Export 72dpi - big file, small pixellated image!

Question

I have a 12MP RAW file from a 5D Mk1. I've cropped the top and bottom off (group shot short and wide) and I'm trying to export it although I've got a much bigger TIFF file (39MP) and it's very pixellated and 72dpi when I zoom in. Is this correct?

Answer 1

"DPI" is a term which mostly only applies to exporting and mostly for purposes of printing.
Images displayed on your screen are scaled based on the zoom factor you choose (or you can display them in 'actual size' if you want to see a one-to-one mapping of each pixel in the image is mapped to a pixel on your screen.  DPI is ignored.
When you 'export' an image, you have the option to specify DPI based on how you plan to use the image.  By default, images are exported based on pre-defined presets
e.g. "full size" means that every pixel is exported - regardless of DPI -- you get all of them, no matter how many there are.
e.g. "50% size" means the image will be rescaled to half as many pixels wide by half as many pixels tall.  Again, DPI really has no meaning here.
e.g. "fit within 1024x1024" means that the "longest" side of the image will be scaled to fit within 1024 and the "short" size will be scaled proportionately (unless your original image really was square -- like that of a Hasselblad or Rollei medium format film camera.)
But you could export an image and say you want an 8 x 10 at 300dpi.  If the image doesn't doesn't have the exact number of pixels required for your width & height and requested resolution, then Aperture will re-sample the image to create an approximation of the image in the new size you require.  This way when it is printed... at 300dpi... it really will be 8" x 10" on the printer.
DPI is handy (even if it's a conceptual value and not a real thing) because it allows you to more accurately output an image to a specific device -- especially when it's a variable resolution device like a printer.  For example if you want to display an image on the web, but you want it to be a 4 x 6 image, you could export 4 x 6 at 72dpi because most monitors are 72dpi.  When the image is displayed elsewhere, it will probably be pretty close to the size you requested.  If, on the other hand, you need to create a print and you want an 8 x 10, you could ask for 300dpi.  While most printers can do better, 300dpi looks pretty good.  If you were to ask for a 16 x 20" print at 1200dpi, Aperture would have to resample the image and invent a lot of data (average or interpolate the missing data) because there's no way your camera _really_ takes 460 megapixel images, but 28 megapixels.... a high end camera might just do that (16x20 at 300dpi = 28.8 megapixels.)
BTW, Aperture wont crop the image automatically... so if you ask for a size that doesn't match the aspect ratio of the image, you'll probably need to select how you want to crop the image so that it's new aspect ratio matches the print size you want.