwTIFF just try to compress the tiff file?




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robertwgross
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Feb 06, 2003 12:26 |  #9

It is hard to understand it that is a question.

TIF holds all of the data in a non-lossy method, but the file takes up a lot of storage space.

LZW TIF holds all of the data, still non-lossy, but it uses computer power to "pack" the data into a tighter file. The good news is that if you've got a fast computer, the computer power is there and it does not slow down the file saving and reading _by_very_much. Note that some people are more interesting in flat out speed, so they won't do this.

The bad news is that the degree of file space saved varies from image to image. If you have an image file with lots of repetitive information (like large areas of constant color), then the LZW TIF "packs" it tightly and you save a bunch of space. For example, an original TIF file at 10MB might get saved as LZW TIF at only 4MB. OTOH, many photo-realistic images do not have much repetitive information, so the LZW TIF ends up being 10.1MB. If I see that happen, then I simply delete the LZW TIF and stay with the straight TIF.

I have about 50GB in stored image filies, so I've been down this road before.

---Bob Gross---




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