Image size and jpg quality

E
Posted By
EBL
Nov 4, 2005
Views
731
Replies
4
Status
Closed
Hi–

I’m starting out with Camera Raw files, cropping a little, opening in PS CS2. I need to size the images to a maximum of 768 pixels (largest dimension), and save the image as a jpg at a maximum quality of 500 KB. However, even when I set the quality at the maximum, the KBs are only about 250. Am I missing something? Seems like, in the past, I’ve made images of the above size but ended up with files of more KBs. Thanks in advance for your help!

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MH
Mike Hyndman
Nov 4, 2005
These are small files from RAW, what is your camera’s pixel count? Re your question, what resolution are the jpegs? What ppi resolution? Are you using maximum or minimum compression when making the jpeg? On my system 12 is the highest quality and therefore lowest in compression/largest in file size. The more ppi for a given image size (physical) the larger the file size.
HTH
MH
E
EBL
Nov 5, 2005
Thanks–but I’m still confused!

Raw file 8.2 megapixels
I’m making maximum resolutions jpgs.
762 Pixels per inch
Maximum quality (12)

Here’s what I’m doing–
Open cropped CR2 file (size 6.62 MB; 3504×2336 dpi)
Convert to 8 bit
Details at this point (2114×2935 pixels; resolution 762 pixels/in)

Change pixel dimensions to the longest side at 768 constraining within (required for submission)
File Save As (or Save for Web–no difference) Choose JPG. Change quality slider to 12
Maximum size is 232K

I know I’ve done this before, and would use ~10 or 11 for file size up to 500 (again maximum allowed for submission).

I want to increase the quality as much as I can–any suggestions?

Thanks!
MH
Mike Hyndman
Nov 5, 2005
EBL,

An image of longest side 768 pixels with a resolution of 762ppi will give you a physical image of just over 1 inch x 0.553 inch
, is this correct?

MH

"Raw file 8.2 megapixels
I’m making maximum resolutions jpgs.
762 Pixels per inch (if this is ppi it seems very excessive, between 200&300 ppi should be sufficient}

Maximum quality (12)

Here’s what I’m doing–
Open cropped CR2 file (size 6.62 MB; 3504×2336 dpi)
Convert to 8 bit
Details at this point (2114×2935 pixels; resolution 762 pixels/in)" =2.77 inches x 3.85 inches

Change pixel dimensions to the longest side at 768 constraining within (required for submission)
File Save As (or Save for Web–no difference) Choose JPG. Change quality slider to 12
Maximum size is 232K
MH
Mike Hyndman
Nov 5, 2005
EBL,

You are definately working in RGB/8, not indexed? If you are your file sizes will be a quarter of the ones below.

Open cropped CR2 file (size 6.62 MB; 3504×2336 dpi)

These are pixels and have a "memory debt" of 8.18 MegaPixels (same as your camera?)

Convert to 8 bit
Details at this point (2114×2935 pixels; resolution 762 pixels/in)
= a "MB" of 6.29MegaPixels. In Photoshop, a new image of these dimensions has a pixel dimension of 17.2MB. If you then reduce the longest side to 768 pixels constrained, the other side will be 553 pixels. At 762 ppi resolution, this will give you an image of 1.008 x 0.726 inches. This gives a pixel dimension of 1.22 MB before compression. These values will differ depending on the complexity of the image.

See below CS2 Help file}

Pixel dimensions and image resolution
The number of pixels along the height and width of a bitmap image is called the pixel dimensions of an image. The number of pixels per inch (ppi) printed on a page determines the image resolution.
The amount of detail in an image depends on its pixel dimensions, whereas the image resolution controls how much space the pixels are printed over. For example, you can modify the resolution of an image without changing the actual pixel data in the image-all you change is the printed size of the image. However, if you want to maintain the same output dimensions, changing the resolution of the image requires a change in the total number of pixels.

Pixel dimensions equal document (output) size times resolution.
A. Original dimensions and resolution B. Decreasing the resolution without
changing pixel dimensions (no resampling) C. Decreasing the resolution at same document size decreases pixel dimensions (resampling). In Photoshop, you can change the resolution of an image. In ImageReady, the resolution of images is always 72 ppi, to optimize the images for online media.

Example of an image at 72 ppi and 300 ppi
When printed, an image with a high resolution contains more, and therefore smaller, pixels than an image with a low resolution. Higher-resolution images can reproduce greater detail and subtler color transitions than lower-resolution images because of the density of the pixels in the images. High-quality images often look good at any print size.
You can’t improve a lower-quality image by printing it at a high resolution. Changing the print resolution of an image simply makes each pixel larger, which results in pixelation-output with large, coarse-looking pixels. Increasing the print resolution of an image doesn’t add any pixel information to the image. You can make a low-resolution image look its best by picking a print size that makes the most of the pixels it has. Important: Video files are displayed only at 72 ppi. Even if an image has a higher resolution than 72 ppi, the quality may not be very good when it’s displayed in a video-editing application.

Printing the same low-resolution image at different sizes
A. Small print size B. Medium print size C. Large print size
File Save As (or Save for Web–no difference) Choose JPG. Change quality slider to 12
Maximum size is 232K

I know I’ve done this before, and would use ~10 or 11 for file size up to 500 (again maximum allowed for submission).

I want to increase the quality as much as I can–any suggestions?

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