the relationship between image quality, size and "dpi"

Your digital image does not have any physical dimensions, unless it is displayed on a screen or printed. Up to then it is just an array of pixels, samples with intensities and colors. The three parameters influence the size and you will be able to display your image.

sizeto : There are several options to set a size on export:

• Fit within pixel: If you pick less pixel than your original image has, you will create an image with less geometrical resolution. You will lose detail. And there is no sense in picking a larger pixel size than your image has. You will only increase the file size, without adding quality.
• Fit within inches/cm: This should be used in combination with the dpi setting. This option will ensure, that the image can be displayed or printed with given physical dimensions in cm or inches, with the given number of pixels per inch.

Quality: Jpeg is a lossy image format. It will represent the the contents of the image only approximately and thus save diskspace. The higher the "Quality" value, the better your image will be represented, but higher quality levels will require more disk space.

The "quality" setting is independent of the "sizeto" setting.

Regards

Léonie

Dpi only has any meaning if you are physically printing the image. It has no meaning it the image is displayed electronically.

DPI will determine how big a given image can be printed at. For example. You want to print an image at 8in X 10in and the final print will be hung in a gallery and be viewed from fairly close range. For this type of viewing a minimum of 200 dpi is needed with 300 dpi being the 'gold standard'.

(the closer an image is to be viewed at the higher the DPI needs to be. If an image is to viewed at long range the dpi can drop.)

So lets go with 300dpi. So the image is 8x10 that means you need 8x300 dots by 10x300 dots or 2400x3000 dots or 7,200,000 pixels (dots and pixels aren't exactly the same but for this discussion they are close enough).

So to print out your image at 8x10 at 300dpi you need to have at least 7.2mp

But again this only holds for printed images. If the image is displayed on a screen the pixel resolution of the screen will determine the dpi.

Dpi only has any meaning if you are physically printing the image. It has no meaning it the image is displayed electronically.

I'd rather say, you can ignore the dpi when viewing an image electronically (if they are present at all) and are free to scale the image and view it at any zoom level you like. But quite a few applications will use the dpi values and display the image initially at the size you set or compute the size from the dpi and pixelsize. For example GraphicConverter or Adobe Photoshop. If I set the GraphicConverter Viewer to "View at original size" it will show the image in the exact length and width computed from dpi and pixelsize. I see the dpi as an attribute of the digital image, a hint, how it is to be viewed. When you insert images into documents, frequently they are added to the page with respect to the size or dpi in the IPTC.

For example, the image below has been exported from Aperture with the size set to "within 4 inches" and a resolution of 200dpi. When I view it in GraphicConverter "at original size" this will display it on the screen in a 4x3 inches window. In the screenshot below the mouse was hovering in the lower right corner. The 36% in the lower right corner are indicating that the resolution of the screen is less that the 200dpi set in the IPTC

So, setting the dpi parameter and/or the size in inches will give directions to other applications on how to display or print the image.

Regards

Léonie

But I guess I'll just go for top quality12.

It will help to know the original quality your image has been taken with. If you took the image with the camera set to a lower quality, then 12 would be wasteful. You cannot recreate quality that is not there originally.

At quality 12 I got a 5.9mb file, dimensions 3000 x 2338. At quality 8 I got 926 kb, but again size 3000 x 2338.

All rather confusing!

The quality slider essentially controls the amount of compression when the image is converted to a jpeg - and thus the accuracy of the approximation to the original image, but also the file size. 12 will give you an image with all details perfectlyrecognizable, but a very large file size, nearly no compression; and about 8 or lower you will start to notice jpeg artefacts and fine structures may be lost.

There is no general recepy how to set the jpeg quality. It will depend on how you will want to use your jpeg.

• If you are mailing the the jpeg, or adding to a web page for download, set the quality to a lower value. Here you will want small files for faster download.
• If you want to export for printing, or for further processing by other aplications, or to archive the jpeg, set the quality very high.

And always inspect the resulting jpeg, if you like it. Zoom in and look at the contours of objects in the image and at plain backgrounds, if you see any artefacts or blurriness.

For example: jpeg quality 4, zoomed in:

I see that Leonie but am wondering how the quality changes but the number of pixels does not! Aren't pixels the basic unit of information?

A grid of pixels is the basic internal representation of an digital image, yes. But jpeg is an external format to store this grid efficiently, so you need less storage. When you store a jpeg, the quality slider will tell the compression algorithm, how much detail and variation you want to keep.

It will store a description of your digital image, that will suffice to recreate the digital image with the exact numbers of pixels, say 4000 by 3000, but not accurately. The jpeg on your disk does not enumerate the pixels one by one exactly, but rather describes blocks of neighboring pixels summarily by the coefficients of mathematical approximation functions. With a low quality setting these blocks will show as large square areas of nearly the same color, but with a high quality setting the bocks will be smaller and more detailed. The higher the quality setting, the more coefficients will be used to get a better approximation of the image, and the larger the file size.