Thank you for taking a look at this with me. Here is what I have done:
There are 14 images shot in RAW with an average size of 25 MB each - 3538 × 2748.
I went to 'Aperture/presets/image export' and the image export box comes up.
I would like to export every bit of info possible.
I set the dpi to 300 and clicked ok. The size that they come out at is only 6 or 7 MB
Should I use tiff instead of jpeg?
If you want to 'export every bit of info possible' you will need to export as a TIFF file, JPG will always loose information as it is a lossy compression format.
However depending on the use of the exported image JPG could be fine. If you describe the intended use of the images once exported we might be able to advise a setting that will give you maximum quality with minimum file size.
Thanks for weighing in on my lack of knowledge here. I am exporting the images to be used on high quality permanent DHPL signage in full color. The sign manufacturer requires 300 dpi and the RAW images that I have will meet that requirement.
Sounds like TIFF will make the grade, right?
TIFF will, but so will JPG with the right settings. 16-bit TIFF will retain more color information than 8-bit TIFF or JPG (which is limited to 8-bit).
An important and difficult point is that the DPI setting in the export preset has no effect on your image size or file size. Image size is measured in pixels. You most likely want to select "original size". File size (in bytes) is determined by Image size (pixels), color-depth (a/k/a "bit depth"), and compression. Compression is determined by the file format (JPG compresses unless set to highest quality -- JPG _is_ an image-compression file format developed for the Web -- TIFF is almost always uncompressed).
DPI is used by the _output device_ -- the printer or monitor on which the Image will be "printed" -- to determine printed size in inches.
You can set DPI in the Aperture print dialog to _anything_ and it will not change the pixel dimensions or the file size of the file you create.
RAW, btw, describes a family of proprietary sensor data file formats. RAW files are not image files -- to create a viewable image file a RAW file must be converted (by, naturally, a program called a RAW converter) to an image-format file. Aperture has a RAW converter built into it.
So, to the task at hand: crop your Image and make whatever adjustments you want. Export your Image as a 16-bit TIFF set to "original size" and put "300" in the DPI field (to make the print house staff happy). Send them the file.
Note that the rule-of-thumb is 300 _pixels_ for every inch of paper to be covered, in order to produce very sharp prints that can withstand scrutiny up close. The further from a picture the viewer stands, the fewer pixels per inch are needed to make the picture appear sharp. Some billboards are probably printed at about 4 pixels per inch.
Note that file size isn't all that useful of a metric. JPEG files compress, and file size depends a LOT on the JPEG settings.
For final export, TIFF doesn't have many benefits over JPEG, honestly. Unless you're going to be re-editing the file (which you wouldn't be after final export), the files will look the same if you don't use high compression on the JPEGs.
You want to be sure you're exporting the full RESOLUTION of the file. In the export preset, ensure "size to" is set to original size. Set the Image Quality to 10 (you can experiment with 11 and 12, which will make MUCH LARGER file sizes, but do compare the output of them to see if you can actually SEE the differences.. I'm pretty sure you won't be able to), and then set the DPI to 300.